UNIVERSITY OF CALIFOR> 
 AT LOS ANGELES
 
 The Fundamentals 
 
 of 
 
 Photography 
 
 <B y C. E. K. Mees, D.Sc. 
 
 Eastman Kodak Company 
 
 Rochester, N. Y. 
 1921 
 
 4- 794G2
 
 PREFACE 
 
 WHILE a knowledge of the theory of photography is by 
 no means essential for success in the making of pic- 
 tures, most photographers must have felt a curiosity as to 
 the scientific foundations of the art and have wished to 
 know more of the materials which they use, and of the re- 
 actions which those materials undergo when exposed to light 
 and when treated with the chemical baths by which the 
 finished result is obtained. This book has been written 
 with the object of providing an elementary account of the 
 theoretical foundations of photography, in language which 
 can be followed by readers without any specialized scien- 
 tific training. It is hoped that it will interest photogra- 
 phers in the scientific side of their work and aid them 
 in getting, through attention to the technical manipulation 
 of their materials, the best results which can be obtained. 
 
 ROCHESTER, N. Y. 
 January, 1921
 
 First Edition August 1920 
 Second Edition January 1921
 
 CONTENTS 
 CHAP. PAGE 
 
 PREFACE . . . . . . . ... 3 
 
 I. THE BEGINNINGS OF PHOTOGRAPHY . .7 
 II. LIGHT AND VISION ...... 14 
 
 III. ABOUT LENSES ...... 20 
 
 IV. THE LIGHT SENSITIVE MATERIALS USED IN 
 
 PHOTOGRAPHY 34 
 
 V. STRUCTURE OF THE DEVELOPED IMAGE . . 40 
 VI. EXPOSURE ....... 49 
 
 VII. DEVELOPMENT 58 
 
 VIII. REPRODUCTION OF LIGHT AND SHADE IN 
 
 PHOTOGRAPHY . . . . . .67 
 
 IX. PRINTING 78 
 
 X. THE FINISHING OF THE NEGATIVE ... 94 
 
 XI. HALATION 106 
 
 XII. ORTHOCHROMATIC PHOTOGRAPHY 110
 
 THE FUNDAMENTALS OF 
 I PHOTOGRAPHY 
 
 CHAPTER I. 
 
 THE BEGINNINGS OF PHOTOGKArHY. 
 
 THE first person to notice that chloride of silver was 
 darkened by light may have been J. H. Schulze, 
 who made the discovery in 1732. It is probable, how- 
 ever, that this had been observed by others. In 1737 Hellot 
 in Paris, was trying to make sympathetic inks, that is, inks 
 that would be invisible when put on paper but which could 
 be made visible afterwards. He found that if he wrote on 
 paper with a solution of silver nitrate, the writing would not 
 be visible until the paper was exposed to light, at which 
 time it would turn dark and could be read. However, no use 
 was made of these discoveries for the purpose of making pic- 
 tures until 1802, when Wedgwood published a paper entitled 
 "An Account of a Method of Copying Paintings on Glass 
 and on Making Profiles by the Agency of Light upon Nitrate 
 of Silver." 
 
 This reference to making profiles is a reference to one of 
 the forms of portraiture which preceded photography. Be- 
 fore portrait photography was discovered, there were people 
 who made what were called "silhouettes", which were pro- 
 file pictures cut out of black paper and stuck on to white 
 paper. Some of these silhouettists were very clever indeed. 
 Others who had not great ability arranged their sitter so 
 that they got sharp shadows thrown by a lamp onto a white 
 screen and this gave them the profile to copy. Wedgwood 
 thought that instead of cutting out the silhouette he might 
 print this profile on the screen by using paper treated with 
 silver nitrate, which would darken in the light. Wedgwood 
 not only used his new process to record these silhouettes, 
 but he tried to take photographs in What was then called
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 the "camera obscura", which was the forerunner of the 
 Kodak of to-day. 
 
 The camera obscura consisted of a box with a lens at one 
 end and a ground glass at the other, just like a modern 
 camera. It was used by artists to make a picture of anything 
 they wanted to draw, as by observing the picture on 
 
 Fig. 1 
 Silhouette Picture from Old Print. 
 
 the ground glass they could draw it more easily. Wedgwood 
 tried to make pictures in his camera obscura by putting his 
 prepared paper in the place of the ground glass. His paper 
 however, was too insensitive to obtain any result; but Sir 
 Humphrey Davy, who continued Wedgwood's experiments, 
 using chloride of silver instead of nitrate, succeeded in mak- 
 ing photographs through a microscope by using sunlight. 
 These are apparently the first pictures made by means of a 
 lens on a photographic material. 
 
 But all these attempts of Wedgwood and Davy failed be- 
 cause no method could be found for making the pictures 
 permanent. The paper treated with silver chloride or silver 
 nitrate was still sensitive to light after part of it had dark- 
 ened, and if it were kept it soon went dark all over and the 
 picture was lost. Davy concluded his account of the ex- 
 periments by saying: "Nothing but a method of prevent- 
 ing the unshaded parts from being coloured by .exposure to
 
 THE BEGINNINGS OF PHOTOGRAPHY 
 
 the day is wanting to render this process as useful as it is 
 elegant." 
 
 This much needed method, however, remained wanting 
 from 1802 until 1839, when Sir John Herschel found that 
 
 Fig. 2. 
 Crystals of Thiosulphate of Soda or "Hypo". 
 
 "hypo", which he had himself discovered in 1819, could dis- 
 solve away the unaltered chloride of silver and enable him to 
 "fix" the picture, as the process has been called ever since 
 Herschel made the discovery, and from that time to this 
 hypo has been the mainstay of the photographer, enabling 
 him to fix his pictures after he has obtained them. 
 
 In the meantime, Niepce in France had been working on 
 an entirely different process, depending on the fact that 
 such substances as resin. or asphalt became insoluble when 
 exposed to light, and he had succeeded in producing results 
 by taking advantage of this property. In France also, 
 Daguerre was working on various methods by which he 
 hoped to make photographs, and entered into partnership 
 with Niepce, but in 1839 Daguerre published the method 
 of photography which was named for him Daguerreotype. 
 This was the first portrait process and became very popu- 
 lar. It depends upon the sensitiveness of plates of metallic 
 silver which have been fumed with iodine so that the sur- 
 face is converted into a thin layer of silver iodide. The 
 plates so treated are exposed to light, and after a very 
 long exposure, as we should consider it now, the plate in 
 the dark is exposed to the vapor of metallic mercury, 
 which deposits itself upon the image and produces a posi- 
 tive image of mercury upon silver.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 The results were very beautiful, but these early processes 
 of photography required very great exposures so that at 
 first the unfortunate subject had to sit for as long as ten 
 minutes in the full sun without moving in order to impress 
 the plate sufficiently. Although many experiments were 
 made in an attempt to find substances more sensitive to 
 light so that the exposure could be reduced, the only real 
 solution was to find some method by which light had to 
 do only a little of the work and the production of the image 
 itself could be effected by chemical action instead of by the 
 action of the light. 
 
 A great step in this direction was taken by Fox Talbot 
 in 1841. He found that if he prepared a sheet of paper 
 with silver iodide and exposed it in the camera he got only 
 a very faint image, but if after exposure he washed over 
 the paper with a solution containing silver nitrate and gallic 
 acid, a solution from which metallic silver is very easily de- 
 posited, then this solution deposited the silver where the 
 light had acted and built up the faint image into a strong 
 picture. This building up of a faint image or, indeed, of an 
 
 image which is altogether 
 invisible, into a picture is 
 what is now called "de- 
 velopment". If we expose 
 a film in the Kodak and 
 then, after the shutter has 
 allowed the light to act for 
 a fraction of a second on 
 the film, look at the film in 
 red light, which will not 
 affect it, we shall not be 
 able to see any change in 
 the film. But if we put the 
 film into a developing solu- 
 tion, the invisible image 
 which was produced by 
 light, and which in photo- 
 graphic books is called "the latent image" will be developed 
 into a black negative representing the scene that was photo- 
 graphed^ 
 
 Fox Talbot was not only the first to develop a faint or 
 invisible image; he was also the first man to make a nega- 
 tive and use it for printing. What is meant by a negative 
 is this: If we look at our film after we have exposed and 
 developed it, we shall find that the sky, which was bright in 
 
 Fig. 3. 
 Negative Image.
 
 THE BEGINNINGS OF PHOTOGRAPHY 
 
 the picture, is shown in our film as very black,while any shad- 
 ows in the picture, which, of course, were dark, will be 
 transparent in the film, so that the light let through the 
 film is in the reverse order of the scene photographed, 
 all the bright parts in the scene being dark in the film and 
 the dark parts bright. For this reason the film is called a 
 "negative," and when it is printed on paper the same 
 reversal happens again and the clear parts in the negative 
 become dark in the print while the dark parts of the 
 negative protect the paper from the action of the light, 
 so that the print which we may call a "positive," represents 
 the scene as it appeared. 
 
 Fox Talbot, then, made two of the great steps in the ad- 
 vancement of photography when he found how to expose 
 his paper for a time insufficient to darken it completely, and 
 then to develop a negative which he could print on paper 
 covered by silver chloride. Of course, the paper was not 
 transparent as our film is, but he made it more transparent 
 by treating it with oil or wax. In this he was followed many 
 years afterwards in the Eastman roll holder, which was the 
 forerunner of all the Kodaks. In this roll holder at first a 
 paper film was used to make 
 the negative and then the 
 paper was made transpar- 
 ent for printing. 
 
 Fox Talbot's paper nega- 
 tives were succeeded by the 
 method known as the wet 
 collodion process, which has 
 survived to the present day. 
 This is the process chiefly 
 used by photo-engravers for 
 making the negatives from 
 which they make the en- 
 graved metal plates for 
 printing pictures. 
 
 Collodion is made by dis- 
 solving nitrated cotton, 
 
 such as is now used for the film base, in a mixture of ether 
 and alcohol. The worker of the wet collodion process had to 
 make his own plates at the time when he wanted to take a 
 picture. He would clean a piece of glass and coat it,with 
 the collodion in which the chemicals were dissolved and 
 then put the plate in a bath of nitrate of silver, which 
 
 Fig. 4. 
 Positive or Print.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 formed silver iodide in the collodion film and made it 
 sensitive to light. Then the glass had to be exposed in the 
 camera while wet, and immediately after exposure it was 
 
 developed by pouring the 
 developer over it. It was 
 then fixed and dried. 
 
 In order to carry out 
 these operations a photo- 
 grapher who wanted to 
 take landscapes had to car- 
 ry with him a folding tent 
 which he could set up in 
 the open air. The tent was 
 dark except for a yellow or 
 red window by which to 
 see to make the plates and 
 develop them. 
 
 All this difficulty in 
 working disappeared with 
 the coming of the gelatine 
 emulsion process, which is 
 the one now used. The 
 sensitive coating on films 
 and papers now consists of 
 a bromide or chloride of 
 silver held in a thin sheet 
 of gelatine, the gelatine 
 being dissolved in hot 
 
 Fig. 5. 
 
 Early Photographer with His Equipment. 
 
 water, the silver salt formed in the solution, and the warm 
 solution of gelatine containing silver then coated on the 
 film or paper. 
 
 The gelatine solution with the silver in it is called an 
 "emulsion" because of the way in which the silver remains 
 suspended in the gelatine. The first gelatine emulsions were 
 made in 1871 by Dr. Maddox. An emulsion made in much 
 the way that we use now was first sold in 1873 by Burgess. 
 
 At first the early experimenters made and sold the emul- 
 sion itself, drying it for sale so that photographers had to 
 take this dried emulsion, melt it up in hot water, and coat 
 it on their plates. After a time, however, people realized 
 that this was a great deal of trouble and that there was no 
 reason why the manufacturer of the emulsion should not coat 
 the glass plates with it, and sell the ready prepared plates. 
 
 In those days all negatives were made on glass plates. 
 These plates were coated with the emulsion by hand and 
 12
 
 THE BEGINNINGS OF PHOTOGRAPHY 
 
 then when the emulsion was spread over them were put 
 on to cold level slabs for the jelly to set before drying. 
 Glass plates are cumbersome and heavy, and for this reason 
 George Eastman continually experimented to substitute 
 a light, flexible support for the brittle and heavy glass. As 
 already mentioned, he first used paper as a support for the 
 negative, waxing it to make it transparent for printing. 
 This was followed by a paper from which the film carrying 
 the image was stripped, the film being transferred to a glass 
 plate coated with gelatine so that this gelatine made a sup- 
 port for the film. 
 
 While experimenting to find a more satisfactory material 
 for coating the film than gelatine it was found that a solu- 
 tion of nitrated cotton would make a clear, transparent and 
 flexible support, and after a period of further experimenting 
 this material was adopted and a roll film was made, the 
 emulsion being carried on the clear, transparent sheet of 
 film support. The only remaining difficulty with this was 
 its tendency to curl owing to the gelatine coating on one 
 side, and this was overcome by coating the other side with 
 plain gelatine, thus producing the non-curling (NC) film.
 
 CHAPTER II. 
 
 LIGHT AND VISION. 
 
 Light is the name which we give to the external agency 
 which enables us to see. In order to see things we must have 
 something which enters the eye and a brain to explain it to 
 us. That which enters the eye is what we call light. 
 
 The eye consists of two principal 
 parts and can best be understood 
 by analogy with the camera. In 
 front it has a lens which forms an 
 image on the sensitive surface, 
 which is called the retina, the 
 retina playing the same part in the 
 eye that the film does in the cam- 
 era. The retina, however, differs 
 from the film in that when light 
 falls upon the film it produces a 
 permanent change, which can be pj g 
 
 developed into a picture, and if Diagram of Human Eye. 
 the light falls upon the film for too 
 
 long a time the film is spoiled, while the retina merely acts 
 as a medium to transmit to the brain the sensation of the 
 light that falls upon it, and when the light stops, the sensa- 
 tion stops and the retina is ready to make a new record. The 
 retina behaves, in fact, like a film in which the sensitive 
 material is continually renewed. 
 
 It is probable that this sensitive material in the eye is 
 really of a chemical nature because it is apparently produced 
 all the time, and when the eye is kept in the dark the sensi- 
 tive material accumulates for some time so that the eye be- 
 comes more sensitive, while when a strong light falls upon 
 the eye, the sensitive substance is destroyed more rapidly 
 than it is produced and the eye becomes less sensitive. 
 
 In this way, the eye has a very great range of sensitive- 
 ness. In bright sunlight it is as much as a million times less 
 sensitive than it is after it has been kept for an hour in the 
 dark, and it changes very rapidly, only a few minutes being 
 necessary for an eye that has been in almost complete dark- 
 ness to adapt itself to the glare of out-door lighting. In 
 order to lessen the shock of changing light intensity, the 
 lens of the eye is provided with an iris diaphragm just like 
 14
 
 LIGHT AND VISION 
 
 that of a camera, but with the additional advantage that it 
 operates automatically, opening and closing according to 
 the intensity of the light. Measurements of the movements 
 
 of the iris of the eye have 
 been made by taking 
 motion pictures of the 
 eye when suddenly il- 
 luminated by a bright 
 light, and these show 
 what a wonderful instru- 
 ment the eye is in its 
 adaptation to changing 
 conditions in the world 
 around it. 
 
 The retina is connect- 
 ed with the brain by a 
 great many nerve fibers, 
 each fiber coming from a 
 different part of the ret- 
 ina, so that when light 
 falls upon any part of 
 the retina, the intensity 
 of the light is communi- 
 cated by the tiny nerve 
 coming from that part of 
 the retina to the brain 
 and the brain forms an 
 idea of the image on the 
 retina by means of the 
 multitude of impressions 
 from different parts of 
 the retina. 
 
 The image on the ret- 
 ina is inverted like all 
 lens images, so that we 
 really see things stand- 
 ing on their heads, but 
 the brain interprets an 
 inverted image on the retina as corresponding to an upright 
 external world, and although the eye sees things upside 
 down, the brain has no idea of it. 
 
 What we observe is the light which falls on the retina, but 
 this light comes originally from some external source which, 
 in the case of daylight, of course, is the sun. The light from 
 the sun is reflected by the objects in the world around us 
 
 15 
 
 Fig. 7. 
 Iris Opening and Closing.
 
 THE FUNDAMENTALS OF PHOTOGRAPHY 
 
 according to their nature, and entering the eye it enables 
 us to see the objects. When we look at a landscape we see 
 that the sky is bright and the roads and fields are less bright, 
 and the shadows under the trees are dark, because much of 
 the light of the sun is reflected from the sky, less from the 
 fields and roads and still less from the shadows under the 
 trees. All these rays from the sun reflected from the natural 
 objects in the landscape enter the eye and make a picture on 
 the retina which is perceived by the brain by means of the 
 tiny nerve fibers coming from the retina to the brain. 
 
 But the eye not only perceives differences in the bright- 
 ness of the light it also observes differences in colour and 
 in order to understand how this can be we must search 
 further into the nature of light itself. 
 
 The nature of light has long been a source of speculation, 
 and at one time it was generally held that the light which 
 entered the eye consisted of small particles shot off from the 
 source of light, just as at one time it was held that sound 
 consisted of small particles shot off from the source of a 
 sound which struck the drum of the ear. This theory of 
 light has the advantage that it immediately explains reflec- 
 tion; just as an india rubber 
 ball bounces from a smooth 
 wall, while it will be shot in ^~ ^ /~"\ 
 
 almost any direction from a / \ f \ 
 
 heap of stones, so the small \./ \ J 
 
 particles of light would re- 
 bound from a polished sur- 
 face at a regular angle, while 
 a rough surface would mere- GREEN 
 
 ly scatter them. x~\ /~\ /^\ 
 
 This theory of the nature ^ V + V -f- *- 
 
 of light was satisfactory un- > / V_X 
 
 til it was found that it was 
 possible by dividing a beam 
 of light and slightly length- 
 ening the path of one of the BLUE 
 halves, and then reuniting (~\ S~\ f~\ /" 
 the two halves together \~T \~J V/ 
 again, to produce alternate 
 
 periods of darkness and light Relatiye Wave {;**; of Red . Green 
 similar to the nodes of rest and Blue. 
 
 produced in an organ pipe, 
 
 where the interference of the waves of sound is taking place. 
 
 It could not be imagined that a reinforcement of one stream 
 
 16
 
 LIGHT AND VISION 
 
 of particles by another stream of particles in the same 
 direction could produce an absence of particles, while the 
 analogy of sound suggested that just as sound was known to 
 consist of waves in the air, so light also consisted of waves. 
 
 BLUE VI 
 
 OLET 
 
 ei 
 
 (M 
 
 GRI 
 
 EN 
 
 0A. 
 YfL 
 
 C 
 
 LOW 
 
 RED 
 
 400 
 
 450 
 
 500 
 
 600 
 
 550 
 
 Fig. 9. 
 Simple Arrangement of Spectrum. 
 
 700 
 
 Light cannot consist of waves in the air, partly because 
 we know that it travels through interstellar space, where 
 we imagine that there is no air but through which we can 
 still see the light of the stars, and also because the ve- 
 locity of light nearly 200,000 miles per second is so great 
 that it is impossible that it could consist of a wave in any 
 material substance with which we are acquainted. It is, 
 therefore, assumed that there exists, spread through all 
 space and all matter, something in which the waves of light 
 are formed, and this something is termed ether, so that it is 
 generally held that light consists of waves in the ether. 
 
 Just as in sound we have wave notes of high frequency, 
 that is, with many waves per second falling upon the ear, 
 which form the high pitched notes, and also notes of low 
 frequency where only a few waves a second fall upon the 
 ear forming the bass notes, so with light we may have dif- 
 ferent frequencies of vibration. Since the velocity of light 
 is the same for waves of different frequencies, it is clear that 
 the waves of high frequency will be of different wave length 
 from those of low frequency, the wave length being the dis- 
 tance from the crest of one wave to the crest of the next, 
 and if we obtain waves of different lengths separated out, 
 we shall find that the color depends upon the wave length. 
 Fig. 8 shows the average length of wave corresponding to 
 light of various colors, the diagram being drawn to scale. 
 
 White light consists of mixtures of waves of various 
 lengths, but if instead of letting the mixture of waves, which 
 forms white light, fall directly on the eye we pass white 
 light through an instrument known as a spectroscope, 
 which changes the direction of the different waves by 
 amounts which differ according to their lengths, we get the 
 white light spread out into a band of colors which we call
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 the spectrum, and we can scale this spectrum by means of 
 numbers representing the lengths of the waves. 
 
 Fig. 9 gives a simple arrangement of the spectrum, the 
 numbers representing the wave lengths in units which are 
 millionths of millimeters. It will be seen that the visi- 
 le spectrum extends from 700 to 400 units, wave lengths 
 of 700 units corresponding to the extreme red and 400 
 to the darkest violet that can be seen, while the. brightest 
 region of the spectrum stretches from 500 to 600 units 
 and includes the green and yellow colors. The spectrum 
 is equally divided into three regions which may be broadly 
 termed red 700-600, green 600-500, and blue-violet 
 500-400. 
 
 If we get a piece of colored glass which lets through only 
 the portion of the spectrum between 600 and 700, then 
 we should have a piece of red glass; a glass which let through 
 from 500 to 600 would be a green glass, and one which 
 let through from 400 to 500 would be blue-violet in color, 
 so that from the spectrum we already derive the idea that 
 light can be conveniently divided into three colors, which 
 we may call the primary colors red, green and blue-violet. 
 It is probable that this is connected with the structure of 
 the retina, and one theorv holds that there are three sets of 
 
 Fig. 10. 
 Portions of Spectrum Transmitted by Primaries. 
 
 18
 
 LIGHT AND VISION 
 
 receiving nerves in all parts of the retina, corresponding to 
 the three primary colors red, green and blue- violet. 
 
 If we let white light fall upon anything, such as a piece of 
 white paper, which reflects all the wave lengths to the same 
 extent, then the reflected light remains white and we should 
 say that the object on which it falls is uncolored, but if the 
 object absorbs some of the wave lengths of the spectrum 
 more than others, then it will appear colored. Thus, a 
 piece of red paper appears red because from the white light 
 falling upon it it absorbs some of the green and blue-violet 
 light, but reflects all the red light and, therefore, appears 
 red. In the same way a green object absorbs both red and 
 blue-violet more than it absorbs the green light and so looks 
 green, and a yellow object absorbs the blue, reflecting the 
 red and green of the spectrum and so appears yellow. 
 
 Light waves differ not only in their length but in their 
 amplitude, that is, in the height of the wave, and the ampli- 
 tude controls the intensity of the light just as the wave 
 length controls the color. The eye, therefore, can detect 
 differences in brightness which depend upon amplitude, and 
 also differences of color which depend upon wave length.
 
 CHAPTER III. 
 
 ABOUT LENSES. 
 
 IN order to take a photograph we use a lens which forms 
 an image of the object we want to photograph upon the 
 film. The simplest lens which we could use would be a 
 small hole. Suppose that we take a sheet of cardboard and 
 make a hole in it with a pin, and then, in a darkened room, 
 hold, the cardboard between a sheet of white paper and an 
 electric lamp ; we shall see on the paper an image of the lamp 
 filament. 
 
 The diagram shows how 
 this image is produced. A 
 ray of light from each por- 
 tion of the filament passes 
 through the pinhole and forms 
 a spot of light on the paper, 
 and all these spots joining to- 
 gether form the image of the 
 filament. 
 
 If we take the lens out of a 
 camera and replace it by a 
 thin piece of metal pierced 
 with a hole made by a needle (a No. 10 sewing needle is 
 about right, and the edges of the hole must be beveled off 
 so that they are sharp), then we can take excellent photo- 
 graphs by giving sufficient exposure. 
 
 If the pinhole is about six inches from the film then an 
 exposure of about one minute for an outdoor picture on 
 film will be required. It is necessary, of course, to make a 
 well fitting cap for the lens aperture so that no light will 
 get in except through the pinhole, and also to make a 
 cover for the pinhole to act as a shutter for exposing. 
 
 But if a pinhole were the only means of forming an image 
 it is very improbable that photography would ever have 
 been developed, since the exposures are so long in conse- 
 quence of the small amount of light which can pass through 
 the pinhole. 
 
 Fig. 11. 
 How an Image is Produced.
 
 ABOUT LENSES 
 
 Fig. 12. 
 Pinhole Image of a Star. 
 
 In order to get more light we could try making the pin- 
 hole larger, but the effect of this is to make the image very 
 indistinct, and even the smallest efficient pinhole can not 
 give as sharp an image as a 
 good lens. 
 
 Suppose we have a small 
 pinhole forming an image of 
 a star, as shown in Fig. 12. 
 
 If we make the hole larger, 
 we shall get a round, spread- 
 ing beam of light and no long- 
 er get a sharp image. (Fig. 
 13.) 
 
 What we need, if we are to 
 use the large hole is, some 
 means of bending the light so that all the light reaching 
 the hole from the star is joined again in a sharp image of 
 the star on the screen, as 
 shown in Fig. 14. 
 
 If a ray of light falls on a 
 piece of glass so that it is not 
 perpendicular to it, it will be 
 bent. There is an interesting 
 experiment which shows this 
 very well. Take a thick block 
 of glass and place it so that it 
 touches a pin (which is mark- 
 ed B in Fig. 15) and stick an- 
 other pin (A) in the board. 
 Now look through the glass 
 and stick a pin (D) between your eye and the glass, and in 
 the same line of sight as A and B, and lastly another pin 
 (C) touching the glass and 
 in the same line of sight as 
 the other three. 
 
 Take away the glass and 
 join up the pinholes with pen- 
 cil lines. You will find that 
 the line DC is parallel to the 
 line AB but is not in the same 
 line; that is, the ray of light 
 marked by the line AB was 
 
 Fig. 13. 
 Effect of Large Pinhole. 
 
 Fig. 14. 
 Need of Means to Bend Light. 
 
 bent when it entered the glass 
 and then bent back again 
 when it left it, so we can bend light by means of glass. 
 
 21 

 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 If we take a triangular piece of glass (called a prism) we 
 can bend a ray when it enters the glass and also more still 
 when it leaves the glass. (Fig. 17.) 
 
 And a lens is really two 
 prisms stuck together base 
 to base (Fig. 18). So that 
 if we put a lens in the hole 
 with which we want to 
 form an image, we can do 
 what we wish to and make 
 all the rays from the star 
 come together again in the 
 image of the star. And this 
 is the purpose of our cam- 
 era tense's, to" form an image 
 sharper than that given by 
 the smallest pinhole and yet 
 
 Fig. 15. 
 Deflecting a Ray of Light. 
 
 much brighter than any pinhole would give. 
 
 Should we place a pinhole, instead of a lens, in the front- 
 board of our camera, we could use the same size of pinhole 
 for making all sizes of pict- 
 ures, because the image 
 formed by a pinhole is al- 
 ways of the same sharpness, 
 whether the pinhole is far 
 from the film or close to it. 
 If we want a large picture 
 we must, of course, use a 
 large camera with a long 
 bellows, so the pinhole will 
 be a long way from the 
 film, while if we want a 
 small picture we shall only Fig. 16. 
 
 need a small camera with Path of Deflected Ray. 
 
 a short bellows, so the pin- 
 hole will be near the film. But if, instead of a pinhole, we 
 use a lens, we shall find that the lens must be placed at a 
 certain distance from the film 
 (depending upon its focal 
 length and its distance from 
 the object photographed) in 
 
 PRI/AA 
 
 Fig. 17. 
 Prism Bending a Ray. 
 
 order to obtain a sharp pic- 
 ture. If it is placed at any 
 other distance from the film 
 the picture will be all blurred. The reason for this is that
 
 ABOUT LENSES 
 
 a photographic lens bends the rays of light that pass through 
 it so that all the light rays from a star, for instance, will 
 meet again to form an image of the star. By placing a sheet 
 of cardboard at the position where the rays of light meet, 
 the image of the star will be sharp, but if we put the card 
 either nearer to or farther 
 from the lens, the image will 
 be blurred into a circle of 
 light. Thejiistance at which 
 the lens must be placed from 
 
 Fig. IS. 
 Rays Bent by Double Prism. 
 
 the film to give a sharp image 
 represents the "focal length" 
 of the lens. 
 
 Fig. 19. 
 
 The longer the focal length of a lens the larger the image, 
 and the shorter the focal length the smaller the image. 
 Suppose we photograph a tree 
 and place the camera at such 
 a distance from the tree that 
 with a lens of three inches 
 focal length we obtain a pic- 
 ture in which the image of 
 the tree is one inch long. 
 
 Now, if with the camera at 
 the same distance from the 
 tree, we had used a six-inch 
 Jens instead ot the three-inch 
 lens, which means that in- 
 stead of the lens being three Lens FormiVgVsharp Image, 
 inches from the film it would 
 be six inches from it, then the image of the tree would be 
 _two inches long instead of one inch long TrTtrie picture. IT 
 
 we were using the same size 
 film with both lenses, of 
 course we should not be able 
 to include as much of the sub- 
 ject we were photographing 
 in the field of view of the pic- 
 ture made with the six-inch 
 Fig. 20 l ens as we should obtain with 
 
 Images formed by a pmhole at various 
 
 distances. _ , 
 
 the three-inch lens, because 
 with the three-inch lens the 
 tree would be, say, a quarter 
 of the length of the picture, p . 2 
 
 While with the six-inch lens it A lens forms an imfge at only one point.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 would be half the length of the picture. In other words, the 
 three-inch lens would give us a smaller image, while the 
 six-inch lens would give us a large image of the tree. 
 
 Fig. 22. 
 Short Focal Length Means Small Image. 
 
 The longer the focal length of a lens, the less subject we 
 include in our picture, and the larger the images of objects 
 are, while the shorter the focal length, the more subject 
 we include in the picture and the smaller the images are. 
 
 In actual practice we must compromise between a lens 
 which will include as large an area as possible in the field 
 of view, and a lens which will give images as large as pos- 
 sible; consequently, for general all-around purposes it is best 
 to use a lens whose focal length is somewhat longer than the 
 longest side of the film. For a 2^2 x 4^ film, for instance, 
 we should use a lens of about 5 inches focal length. 
 
 It is most important not to use a lens of too short a focal 
 length for the size of the film employed. There is a great 
 temptation to do this. While a lens of 4}/ inch focus as com- 
 pared with a lens of three inch focus means a big lens in place 
 
 FOCflL LENGTH 
 
 Fig. 23. 
 Long Focal Length, Larger Image. 
 
 of a little lens, and a larger shutter and a somewhat larger 
 camera in place of a smaller shutter and an extremely com- 
 pact camera, it also means (and this is vastly more impor- 
 tant than mere camera compactness) the making of pictures 
 having good perspective instead of pictures with bad per- 
 24
 
 ABOUT LENSES 
 
 spective; in other words, it means pictures the drawing in 
 which looks right instead of pictures whose drawing looks 
 
 wrong. The reason for 
 this is that the per- 
 spective of a picture 
 is determined by the 
 point of view from 
 which the lens makes 
 the picture. If this 
 perspective is not 
 pleasing to the eye it 
 will not be pleasing 
 in the picture. 
 
 Fig. 24 shows a pic- 
 ture made with a very 
 short focus lens used 
 close to the subject. 
 This is a faithful ren- 
 dering of the perspec- 
 tive that the eye saw 
 from the viewpoint of 
 the lens, but it is far 
 
 Fig. 24. 
 Made With a Very Short Focus Lens. 
 
 Fig. 25. Made With a Long Focus Lens. 
 
 from pleasing. 
 
 In Fig. 25 the same 
 subject is shown 
 photographed with a 
 long focus lens, and 
 in this picture the 
 perspective is satis- 
 factory. It likewise 
 represents the per- 
 spective that the eye 
 saw from the view- 
 point of the lens. 
 
 It is a good rule to 
 secure a lens which 
 has a focal length at 
 least equal to the 
 diagonal of the film. 
 A little more focal 
 length is still better. 
 
 Lenses differ in an- 
 other respect than 
 their focal length. 
 They differ in the
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 amount of light they admit, and this is very important, 
 because the more light admitted, the shorter the exposure 
 
 can be. The chief object in 
 using a lens instead of a pin- 
 hole is to transmit more light 
 to the film, and the amount 
 of light that is transmitted 
 depends upon the area of the 
 glass in the lens. 
 
 Suppose we place a piece 
 of cardboard, instead of a 
 Fig. 26. "" S film, in the back of a camera, 
 
 Visible Area with Long Focus, and have a pinhole in the 
 card through which we can 
 
 look at the lens; then point the lens toward a window; the 
 amount of light that reaches the eye through the hole in 
 the card depends upon how much of the light from the 
 window is passing through the lens; that is to say, it will 
 depend on the area of the window which we could see if 
 there was no glass in the lens. Of course, since the visible 
 area of the window is bounded by the edges of the lens 
 mount, we_could see more 
 if the_ lens were of shorter 
 focal length so that the eye 
 was closer to it. With a 
 lens of long focal length 
 only a small part of the 
 window area is visible. 
 
 With a lens of half the 
 focal length but of the 
 same diameter as that 
 shown in Fig. 26, four times 
 as much of the window area 
 is visible. 
 
 The brightness of the image projected by lenses of the 
 same diameter varies inversely as the square of the focal 
 length of the lens. It also varies as the area of the lens 
 surface (aperture) which admits the light. The greater 
 the lens aperture the more light it admits. Now the area of 
 the lens aperture, of course, is proportional to the square 
 of its diameter, so that all lenses in which the diameter 
 .of the aperture bears the same ratio to the focal length 
 will give equally bright images. This means that the bright- 
 ness of the image is determined not solely by the focal 
 length, nor solely by the diameter of the lens aperture, but 
 
 26 
 
 Fig. 27. 
 Visible Area with Short Focus.
 
 ABOUT LENSES 
 
 by the relation that exists between the lens aperture and 
 the focal length of the lens, so that all lenses in which the 
 diameter of the opening is, say, one-sixth of the focal length, 
 will give equally bright images. Thus, in a lens of one-inch 
 aperture and a focal length of six inches, the opening is 
 one-sixth of the focal length, and in a lens of twelve inches 
 focal length and two inches aperture, the opening is like- 
 wise one-sixth of the focal length. Both lenses are of the 
 same / value. This means that both give an image of the 
 same brightness, and will require the same exposure. . Lens 
 "apertures" are, therefore, rated according to the ratio be- 
 tween their diameter and their focal lengths; thus, one in 
 which the opening is one-sixth of the focal length is marked 
 /.6; one in which the opening is one-eighth, /.8, and so on, 
 and the larger the aperture, the more light the lens trans- 
 mits, and the more light it transmits the shorter the expos- 
 ure needed. 
 
 But while large lens apertures have the advantage of 
 permitting shorter exposures, they have some disadvant- 
 ages. In the first place, to get a large aperture we must 
 have a large lens, and this means an expensive lens; also, 
 the errors of definition, which are called the "aberrations" 
 of lenses, increase very much as the apertures increase, so 
 that only the very best types of lenses in which these aberra- 
 tions are removed to as great an extent as possible, can be 
 made of large aperture and still give good definition. Large 
 aperture lenses are therefore costly. 
 
 But even when we have a lens with a large aperture we 
 shall have to regard this as a reserve power for use in 
 special circumstances, and we shall not by any means be 
 able to use it at its largest aperture all the time. 
 
 From the construction of a lens it follows that only the 
 rays from a mathematical point can come together in a 
 point again, and that the rays from any point nearer or 
 farther than the point focused can not meet in a point 
 image on the film, but must produce a small disc of light 
 instead of a sharp point of light. (See Fig. 21.) 
 
 The disc is termed the circle of confusion. If the circle 
 of confusion is small enough we shall not be able to dis- . 
 tinguish it from a point, and the picture will appear to be 
 sharp. 
 
 With what are known as "fixed focus" cameras, such as 
 
 the Vest Pocket Kodaks and the Box Brownies, no attempt 
 
 is made to secure a wholly sharp focus for objects at all 
 
 distances, but the cameras are sharply focused on the near- 
 
 27
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Focu of Foct 
 Distant Nea 
 Object 01.] 
 
 est point to the camera which will still enable distant ob- 
 jects to appear approximately sharp in the pictures, and in 
 this way objects in the middle distance are perfectly sharp, 
 and near objects are also sharp, provided they are not too 
 near. 
 
 The following table of these distances, beyond which 
 everything is sharp when the largest stop is used, may be 
 useful : 
 
 Vest Pocket Kodak ... . 9 feet 
 No. Brownie . . . . . 9 
 No. 2 Brownie . . . . . 133/2 " 
 No. 2A, 2C and No. 3 Brownie . 15 
 
 If we are using a No. Brownie, for instance, as long as 
 everything is farther off than nine (9) feet we can rely on 
 getting a picture with everything focused sharply. 
 
 With the focusing Kodaks we must judge the distance 
 of the object on which we wish the focus to be sharpest 
 and set the scale to that; then we shall find that objects 
 somewhat nearer, and also objects a good deal farther from 
 
 the camera are also sharp, 
 and the distance from 
 -j- i i the nearest to the far- 
 thest objects that ap- 
 pear sharp in the nega- 
 tive is called the "depth 
 of focus." This depth of 
 focus depends on the 
 focal length of the lens and on the size of stop used in the 
 lens; the greater the focal length the less the depth of focus, 
 and the bigger the stop the less the depth of focus. Thus in 
 Fig. 28, we have a lens 
 focusing near and far 
 points at full aperture -- 
 and producing large cir- 
 cles of confusion. In Fig. p. 9 
 29 a smaller stop is used Depth of Focus wi ' t g h ' Smaller Aperture, 
 in the same lens, and the 
 
 circles diminish in size in proportion to reduction in the 
 size of the stop. 
 
 Sometimes we have to focus near objects at the same 
 time as distant ones, so that it is necessary to "stop the 
 lens down" to some extent. 
 
 StQEsare marked in two different systems, though both 
 are baseoTon the fundamental ratio of the diameter to the 
 28 
 
 Fig. 28. 
 Depth of Focus with Full Aperture.
 
 ABOUT LENSES 
 
 focal length of the lens. In the one system the stop is ex- 
 pressed simply as a fraction of the focal length; thus F./8 
 (commonly written /.8) means that the aperture is one- 
 eighth of the focal length of the lens;/.16, one-sixteenth, 
 and so on. The rectilinear lenses fitted to Kodaks are, how- 
 ever, marked in the "Uniform System" (U. S.) in which 
 the numbers are proportional to the exposure required, /.4 
 being taken as unity, so that the scale is as follows: 
 
 F. /.4 /.5.6 f.6.3 /.8 /.ll /.16 /.22 /.32 /.45 
 U. S. 1 2 2Y 2 4 8 16 32 64 128 
 
 The U. S. numbers give the relative exposure that is re- 
 quired with the /. system stops, the exposure varying as 
 the square of the/, value, so that /.1 1 requires twice the 
 exposure of /.8;/.16 twice that of/. 11 and so on. 
 
 Kodaks, Premo and Brownie cameras are listed with 
 several different kinds of lenses, the smaller cameras being 
 listed with either Meniscus, Meniscus Achromatic, Rapid 
 RectilineaV or Anastigmat Lenses. The larger cameras have 
 either Rapid Rectilinear or Anastigmat Lenses, while the 
 Special Kodaks and Graflex cameras have Anastigmats only. 
 The Box Brownies are equipped with Meniscus or Meniscus 
 Achromatic Lenses, while with the Folding Brownies there 
 is a choice between Meniscus Achromatic and Rapid Recti- 
 linear lenses. 
 
 Many people do not understand the meaning of these 
 terms, and while it is a safe rule to choose the best lens 
 which can be afforded, certain that the better lens is worth 
 the extra cost, it is still better to understand the properties 
 of the different kinds of lenses and what advantages can be 
 gained from the use of the higher grades. 
 
 The simplest lenses which can be used are made of a single 
 piece of glass, the form of the lens being of the type which 
 gives the best definition; that is, a Meniscus or crescent 
 shape, and the lenses are called Meniscus (not Meniscus 
 Achromatic) lenses. Such a Meniscus lens can only be 
 used in a fixed focus camera where the maker of the camera 
 has put it in the correct position for forming a sharp image 
 upon the film, but if such a lens were used in a focusing 
 camera we should find that however carefully we focused 
 the picture on the ground glass the negatives would not be 
 sharp, unless the difference between the focusing point 
 of the visual rays by which we focus, and the chemical rays 
 which affect the film, was provided for. 
 29
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 This is because a non-achromatic lens bends the rays of 
 light of different colors to different extents, so that the 
 yellow rays which we use for focusing do not come to a 
 focus in the same place as the blue rays which affect the 
 film, because the blue rays are bent more than the yellow. 
 
 In 1752, Dollond, an 
 English optician, showed 
 ^ that by combining two dif- 
 
 BU/E poo/5 ferent kinds of glass to 
 2-YELuw" make a lens he could get 
 
 . _. Fi s- 30. the blue rays to focus at 
 
 Focus of Blue and Yellow Rays. ^he same point ^ thg ye ,_ 
 
 low rays, and lenses made in this way were called "achro- 
 matics," from the Greek words "a" meaning not, and 
 "chroma" meaning color. The best shape of achromatic 
 lens to use is shown in Fig. 31, and since this is also of a 
 "meniscus" or crescent shape the lenses are called meniscus 
 achromatics. If a single achromatic lens is used, it is neces- 
 sary to "stop it down" so that only a small 
 portion of the lens is used, because the rays 
 which come through the edges do not focus 
 together as well as those which comethrough 
 the center, and so the image is not quite sharp 
 if the whole lens area is used. 
 
 This stopped-down meniscus lens has the 
 effect of producing slight curvature of the Fig. 31. 
 edges of the picture, which does not matter Achromatic Lens. 
 in landscape work or portraiture; but if subjects containing 
 straight marginal lines are photographed with such a lens, 
 their outer lines appear slightly curved so slightly, how- 
 ever, that the effect is negligible unless the image of the 
 subject so crowds the picture area that its outer lines are 
 very near the margins of the picture, as shown by figures 
 32 and 33, which represent a window sash photographed 
 with a meniscus lens at short range. 
 
 If the stop is in front of the lens the curvature is in one 
 direction, and if it is behind the lens the curvature is in the 
 opposite direction, so that if we put two lenses to- 
 gether with the stop between them, the curvature is neutra- 
 lized and we get a lens which gives no curvature at all. 
 
 Such a lens is called a "Rapid Rectilinear" rectilinear 
 because it gives straight-line images, and rapid because 
 having a focal length half that of either of the component 
 lenses with a stop of the same diameter, it passes 
 four times as much light and only requires one-quarter of 
 
 3
 
 ABOUT LENSES 
 
 the exposure. Rapid Rectilinears are sometimes called by 
 other names, such as "Rapid Aplanats," "Planatographs," 
 and so on. Now, it so happens that the two kinds of glass 
 used in an achromat must fulfill certain conditions to bring 
 the blue and the yellow rays to the same focus, and must 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 Made w 
 of 
 
 Fig. 32. 
 th Stop in Front 
 leniscus Lens 
 
 Fig. 33. Fig. 34. 
 
 Made with Stop Behind Made with Rectilinear 
 Meniscus Lens. Lens 
 
 fulfill certain other conditions to get a picture which is flat, 
 that is, a picture that is sharp on a flat plate or film; and 
 the ordinary glasses which are used for making achromats 
 will not fulfill all these conditions at once, so that the lenses 
 made with "old" achromats will not give flat field images, 
 the image being saucer-shaped. These lenses are, therefore, 
 said to be "astigmatic," which means that they do not give 
 sharp-point images of points. 
 
 About thirty years ago, Professor Abbe and Otto Schott, 
 working together at Jena, found out how to make new kinds 
 of optical glass from which lenses could be made which 
 would give flat field images with the blue and yellow rays 
 of the same focus. 
 
 By the use of these new glasses the opticians have been 
 able to make lenses that give sharp images on a flat field to 
 the very edge of the picture and, therefore, these lenses are 
 called "Anastigmats," meaning "not astigmatic," but this 
 better defining power can, however, only be obtained by 
 the most careful and skilled work in making the lens, this 
 work being of a far higher quality than that employed on 
 the older types of lenses, which accounts for the higher cost 
 of anastigmats. 
 
 Anastigmat lenses can be used with larger stops than 
 any of the older lenses, so that if an Achromatic working 
 
 3 1
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 at/. 16 requires a 1/5 second exposure, a Rapid Rectilinear 
 working at /.8 will require a 1/20 second exposure, and an 
 Anastigmat working at f.6.3 will require a 1/32 second ex- 
 posure. 
 
 To summarize the advantages and disadvantages of the 
 three types of lenses discussed in the preceding pages : 
 
 The single lenses (meniscus and meniscus achromatic) 
 must be used with a relatively small stop, which means 
 that they are somewhat slow. They are fast enough for 
 snapshots in good light, the shutters they are fitted with 
 being adjusted for the making of moderately slow "snaps". 
 The very fact that they require a small stop gives them 
 great depth of focus, however, and for that reason errors in 
 focusing are largely compensated for, resulting in a high 
 percentage of successful pictures. 
 
 The Rapid Rectilinear Lenses have more speed than the 
 single lenses, and are also better for architectural work. 
 
 The Anastigmat, f.6.3, lenses are about sixty per cent 
 faster than the Rapid Rectilinear lenses and are corrected 
 for the finest definition (sharpness). When used at their 
 full speed that is, with the largest opening they require 
 accurate focusing, although it should be borne in mind that 
 both the length of focus and the stop opening affect this 
 matter of depth of focus. That is why the 3A,the largest of 
 the Kodaks, requires more accurate focusing than the 
 smaller ones, and is why, when we get down to the Vest 
 Pocket size, it is possible to use an Anastigmat lens with a 
 fixed focus. 
 
 An Anastigmat lens does not require any more accurate 
 focusing than any other lens when used with the same stop. 
 Take, for instance, an average landscape with a prominent 
 object in the foreground. The correct stop would be/.16 
 and, if the sun were shining, the correct exposure 1/25 of a 
 second. This same stop and exposure should be used 
 with a Single lens, a Rapid Rectilinear or an Anastigmat, 
 and the depth of focus with the same focal length of lens 
 would be the same in all cases no more accurate focusing 
 would be required with one lens than with another. 
 
 But when the light is weak and an Anastigmat is used 
 at its full opening, or nearly its full opening, in order 
 to get a well timed snapshot, there will be a gain in 
 speed but a loss in depth of focus. The object at the 
 focused distance may photograph even sharper than it 
 would with the Single or Rapid Rectilinear lenses, but 
 objects a little nearer the camera or a little farther away 
 
 ^ 32
 
 ABOUT LENSES 
 
 will not be so sharp because depth of focus has been sacri- 
 ficed for speed. And, of course, this same thing is true in 
 using a large stop in order to arrest the motion of moving 
 objects. With a fixed-focus camera working at a fixed 
 shutter speed, all still objects at, say, fifty feet away, 
 would be sharp and, with a good light, fully timed, but 
 moving objects might show a blur. With an Anastigmat 
 lens opened to/.6.3 and a shutter speed of 1/200 of a second, 
 it is possible to arrest moderately fast motion and get a 
 fully timed negative (with good light), but in such case care 
 must be taken to focus accurately. 
 
 33
 
 CHAPTER IV. 
 
 THE LIGHT SENSITIVE MATERIALS USED IN PHOTOGRAPHY. 
 
 AS was explained in Chapter I, the sensitive coating on 
 films and papers consists of bromide or chloride of silver 
 held in a thin layer of gelatine, and thus, photography de- 
 pends upon the fact that the shiny, white metal silver when 
 combined with certain other substances forms compounds 
 which are sensitive to light and which are changed in their 
 nature when they are exposed to light. 
 
 Chemical compounds are formed by the combination in 
 definite proportions of a limited number of elements, of 
 which about eighty exist. 
 
 These elements may be divided into the two classes of 
 metals and non-metals, and the metals combining with the 
 non-metals form compounds called salts. These salts are 
 not usually formed by the direct combination of the metal 
 and the non-metal but by the agency of acids. 
 
 !g- 
 
 Crystals of Silver Nitrate. 
 
 Thus, the first step in making a light sensitive compound 
 of silver is to dissolve the silver in nitric acid. After the 
 silver has been dissolved by the acid, and then dried up 
 we get flat, plate-like crystals of silver nitrate. These 
 
 34
 
 LIGHT SENSITIVE MATERIALS IN PHOTOGRAPHY 
 
 crystals of silver nitrate dissolve in water quite easily, 
 but if some cooking salt solution is added to the silver 
 nitrate solution, the silver combines with one of the com- 
 ponents of the salt, called chlorine, and the silver chloride 
 that is produced is not soluble in water, so that it will be 
 visible as a sort of white mud in the solution. 
 
 Chlorine is one of a group of elements which, because 
 they occur in sea salt, are called halogens, from the Greek 
 name for the salt sea. Two others of these elements are 
 bromine and iodine, and the silver compounds with these 
 three elements are distinguished by their extreme insolu- 
 bility in water and their sensitiveness to light. Silver 
 bromide is more insoluble than silver chloride and is pale 
 yellow in colour; silver iodide is still more insoluble and is 
 strongly yellow. 
 
 These silver compounds are formed by simply adding a 
 solution of a chloride (such as cooking salt), bromide or 
 iodide to a solution of silver nitrate. If this is done in a 
 water solution, the silver compound will settle down to the 
 
 Fig. 36. 
 Two Flasks Containing Precipitated Silver Bromide: Left, Without 
 
 Gelatine: Right, With Gelatine. 
 
 The flask on the left shows that silver bromide without gelatine settles to the bottom 
 
 of. the solution. The one on the right shows the silver bromide held 
 
 evenly in suspension by gelatine. 
 
 35
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 bottom of the vessel, but this may be prevented by adding 
 to the water some gelatine, like that used for cooking. 
 
 The gelatine is soaked in water, and then when it is swol- 
 len it is dissolved by putting it in warm water and gently 
 warming and shaking until it is all dissolved. Then there is 
 added to this the right quantity of bromide. The bromide 
 dissolves in the gelatine solution just as salt would, and is 
 stirred up to get it evenly distributed. Meanwhile, some 
 silver nitrate has been weighed out so that the right amount 
 is taken to act with the amount of bromide chosen and is 
 dissolved in water, in which it dissolves very easily. This 
 silver nitrate solution is then added slowly to the bromide 
 dissolved in the gelatine, and produces at once a precipitate 
 of silver bromide. This silver bromide is sensitive to light 
 so that before adding the silver nitrate to the bromide and 
 gelatine all the white lights are turned out and the silver is 
 added by the light of a photographic red lamp. 
 
 As the silver is added a little at a time, the solution 
 being stirred meanwhile, the gelatine becomes full of the 
 smoothly, evenly precipitated silver bromide distributed 
 through the solution. 
 
 If the emulsion of silver bromide in gelatine is coated on 
 the film and then cooled, the gelatine will set to a jelly, still 
 containing the silver bromide suspended in it, and then 
 when this layer is dried, we get the smooth yellowish coat- 
 ing, which is familiar to those of us who have looked at an 
 undeveloped film in the light. 
 
 If we look at the silver bromide film through a very high 
 power microscope, we shall find that the silver bromide is 
 distributed throughout it in the form of tiny crystals. These 
 crystals are in the form of flat triangular or hexagonal 
 plates, and careful investigation has shown that they be- 
 long to the regular system of crystals. When these crystals 
 are exposed to light, no visible change takes place, but there 
 must be some change because when a crystal of silver bro- 
 mide, which has been exposed to light, is put into a devel- 
 oper, the developer takes the bromine away from the silver 
 and leaves instead of the crystal what looks under a 
 microscope like a tiny mass of coke, which is, really, the 
 metallic silver itself freed from the presence of the bromine. 
 
 It may seem strange that silver, which we always think 
 of as a bright, shiny metal should look black, but when it 
 is divided up in this irregular way, it looks black, although 
 it is the same thing as the shiny metal we are familiar with, 
 
 36
 
 LIGHT SENSITIVE MATERIALS IN PHOTOGRAPHY 
 
 just as a black lump of coke is the same thing as the bright 
 gleaming diamond. 
 
 If the silver bromide has not been exposed to light, then 
 the developer has no power to take away the bromine from 
 
 Crystals of Silver Bromide before (left) and after (right) 
 Development. 
 
 The photographs above, taken through a very powerful microscope, show crystals of 
 silver bromide before development (on the left) and (on the right) some crystals after 
 they have been changed into metallic silver by development. The crystals before 
 development are transparent except where they are seen sideways or where their 
 edges appear darker. After development the clear yellow silver bromide is turned into 
 a black coke-like mass of silver in exactly the same position as the crystal from which 
 it was formed. 
 
 the silver and leave the black silver behind, so that we see 
 a developer is a chemical that has the power to take away 
 the bromine from the silver in a grain of silver bromide 
 which has been exposed to light but will not affect one which 
 has not been exposed to light. 
 
 Wherever, then, the light in the Kodak acts upon the 
 silver bromide crystals in the emulsion, the developer turns 
 them into black grains of silver and we get an image, and 
 where the light has not acted the developer has no action 
 and no image is produced. The chemical part played by a 
 developer, therefore, is the freeing of the metallic silver from 
 the bromine associated with it. 
 
 This liberation of metals from their compounds is the 
 most important chemical process in the history of the human 
 race. 
 
 The great thing which has distinguished man from the 
 other animals has been his ability to make and use tools 
 and weapons, and man has progressed step by step from the 
 earliest days when he used a flint fastened to a stick, to the 
 present time, when he employs the marvelous machinery 
 
 37 
 
 
 79462
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 of modern civilization ; but the greatest step in all that pro- 
 gress came when men found out how to get metals to use in 
 the place of stone. All the earliest weapons were made of 
 stone, and then men found a way of getting tin from its 
 ores, and found that when this tin was combined with cop- 
 per, which they found in the ground, they could get bronze, 
 and for a long time all the weapons and tools were made of 
 bronze, and then came the greatest discovery of all they 
 found that by taking iron ore and heating it with charcoal 
 they could get the metal iron, which made such beautiful 
 tools and weapons; and from the time that men found out 
 how to get iron, they ceased to be savages and began to be 
 civilized. 
 
 Iron is got from the ore by heating it with charcoal or 
 coke, which takes away the other components of the ore 
 and leaves the metallic iron free. Metals can be got out of 
 their compounds in different ways. Quicksilver, for in- 
 stance, can be got by merely heating its oxide. If the red 
 oxide of quicksilver be heated the quicksilver will boil off, 
 and can be collected quite pure at once. Silver is rather 
 easy to get, and, indeed, if we take a solution of silver nitrate 
 and add some iron sulphate to it the metallic silver will be 
 thrown out as a black sludge. 
 
 The developers that we use in photography play the same 
 part for the silver that the charcoal does for the iron ; they 
 take away the bromine from the silver bromide and leave 
 the metallic silver behind. 
 
 The emulsion coated on films and used for making the 
 negative contains silver bromide with a small addition of 
 silver iodide. The different degrees of sensitiveness are ob- 
 tained by the amount and duration of heat to which the 
 emulsions are subjected during manufacture, the most sen- 
 sitive emulsions being heated to higher temperatures and 
 for a longer time than the slower emulsions. 
 
 If a slow bromide emulsion is coated upon paper, the 
 material is known as bromide paper and is used for printing 
 and especially for making enlargements. The less sensitive 
 papers which are commonly used for contact printing by 
 artificial light, contain silver chloride in the place of silver 
 bromide. 
 
 Materials which are to be used with development must 
 not contain any excess of soluble silver, and the emulsion 
 must be made so that there is always an excess of bromide 
 or chloride in the solution, since any excess of soluble silver 
 will produce a heavy deposit or fog, over the whole of the 
 
 38
 
 LIGHT SENSITIVE MATERIALS IN PHOTOGRAPHY 
 
 surface as soon as the material is placed in the developer. 
 In the case of Solio paper, however, which is not used for 
 development but which is printed out, a chloride emulsion 
 is made with an excess of silver nitrate, this having the 
 property of darkening rapidly in the light, so that prints 
 can be made upon Solio paper without development, a visi- 
 ble image being printed which can be toned and fixed. Solio 
 paper can be developed with certain precautions, but only 
 by the use of acid developers or after treatment with 
 bromide to remove the excess of silver nitrate. 
 
 In the early days of photography prints were usually 
 made on printing-out papers, but at the present time most 
 prints are made on developing-out chloride and bromide 
 papers, which are chemically of the same nature as the nega- 
 tive making materials, and which are coated with emulsions 
 containing no free silver nitrate. 
 
 39
 
 CHAPTER V. 
 
 THE STRUCTURE OF THE DEVELOPED IMAGE. 
 
 THE silver grains which form the developed image are 
 held in a layer of gelatine. This gelatine is used in 
 making the emulsion which is coated on the support to make 
 the sensitive film. 
 
 Gelatine is a very interesting substance, and its charac- 
 teristics are markedly different from those of most other 
 chemical substances. Most chemical substances form crys- 
 tals, and many of them are soluble in water. When they 
 are dissolved in water, the solution is quite homogeneous, 
 
 that is to say, alike in its 
 properties in all its parts. 
 Substances generally will 
 dissolve in water to a fixed 
 extent, dependent on the 
 temperature. We say of one 
 material, for instance, that 
 it is soluble to the extent of 
 30%, meaning that a hun- 
 dred parts of water will take 
 up 30 parts of the material. 
 If we heat the solution it will 
 usually dissolve more, but 
 then when it cools again 
 the material will crystallize 
 out so that whatever we do 
 we can only obtain the fixed 
 30 parts per hundred re- 
 maining in solution. 
 
 Gelatine behaves quite dif- 
 
 Swelling of Satine Cube. ferentl y *> this * ^. C ? ld 
 
 water it does not dissolve 
 
 but it swells, as if, instead of the gelatine dissolving in the 
 water, the water dissolves in the gelatine. If the water is 
 heated, the gelatine will dissolve in it, and it will dissolve 
 to any extent. You cannot say that there is a definite solu- 
 bility of gelatine in water. The more gelatine is added, the 
 40
 
 STRUCTURE OF THE DEVELOPED IMAGE 
 
 thicker the solution becomes, but there is no point at which 
 the gelatine will refuse to dissolve. 
 
 Fig. 39. 
 Reticulation. 
 
 If we heat a gelatine solution it will become thinner and 
 less viscous when hot, and will not recover completely when 
 cool; it will remain thinner than if it had not been heated, 
 so that the heating of the gelatine solution produces a per- 
 manent change in its properties. If we cool a gelatine solu- 
 tion, the gelatine will not separate from the solution in a 
 dry state, but the whole solution will set to a jelly, which 
 we might consider a solution of water in the gelatine. If we 
 heat the jelly it will melt again, and we can melt and reset 
 a jelly many times, but in doing so we shall produce a pro- 
 gressive change in the jelly, and if we continue the process 
 too long, sooner or later it will refuse to set and will remain 
 as a thick, gummy liquid. 
 
 Gelatine belongs to the class of substances which are 
 called colloids, the name being derived from a Greek word 
 meaning gummy. 
 
 When a gelatine jelly is dried, it shrinks down and forms 
 a horny or glassy layer of the gelatine itself, smooth and 
 rather brittle, and this dry gelatine when placed in water 
 will at once absorb the water and swell up again to form a 
 jelly. 
 
 41
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 An interesting and important property of the drying and 
 swelling of gelatine is that it swells almost entirely in one 
 direction, namely, that in which it was dried. This is illus- 
 trated in Fig. 38. In this, A represents a small cube cut out 
 of a sheet of gelatine which was originally dried in the 
 
 horizontal plane when it was 
 made. If this cube is placed 
 in water, it will not swell in 
 all directions, becoming a 
 bigger cube, but it will swell 
 almost entirely in the direc- 
 tion in which it dried down, 
 and will take the form B 
 and, finally, the form C. 
 
 The explanation of this 
 directional swelling of the 
 Fig. 40. gelatine jelly, and also of the 
 
 Spot on Gelatine f ac t that gelatine solutions 
 
 Caused by Moisture. change permanently with 
 
 heating, lies in the fact that gelatine is not a uniform sub- 
 stance but has an internal structure. Probably, gelatine has 
 a structure somewhat like that of a sponge, but the structure 
 is very small and has not the elasticity of the sponge. 
 
 When the gelatine is in the jelly state, it is as though the 
 sponge were full of water, and then it is fairly rigid, because 
 of the water contained in the pores. When the water is 
 dried out, the sponge structure shrinks down, and if it is 
 stretched out in one direction by being coated on film or 
 paper, for instance, it will shrink down vertically just as a 
 sponge without elasticity would fall into a flat mass if placed 
 on the table. 
 
 When the gelatine solution is heated and the gelatine 
 dissolves, it seems at first to retain a certain amount of its 
 structure, as if the sponge had disintegrated and was dis- 
 tributed through the solution but the sponge structure had 
 not entirely disappeared. Then, if the temperature is raised, 
 it behaves as if the structure were slowly breaking up and 
 dissolving, so that after a considerable heating at a high 
 temperature the whole solution becomes homogeneous. 
 When this solution is cooled and, finally, set to a jelly, it 
 has to re-establish a new sponge structure, and this will be 
 different to the original one and probably of less strength. 
 This explanation of the behavior of gelatine, that it has 
 an internal structure which can persist even in solution, 
 seems to account for most of its properties and behavior.
 
 STRUCTURE OF THE DEVELOPED IMAGE 
 
 When a gelatine jelly contains only such an amount of 
 water that it still contains a considerable proportion of 
 gelatine, over 10% for instance, the jelly will be strong and 
 tough, but if the jelly contains much less gelatine than this, 
 it will be weak and likely to rupture on any kind of strain. 
 This is a very important matter in dealing with photo- 
 graphic films. When the film is first placed in the developer 
 the gelatine at once commences to swell. As long as it does 
 not swell too much it is easily handled, but if it swells too 
 far, then it becomes very tender and is likely to be damaged 
 by touch, and in extreme cases will swell so much that it 
 will loosen from its support or wrinkle up in what is called 
 "reticulation". 
 
 The swelling of a gelatine film is influenced by the tem- 
 perature of the solution in which it is placed and also by the 
 presence of other substances in the solution. A small 
 amount of either acid or alkali will produce a considerable 
 
 increase in the swel- 
 lingi and since the 
 developer is alkaline 
 uiwwHwvg., and the fixing bath 
 is acid, both these so- 
 lutions have a great 
 tendency to swell 
 the gelatine, especi- 
 
 __^_^^ a ^y wnen they are 
 
 - - - =^ -.--i,,v- -,.!,,. *-.",-.,,--,- warm On the other 
 
 hand, sulphites tend 
 to prevent swelling, 
 so that an increase 
 in the concentration 
 
 ...,._ ...... ,^-,/^r-^- -V^UT.. of the sulphite in a 
 
 developer or fixing 
 
 bath will diminish it. 
 
 An even greater aid 
 in preventing swel- 
 
 A , ^ ling is the hardener 
 
 in the fixing bath. 
 
 The hardening 
 
 Fig. 41. agents used in fixing 
 
 The Way a Waterspot Dries. baths are the alums, 
 
 which not only pre- 
 vent the swelling of the gelatine temporarily but which per- 
 manently harden the structure of the gelatine so that it will 
 not easily swell. The alum is introduced into the fixing 
 
 43
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 bath so that after fixing the film will not become soft and 
 disintegrate in washing. 
 
 Reticulation is due to local strains in the gelatine, and a 
 sudden change in the temperature of solutions will some- 
 times produce this effect. If a film is transferred for instance 
 from a cold fixing bath containing a hardener to very warm 
 wash water, the whole film will sometimes pucker into tiny 
 reticulations, a good example of which is shown in Fig. 39. 
 If one part of the film contains much more moisture than 
 another, the silver image itself is liable to become distorted 
 by the movement of the gelatine, and of the silver grains in 
 it. If a drop of water, for instance, falls on a film and this 
 is dried rapidly, it will often produce a curious ring-shaped 
 mark, the middle of the drop being lighter and the edge of 
 the drop darker than the surrounding negative, Fig. 40. 
 The explanation of this is shown in Fig. 41. The gelatine 
 swells up where the spot of water fell on it, and as it dries 
 again a strain is produced by the collapse of the center of the 
 swollen spot, and so the gelatine and silver grains are pulled 
 in to the edges of the spot and there produce the dark ring. 
 
 APPEARANCE 
 
 OF 
 
 DEVELOPED 
 EMULSION 
 
 WHEN 
 MAGNIFIED 
 
 20 diameters 
 
 400 diameters, 
 
 100 diameters 
 
 900 diameters 
 
 Fig. 42. 
 Appearance of Emulsion After Development When Magnified. 
 
 The developed image consists of grains of silver, each 
 grain under sufficient magnification looking like a little mass 
 
 44
 
 STRUCTURE OF THE DEVELOPED IMAGE 
 
 of coke, replacing one of the silver bromide crystals which 
 were originally formed in the emulsion and keeping the 
 same position. See Fig. 37. When we look at a negative it 
 appears perfectly smooth to the eye, but under a small 
 degree of magnification it begins to show an appearance of 
 graininess. 
 
 It must not be thought, however, that with a magnifying 
 glass we can see the silver grains themselves. The silver 
 grains are so small that to make them visible requires power- 
 ful magnification. What we see through the magnifying 
 glass are clumps of grains. 
 
 Suppose that an aviator is flying over country dotted with 
 occasional woods and clumps of bushes. If he is flying near 
 to the ground, he will be able to distinguish the separate 
 trees and bushes. If he goes higher, he will no longer be 
 able to see them separately but he will see them in little 
 clumps of two and three where they are close together with 
 
 the spaces where they are 
 
 ^^'^f'.^ff^^f.r^'.^^i^-^^i*. farther apart showing be- 
 *~ '."'""' ""'' ' - >.* "".* tween them, and then as he 
 
 goes higher still, he will no 
 longer be able to see these 
 small clumps, but will be 
 able to see only the large 
 A masses of woodland or for- 
 
 est. In the same way when 
 we look at a negative under 
 a low magnification, we see 
 the larger masses of clumps 
 of grains, and then as we 
 increase the magnification 
 we see the smaller clumps 
 of grains, and then finally 
 at a very high magnifica- 
 tion we see the grains them- 
 selves, Fig. 42. 
 
 These clumps of grains 
 which we can see under low 
 magnification are made up 
 of grains which are not all 
 in the same layer. This can 
 be seen by first of all photographing an image from above 
 and then cutting a section down through it so as to see how 
 the grains lie one below the other. In Fig. 43 A it will be 
 seen that the image is as much as six grains deep so that 
 
 45 
 
 Vertical Section Showing Grain 
 Deposit. 
 
 Horizontal Plan of Same 
 Grain Deposit. 
 
 Fig. 43.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Exposed 1 Unit of Time Exposed 16 Units of Time. 
 
 * *V'ff- : ."*"' .' r --V- 1'- r . *&-*" b5$ 
 
 Exposed 4 Units of Time. 
 
 Fig. 44. 
 
 Exposed 64 Units of Time. . 
 
 many of the clumps of grains seen in Fig. 43 B are not made 
 up of grains in the same layer but of grains in different 
 layers, some on the top and some below. 
 
 The distribution of the grains in the depth of the film is 
 interesting. It might be thought that with short exposures 
 the image would be on the top of the film and that as the 
 exposure was continued, the light would penetrate farther 
 and farther into the film, making the grains in the lower 
 layers more and more developable. This sometimes seems 
 to be the case, but with some emulsions it is not so, as is 
 proved by the photographs of sections shown in Fig. 44, 
 which are cut from an N. C. film. These are fully developed 
 so that the effect of development is eliminated, and they 
 show that the grains are exposed at all parts of the film to 
 
 
 2 4 
 
 Fig. 45. 
 
 Showing Progress of Development from Surface to Base of Emulsion. 
 46
 
 STRUCTURE OF THE DEVELOPED IMAGE 
 
 an almost equal extent, though in the second and third 
 prints there is a slight tendency for the image to be more 
 on the top of the film. It looks as though the emulsion con- 
 tains grains of various degrees of sensitiveness and the more 
 sensitive grains are made developable first. Further, since 
 there is certainly more light at the surface of the film, it 
 
 r----. ->-- --- ~- T . ....--., . 
 
 Strong or Concentrated Developer 
 
 Weak or Diluted Developer. 
 Fig. 46. 
 
 must be a fact that the more sensitive grains are found in 
 the lower parts of the film. 
 
 During development, however, there is an appreciable 
 effect due to the penetration of the developer into the film. 
 This is shown in Fig. 45, where it is seen that at the be- 
 ginning of development only the surface of the emulsion is 
 developed, and then as development continues the develop- 
 er penetrates into the film and develops more and more 
 deeply in it. In the case of a strong developer this effect is 
 accentuated, because a strong developer will develop the 
 surface to good density before it has penetrated through 
 the emulsion, while a weak developer will penetrate at the 
 same rate as the strong developer and will not develop so 
 rapidly, so that with a strong developer there is a tendency 
 for the image to be confined to the surface of the emulsion, 
 and with a weaker developer for it to penetrate through the 
 whole emulsion. This effect is well shown in Fig. 46, where 
 two photographs are shown of the edge of an exposed image, 
 the image being shown as the dark part on the left, while on 
 the right we have the light deposit of grains due to fog. 
 The broad black line at the bottom of each illustration 
 represents the film on which the emulsion is coated. In 
 the upper picture, the image was developed with a very 
 strong developer, while in the lower picture it was devel- 
 oped with a much weaker developer, and it will be noted 
 that the weak developer has penetrated right through the 
 image to the back, while with the strong developer the image 
 has not developed through to the back of the film, although 
 
 47
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 care was taken to develop the images to the same apparent 
 density. 
 
 There is a curious effect shown in these photographs at 
 the point marked A, where it is seen that at the edge of the 
 developed image the fog grains are not developed in the 
 lower part of the film; it is as if they had been eaten away. 
 There is no doubt that the reason for this is that the bro- 
 mide liberated during development of the heavy image has 
 prevented the fog grains close to the edge of the image from 
 developing. In extreme cases this will sometimes surround 
 a dense image with a white line. 
 
 48
 
 CHAPTER VI. 
 
 EXPOSURE. 
 
 IN order to get a satisfactory photograph of any scene it 
 is necessary that the exposure should be correct. The time 
 of exposure required will, of course, depend upon the bright- 
 ness of the image formed by the lens on the film, and this 
 in turn will depend upon the aperture of the lens used and 
 on the brightness of the scene photographed. The bright- 
 ness of a lamp is measured in terms of its candle power; 
 that is, a lamp is stated to be equivalent to 1, 5, 10, or 
 100 candles, the original ,unit being an actual candle, 
 though nowadays the practical standards used are electric 
 lamps which have been carefully measured and which are 
 kept for use only as standards. 
 
 When the light of one candle falls upon an object at a 
 
 distance of one foot, the brightness falling on the object is 
 
 said to be one foot-candle. When we have 3 candles at a 
 
 foot distance, the 
 
 A brightness would, of 
 
 course, be 3 foot-can- 
 dles, and if we use a 25 
 candle power lamp, the 
 brightness will be 25 
 foot-candles. (Fig. 47). 
 If we change the dis- 
 tance, the brightness 
 will vary inversely as 
 the square of the dis- 
 tance, because the 
 cone of light which 
 covers one square at a 
 
 pj 47 foot will embrace 4 
 
 squares of the same 
 
 size at 2 feet, 9 squares at 3 feet, and so on; and since the 
 same light that falls on one square at one foot is spread 
 over 4 squares, at 2 feet distance, it is naturally Y of the 
 strength, so that a 25 candle power lamp at one foot dis- 
 
 49 
 
 3 FOOT WHOLES
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 tance gives a brightness of 25 foot-candles, and at 5 feet 
 distance it gives only one-foot-candle brightness (Fig. 48). 
 The brightness of natural objects can be measured by 
 means of a photometer, in which the brightness is matched 
 with a lamp of known brightness. A convenient form of 
 the instrument is shown in Fig. 49. In this, the scene is 
 viewed through a hole in a piece of white paper, and the 
 white paper, which must be backed on metal so that it is 
 opaque, is illuminated by a small movable lamp of which 
 the distance from the paper can be varied. In order to use 
 the instrument, it is 
 held up to the eye so 
 that the brightness to 
 be measured can be 
 seen through the hole 
 in the paper, and then 
 the lamp is moved un- 
 til the brightness on 
 the paper is the same 
 
 as that seen through 
 the hole. Now, the Fig. 48. 
 
 brightness which the 
 
 lamp throws on the paper can be calculated from the dis- 
 tance of the lamp, and consequently we can read on the 
 instrument the brightness of the object to be measured. 
 
 White Paper 
 
 Movable Lamp 
 
 Fig. 49. 
 Photometer to Measure Relative Brightness. 
 
 In Figs. 50 and 51 are shown two landscapes which were 
 photographed and at the same time were measured with 
 the photometer, and it will be seen that the sky in these 
 has a brightness of about 1500 foot-candles, while the 
 deepest shadows in the foreground have a brightness of 
 about 60 foot-candles. 
 
 5
 
 EXPOSURE 
 
 It is often believed by photographers that the range of 
 light intensities occurring in natural objects is very great, 
 and that in an ordinary landscape, for instance, the sky 
 will be enormously brighter than the shadows, but this idea 
 
 xHSOOF.C. 
 
 60 F,C. 
 
 Fig. 50. 
 
 is quite incorrect. In a bright landscape with heavy shad- 
 ows, the sky is only about 30 times as bright as the deepest 
 shadows, while in the case of open landscapes in which 
 there are no close objects in the foreground, the range of 
 intensities will be much less than this, the sky often being 
 only 5 or 6 times as bright as the shadows. The range of 
 light intensities, therefore, with which it is necessary to 
 deal in ordinary photography will vary from, perhaps, 1 to 
 4 at the least up to 1 to 50 as a maximum, and the bright- 
 est part of a landscape the sky will have a brightness of 
 from 1000 to 3000 foot-candles. This is the photometric 
 brightness of the sky itself; but when we take a photograph, 
 we are concerned not with the brightness outside but with 
 that inside the camera; that is, with the brightness of the 
 image which falls upon the film. This brightness depends 
 upon the aperture of the lens, and we can calculate it from 
 the fact that at an aperture of f.8 the photometric bright-
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 ness of the image is about I/ 100th of the brightness of the 
 object outside, so that the light from the sky falling upon 
 
 80 F.C. 
 
 Fig. 51. 
 
 the film will have a brightness of, at most, 30 foot-candles, 
 and the shadows will be represented by a brightness of 
 about one foot-candle in a photograph of a landscape having 
 a brightness range of 30 to 1. 
 
 Now, let us consider how much time of exposure will be 
 required for the film to reproduce the shadows, of which we 
 see that the image formed on the film has a brightness of 
 one foot-candle. In order to do this, we must know how 
 much exposure is required by a film to make it developable. 
 We can find this by exposing the film to a candle at a fixed 
 distance and giving it a series of different times of exposures. 
 It is convenient to have each of these exposures double the 
 next one, so that one part is exposed for one second, the 
 next for 2 seconds, the next for 4, and so on. If, now, we 
 develop and fix the film, and then after it is dry find out 
 how much silver per square inch we have produced in each 
 exposed part we shall find that each time the exposure was
 
 EXPOSURE 
 
 doubled we added almost the same amount of silver (Fig. 
 52.) 
 
 It is rather hard to measure the amount of silver by actual 
 analysis, but it can easily be done optically by measuring 
 
 EXPOSURE 
 
 Fig. 52. 
 
 the blackness of the deposit, and this measurement of the 
 blackness, which is proportional to the amount of silver 
 per square inch, is called the "density". A density of unity 
 is taken as the standard and represents the blackness of a 
 deposit which lets through only l/10th of the light; it 
 corresponds to a very small amount of silver only about 
 l/100th of a grain per square inch. 
 
 The relation between the density and the exposure of 
 the plate can easily be represented as a curve, and for most 
 of this curve the density is increased proportionally as the 
 exposure is doubled (See Fig. 53). This condition is that 
 which produces a correct rendering of the original in the 
 print, and for that reason the parts of the curve for which 
 it is true is known as the region of correct exposure. But at 
 the beginning and the end, the curve is not straight; at the 
 beginning, the density increases more rapidly; this is known 
 
 53
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 as the region of under-exposure, and at the top of the curve 
 the density falls off and finally fails to increase at all when 
 the exposure is increased; this is the region of over-exposure. 
 We may note that in the example taken the under-exposure 
 region persists while the exposure increases from unity to 
 three and one-half units; then we have correct exposure 
 until the exposure becomes about one hundred and twenty- 
 eight units, and then the over-exposure region appears, dif- 
 ferences in exposure failing to grow in density after about 
 five hundred units of exposure. For a rapid film, the point 
 marked unity on this curve represents an exposure of about 
 l/50th of a candle-foot-second; that is, this film requires 
 an exposure of l/50th of a second to a candle at one foot 
 distance in order to give the first visible trace of deposit. 
 
 When photographing a landscape, we want to obtain in 
 our negative just a trace of deposit for the shadows, and we 
 have already seen that the image of the shadows on the 
 film will have a brightness of one foot-candle, so that the 
 correct exposure time to give for such a landscape will be 
 l/50th of a second with the lens working at an aperture of 
 /.8. The exposure given for such a landscape will therefore 
 vary from l/50th candle-foot-second in the shadows to 
 30/50th or 3/5 th of a candle-foot-second for the sky. 
 
 This reasoning applies to an ordinary film, but photo- 
 graphic materials are of various speeds, and we can clearly 
 define the speed according to the exposure required to give 
 an impression upon it. The shorter the exposure required, 
 the "faster" the film; and from the exposure which we find 
 to be required, we may calculate a number which will repre- 
 sent the "speed" of the film. 
 
 A film might be said to have a speed of unity which re- 
 quires the exposure of l/50th of a second to give a deposit 
 equal to that given by the light of an intensity of 1 foot- 
 candle, such as is reflected from the darkest shadows of a 
 landscape. But it would be inconvenient to choose unity 
 as the speed of our film, because the speeds of all slower 
 materials would have to be expressed in fractions, and in 
 practice such a film is said to have a speed of 250 in the 
 units generally used by photographic workers. 
 
 We see, therefore, that for a film of speed 250 at /.8 
 which reduces the light by about 100 times, we shall 
 require an exposure of l/50th of a second if the light 
 reflected from the darkest portion is about 100 foot-candles. 
 If the light reflected from the darkest shadow of the object 
 
 54
 
 EXPOSURE 
 
 is one foot-candle, we shall require an exposure of 1 second 
 on a film of speed 500, or 500 seconds with a film of speed 
 one. Or, generally, if L is the light intensity from the dark- 
 est part of the subject, P is the speed of the film or plate, 
 and E is the exposure at/.8, then 
 
 E = -^- | X~ 
 
 L x P 
 
 E being exposed in seconds, L in foot-candles, and P in 
 the usual speed units. 
 
 It will be seen that this method of calculating exposures 
 assumes that the exposure is made for the shadows, and in 
 practical photography this is almost always true; one ex- 
 poses to get shadow detail and trusts to the latitude of the 
 emulsion being sufficient to render the whole scale of grada- 
 tion of the subject. 
 
 If, instead of a landscape with foreground, we photo- 
 graph a quite open landscape with sea or open country in 
 the distance, then the darkest part of the picture will re- 
 flect perhaps 1/5 th of the sky light or about 500 foot-candles. 
 Using a film of speed 250, we should have to give an expos- 
 ure of only l/250th second at/.8 or about l/60th of a second 
 at/.16. 
 
 Using this line of reasoning, let us consider what the 
 shortest exposures practicable for figures in rapid motion 
 are likely to be. The range of contrast when taking a photo- 
 graph of an athlete jumping, for instance, will be much 
 smaller than in our typical landscape, and probably 1 to 10 
 would be a fair approximation; if the scene is in sunlight, 
 the shadow detail may be represented by 250 foot-candles. 
 Using the most rapid lenses available for such work, we may 
 reckon on having 3 times as much light as a lens at/.8 will 
 give, and we can use a Seed Graflex plate of speed 500; the 
 exposure required will therefore be 500/250 x 3 x 500 or 
 l/750th of a second. We see, therefore, that unless the 
 light conditions are of the very best, the use of such high 
 shutter speeds will involve some degree of under-exposure, 
 and this fact illustrates the advantage well-known in prac- 
 tice of taking very rapidly moving objects as silhouettes 
 against the sky. 
 
 When photographing in the streets of cities, a consider- 
 ably greater exposure is permissible than in landscape work, 
 because there are always deep shadows outside the main 
 
 55
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 range of contrast, in which an increase of exposure will give 
 detail at the expense of the highlights, and an increase of 
 exposure therefore means a shifting of the center of the 
 scale of gradation from the highlights to the shadows. In 
 practice topographical views are usually made at the shorter 
 exposures, while the pictorial photographer prefers the 
 longer exposures which concentrate interest on the lower- 
 toned portions of the picture. 
 
 When using color filters, their factors must be allowed 
 for in considering exposure; thus, taking the speed of film 
 as 250, the use of the color filter requires an increase of 
 five times in the exposure, so that for our typical landscape 
 when using a color filter at/.8 we shall need an exposure of 
 l/10th of a second. 
 
 POSITIVE PRINTING. 
 
 When printing positives either on paper or on plates for 
 lantern slides, working conditions are somewhat different, 
 no camera being used and the object reproduced being a 
 negative instead of the original subject. The range of 
 contrast in negatives is frequently much greater than in 
 natural objects, but the exposure is governed by the same 
 conditions as those which apply in the negative making. 
 Such an exposure must be given that the greatest opacity 
 which it is desired to print through at all just produces a 
 visible deposit. Usually the highest light of all should be 
 printed free from any deposit, and the next tone to this 
 should be taken as the one to be printed through. 
 
 Turning to the curve shown in Fig. 53 and considering 
 
 a bromide paper, 
 this will have a 
 speed of about 5 on 
 the speed scale, so 
 that it is 50 times 
 slower than the film 
 
 which we considered 
 first and the point 
 marked 1 on the ex- 
 posure axis corre- 
 sponds to an expos- 
 ure of a candle-foot- 
 second. Now the 
 highlight which we 
 
 -^ shall want to print 
 
 E.XP05URJL through in an aver- 
 
 Fig. 53. age negative will let
 
 EXPOSURE 
 
 through only about l/20th of the light, so that we shall 
 have to give such a bromide paper an exposure of 20 candle- 
 foot-seconds behind such a negative, and for a paper or 
 plate of any speed we may write 
 
 5x0 
 
 P 
 
 where P is the speed of the paper and is the opacity of 
 the highlight in the negative which it is desired to print 
 through. If the highlight in a negative lets through l/20th 
 of the light, then the opacity of that negative is said to be 
 20. If it lets through l/100th, it is 100, and so on. 
 
 57
 
 CHAPTER VII. 
 
 DEVELOPMENT. 
 
 IN chapter IV we saw that the chemical process of devel- 
 opment consists of the removal of the bromine from the 
 silver bromide in the emulsion so as to leave the grains of 
 silver behind. 
 
 There are many chemicals which will remove bromine 
 from silver bromide in this way, but in order to act as a 
 developer, it is necessary that a chemical should be chosen 
 which has the power of turning the exposed silver bromide 
 into metallic silver, but which will not act on unexposed 
 silver bromide, since, if the developer acted on the unexpos- 
 ed, as well as on the exposed grains, we should not get an 
 image at all, but the whole film would go dark when put in 
 the develo'per, just as if it had all been fogged by exposure 
 to light. Only a very limited number of chemicals have this 
 power of distinguishing between exposed and unexposed 
 grains of silver bromide and, consequently, there are only 
 a few substances which are suitable for use as developers. 
 
 The chief of these developing substances are pyrogallol, 
 or "pyro" as the photographer calls it, hydroquinone and 
 elon, all of which are chemically related to aniline, which is 
 used as the base of coal tar dyes. Hydroquinone and elon, 
 indeed, are made by the same methods as those used for 
 making dyes, but pyro is made by distilling gallic acid, 
 which is produced by fermenting gall nuts, so that, although 
 pyro is really a cousin of hydroquinone, it is made quite 
 differently, from a vegetable product, while hydroquinone 
 itself is made from aniline. 
 
 Now, if we take a solution of one of these chemicals, let 
 us say pyro, and put an exposed film into it, we shall get no 
 development at all; the developing agent by itself having 
 no power to develop. In order to make it develop we must 
 add a little alkali to the solution. Any kind of alkali will 
 make it develop, but the most convenient one to use is 
 carbonate of soda which, in its crude form, is called sal-soda 
 
 58
 
 DEVELOPMENT 
 
 and is used to make water alkaline for washing. If, then, 
 we take a solution of pyro and add some sodium carbonate 
 to it it will develop our exposed films; but a solution con- 
 taining only pyro, carbonate and water will not keep and, 
 if we leave it in the air, it will very soon darken and lose its 
 developing power. 
 
 In order to make it keep, there is added to the developer 
 some sulphite of soda because the developer is spoiled by 
 taking up oxygen, and sulphite is so greedy for oxygen that 
 it will take it away from the oxidized pyro or take it in 
 preference to the pyro, and thus protects the pyro from the 
 oxidizing action of the air and enables it to keep its develop- 
 ing power, although the sulphite itself has no developing 
 power at all. 
 
 The essential constituents of a developer therefore are: 
 The developing agent pyro or hydroquinone or elon or 
 Kodelon which is a relative of elon the alkali, which is 
 generally carbonate of soda, and the preservative, which 
 is sulphite of soda. Very often a developer which contains 
 only these constituents will prove difficult to handle. It 
 will tend to give fog, that is, to develop unexposed silver 
 bromide as well as exposed silver bromide, and so, in order 
 to regulate it, there is put in a little potassium bromide to 
 act as a restrainer. 
 
 The various developing agents behave somewhat differ- 
 ently. Suppose, for instance, that we make up two devel- 
 opers, one with hydroquinone and the other with elon, and 
 start to develop a film in each at the same time. In the 
 elon developer the image will appear very quickly on the 
 film and will appear all over the film at the same time, the 
 less exposed portions which, of course, were the shadows in 
 the picture, appearing at the same time as the highlights. 
 On the other hand, with the hydroquinone the image will 
 appear more slowly, and the most exposed portions, or the 
 highlights, will appear first, so that by the time the shadows 
 have appeared on the surface of the film the highlights will 
 have acquired considerable density. If development is stop- 
 ped as soon as the whole image is out, then the negative 
 developed in elon will be very thin and gray all over, while 
 that developed in hydroquinone will have a good deal of 
 density in the highlights. Thus, of these two developers 
 we may say that elon gives detail first and then slowly 
 builds up density, while with hydroquinone the detail comes 
 only after considerable density has been acquired. It is for 
 this reason that these two developing agents are used in 
 
 59
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 combination; the hydroquinone gives the density and the 
 elon the detail, and together they make a well balanced 
 developer. 
 
 These differences in the behavior of developing agents 
 are due to a property of the developer which can be ex- 
 plained very easily by an analogy. Suppose that we had 
 two automobiles of the same kind, one of 20 horse power 
 and the other of 100 horse power. What would be the dif- 
 ference between them? Naturally, the high horse power 
 automobile would be able to go faster than the other; but 
 in a city, at any rate, either of them would be able to go 
 as fast as was safe, and no one would wish to use the higher 
 horse power for increased speed; but the advantage of the 
 high horse power would be found whenever the auto- 
 mobiles were used against adverse circumstances, as, for in- 
 stance, against high winds, in snow or in climbing hills, 
 when the high-power machine would be able to keep up its 
 speed against the difficulties, and the lower power machine 
 would be slowed and might even be unable to get ahead. 
 The difficulties which affect development in a manner cor- 
 responding to the effect of hills or winds for an automobile 
 are cold and bromide. The addition of bromide has the 
 same effect on a developer that a hill has on an automobile 
 it slows it down; but bromide has far more effect on 
 a low power developer like hydroquinone than it has on a 
 high-power developer like elon; the effect of bromide on 
 elon is very small, while on hydroquinone it is very great. 
 In the same way, hydroquinone develops very slowly when 
 it is cold, while elon is not nearly so much affected by tem- 
 perature. 
 
 The analogy between the horse power of the automobile 
 and the power of the developer is really very close. The 
 high horse power automobile will start from rest very much 
 more quickly than the machine of lower horse power, just 
 as the elon developer forces out the image all over the film 
 much more rapidly than the hydroquinone developer. Just 
 as the horse power of an automobile could be measured by 
 the effect of a hill on its speed so the power of a developer 
 can be measured by the reduction of density produced by 
 the addition of bromide, and just as one would not wish to 
 have an over-powered automobile, hard to handle and al- 
 ways picking up speed very rapidly, so it is difficult to use 
 the very high-power developers, and elon, for instance, is 
 rarely used alone, but is generally adjusted by admixture 
 with the slower hydroquinone. 
 60
 
 DEVELOPMENT 
 
 Pyro is an almost ideal developer for negative making. 
 Owing to the fact that the pyro is changed during devel- 
 opment into a yellow colored substance, some of which re- 
 mains with the silver in the image, pyro tends to give a 
 slightly yellowish or brownish image. The yellowish stain 
 is prevented from forming by sulphite, so that the more 
 sulphite there is in a developer the less tendency to warmth 
 the deposit will show. Pyro is not used for papers, for 
 which the blue-black image obtained with elon and hy- 
 droquinone is preferred. 
 
 When a film is developed, it is only the grains of silver 
 bromide which have been changed by the action of light 
 that are affected by the developer. The grains that have 
 not been changed are not affected; at the beginning of 
 development there are a great many exposed grains ready 
 to be developed, and then as development proceeds, these 
 exposed grains are turned into grains of black silver, so that 
 the number of developable grains decreases during develop- 
 ment until at last there are no developable grains left; all 
 those which can be developed have been acted upon, and 
 development ceases. 
 
 The rate at which the development proceeds can best be 
 understood by an analogy from fishing. Suppose one went 
 out fishing and found a pond where there were about four 
 hundred fish. In the first day's fishing one might catch half 
 the fish in the pond, or two hundred fish, but the second 
 day one would not expect to catch the other half; all one 
 could expect to catch would be the same proportion of the 
 remaining fish, that is, half of what were left, or one hun- 
 dred fish, and the third day one might catch half of what 
 were left again, or fifty fish, and the fourth day half of 
 what were left again, or twenty-five fish, and so on, the 
 catch growing smaller as the number left decreased /until 
 finally no fish were left to catch, or more probably until 
 one got tired of trying to get the few remaining fish. 
 
 This is what happens in development. The rate at which 
 the grains develop depends upon the number of undevelop- 
 ed grains left, and as the grains are developed up and the 
 number of undeveloped grains remaining become less, fewer 
 and fewer grains develop in each minute, until finally, it is 
 not worth while to prolong the development in order to 
 get any more density. (See Fig. 54.) 
 
 If the development is prolonged beyond the point at 
 which all the exposed grains are developed, then there is a 
 61
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 danger of developing some of the unexposed grains, which 
 produces a veil over the whole negative exposed and un- 
 exposed portions alike and this veil is known as fog. 
 
 At Beginning After After After After 
 
 1 Minute 2 Minutes 3 Minutes 4 Minutes 
 Development of exposed grains in a film which is half developed in o 
 
 After 
 5 Minutes 
 
 Fig. 54. 
 
 The growth of the image during development is referred 
 to as a growth of density, that is to say, the density is a 
 measure of the number of grains of silver which are produced 
 at any given point because these grains of silver, after the 
 film has been cleared by the fixing bath, obstruct the passage 
 of light through the film. We have seen that the density of 
 an image is measured in units which are based on the 
 amount of silver which will let through l/10th of the light, 
 so that if only l/10th of the light falling on the negative 
 gets through a certain part of it, that portion of the negative 
 62
 
 DEVELOPMENT 
 
 is said to have a density of 1. The blackest part of a nega- 
 tive may have a density of perhaps 2, the middle tones 1 
 or less, and the shadows, perhaps l/10th. (Fig. 55.) 
 
 The difference of density between the darkest portion 
 and the lightest portion of the negative is called its contrast. 
 In most negatives the shadows are nearly clear so that the 
 contrast depends chiefly on the density of the darkest por- 
 tion, but this is not necessarily so because an over-exposed 
 negative, or one taken of a very flat subject, may have no 
 clear portion in it and may be even very dense owing to 
 over-exposure, and yet not contrasty at all because there is 
 very little difference between the density of the most ex- 
 posed portion and that of the least exposed portion, the 
 negative being very dense all over. It is necessary to keep 
 clearly in mind this difference between the density and the 
 contrast. 
 
 Since the contrast depends chiefly 
 upon the density of the highlights, 
 it grows during development just as 
 the density does. It grows rapidly 
 at first, when there are many grains 
 to be devel- 
 oped, and then 
 more slowly 
 until, finally, 
 
 JDensrtyTT? 
 
 Shadows Half Tones Highlights 
 
 Fig. 55. Densities of Various Parts of a Negative. 
 
 when the grains are all developed, the negative will not 
 give any more contrast however long development may be 
 prolonged, and a continuation of development will only 
 result in the production of fog. (Fig. 56.) 
 
 After 1 Minute After 2 Minutes After 3 Minutes 
 
 Fig. 56. Growth of Contrast During Development. 
 
 63
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 The final contrast which can be obtained depends upon the 
 kind of emulsion used. The fast emulsions, such as the 
 film emulsions, give moderate contrast, but the slow emul- 
 sions, such as those used for copying purposes or for making 
 lantern slides, are specially made to give great con- 
 trast when development is prolonged. (Fig. 57.) 
 
 Highly Sensitive Film Medium Speed Plate Slow Lantern Plate 
 
 Fig. 57. 
 Greatest Contrasts With Different Emulsions. 
 
 It would be convenient if the manufacturer could make 
 the film so that it would be impossible to over-develop it, 
 but this is not practicable. It would be possible if a film 
 developed at an even rate and then stopped developing 
 when it was correctly developed as is shown in Fig. 58, 
 where development is supposed to go straight on for a 
 given time and then stop altogether, the film not changing 
 after that time. But the film does not develop like this; 
 the growth of the image gets slower as time goes on but it 
 takes a very long time indeed to stop completely, so that 
 the growth of the image occurs as shown in Fig. 59. If 
 
 TIME OF DEVELOPMENT 
 
 Fig. 58. 
 
 Growth of Image During Development 
 (to be desired) 
 
 TIME OF DEVELOPMENT 
 Fig. 59. 
 
 Growth of Image During Development 
 (actual) 
 
 64
 
 DEVELOPMENT 
 
 a film were made so that we had to develop it as far as we 
 could, it would take too long to develop, and therefore it is 
 necessary to make a film that is capable of giving more 
 density than is required in order that it may be developed 
 in a sufficiently short time; this means that we must be 
 able to stop development at the right time to get enough 
 density and contrast, the density being the blackness of 
 the image and the contrast. 
 
 Old-time photographers used to take pride in the accuracy 
 with which they could judge the progress of the develop- 
 ment of negatives, and it was regarded as quite wrong when, 
 in recent years, people insisted that negatives could be 
 developed just as well by timing the development as by 
 watching it, and that it was better for the negatives not to 
 be watched. 
 
 The customary way of judging the progress of develop- 
 ment in a negative is to hold it up to a lamp and look 
 through it, but unless one has had a lot of experience he is 
 very likely to be deceived because the apparent density of 
 a negative held up to a light is very difficult to judge. The 
 emulsion which has not been developed makes it appear 
 stronger than it really is, and beginners almost always 
 under-develop negatives if they try to judge when to stop 
 development. If for some reason it is necessary to judge 
 the progress of development by inspection (and this applies 
 particularly to lantern slides), the best way is to turn the 
 emulsion side to the light and look through from the back. 
 This is much less misleading than if they are examined from 
 the front. 
 
 There is no doubt, however, that the best method of 
 judging development is simply to develop for a fixed time. 
 
 Films are best developed in a film tank, and the time of 
 development, at a temperature of 65 for the tank developer, 
 is 20 minutes. This time depends on the temperature. If 
 the temperature is lower than 65 the time must be in- 
 creased, and if it is higher than 65 the time must be reduced. 
 
 Instructions for development are furnished with each 
 Kodak or Premo tank. It might be thought that if the 
 film were over-exposed and so gave density easily it should 
 be developed for a shorter time than if it had received less 
 exposure, but this idea is quite wrong, because what is 
 wanted in a negative is not correct density, which only 
 affects the time of printing, but correct contrast, and the 
 contrast is controlled by the time of development. An over- 
 
 65
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 exposed film will tend to have too little contrast, and if the 
 development is lessened the contrast will be still further re- 
 duced and the negative will be flat. On the other hand, an 
 under-exposed film tends to be too contrasty, and must not 
 be forced in development or it may be unprintable, and so 
 whatever the exposure, the best result will be obtained by 
 the use of the normal time of development. Of course, the 
 best negative can only be obtained by correct exposure as 
 well as by correct development, and it is a mistake to think 
 that we can correct errors in exposure by deviation from 
 the correct time of development. 
 
 66
 
 CHAPTER VIII. 
 
 THE REPRODUCTION OF LIGHT AND SHADE IN 
 PHOTOGRAPHY. 
 
 PHOTOGRAPHY is the art of making representations of 
 natural objects by mechanical and chemical processes. 
 These representations deal with differences of brightness, 
 color being ignored, except in color photography, and the 
 object of the photographic process is to translate, as ac- 
 curately as possible, the degrees of brightness which occur 
 in natural objects into corresponding degrees of brightness 
 in a photographic print. 
 
 It is not possible to convey any impression in a photo- 
 graph of the brightness of an object of even brightness; a 
 piece of black velvet seen in bright sunlight is brighter than 
 a piece of white paper in a dark room, so that it is impos- 
 sible to speak of the brightness of paper or the blackness of 
 velvet unless there is some standard oLcomparison by which 
 it can be measured. If black marks rare made on the white 
 paper and then photographed, the resulting print will re- 
 produce the relative intensity of the black marks and of 
 the white paper. 
 
 When a representation of a natural object is made on a 
 flat surface, the form can be represented only by differences 
 
 Fig. 60. 
 Two Tones. 
 
 Fig. 61. 
 Three Tones.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 of brightness or color. Shape is only possible in sculpture. 
 The painter uses differences of brightness and of color, 
 while the black and white draftsman uses only the differ- 
 ences of brightness. Except in the special branch of color 
 photography, photographs deal only with the reproduction 
 of objects in their degrees of brightness. 
 
 The different degrees of brightness are spoken of by 
 artists as "tones." If a piece of white paper on which black 
 marks have been made is photographed the result will be 
 a picture in two tones (Fig. 60). Between these extremes 
 are other tones spoken of as halftones. Figs. 61, 62, and 
 63 show the effects of additional tones. In Fig. 64 the six 
 
 tones complete the repre- 
 sentation of a solid object, 
 from which it will be seen 
 that form and substance 
 are shown by degrees of 
 brightness. In the mind 
 the forms of natural objects 
 are comprehended by the 
 degrees of brightness that 
 occur in them. It is the 
 business of photography to 
 reproduce these different 
 degrees of brightness,which 
 may vary from white to 
 black. 
 
 Differences in brightness 
 which occur in nature may 
 
 Fig. 62. 
 Four Tones. 
 
 Fig; 63. 
 Five Tones. 
 
 Fig. 64. 
 Six Tones. 
 
 68
 
 REPRODUCTION OF LIGHT AND SHADE 
 
 Fig. 65. 
 Front Lighting (Flatness). 
 
 Fig. 66. 
 
 Side and Top Lighting (Tone 
 Graduations). 
 
 be produced by differences in the illumination of the object. 
 If a plaster cast is lighted directly from the front the out- 
 lines will be visible but there will be no variation in tone. It 
 will have a flat, even appearance (Fig. 65). If the cast is 
 lighted from one side shadows will be formed, there will be 
 variations in illumination, and in this way tones will be pro- 
 duced by shadow (Fig. 66). 
 
 The brightness of an object depends not only upon the 
 illumination falling upon it, but also upon the reflecting 
 power of the object itself. Things differ very much in re- 
 flecting power. If a piece of white paper represents a re- 
 flecting power of 80%, a piece of gray paper may reflect 
 only 44% of the light falling upon it, and so on down the 
 scale, a piece of black paper reflecting only about 5%. The 
 brightest thing known is white chalk, which reflects 90% of 
 the light falling upon it; that is, of all the light falling on 
 the white chalk 90% is reflected back. Snow does not re- 
 flect quite as much light as chalk. The ordinary red brick 
 69
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 wall reflects only about 20%. Good black printers' ink 
 reflects about 10%, and the blackest thing, black velvet, 
 will reflect about 1% or 2% of the light falling upon it. 
 
 Since in natural scenes both the reflecting power and the 
 illumination vary, some parts of a landscape consisting of 
 clouds in sunlight, and others of dark rocks in the shade, 
 the range of contrast is often very considerable. For photo- 
 graphic purposes a scale, or contrast of 1 to 4, in which the 
 brightest thing is only four times as bright as the darkest, 
 is very low, and such a subject would be called flat; a con- 
 trast of 1 to 10 is a medium soft contrast; 1 to 20 a strong 
 contrast; 1 to 40 very strong and 1 to 100 an extreme de- 
 gree of contrast. All these degrees of contrast occur in 
 subjects such as landscapes, street and seashore scenes. 
 
 Since the more nearly we can reproduce in our picture 
 the range of brightnesses which were present when the pic- 
 ture was taken, the better the picture will represent the 
 original scene, our object in photography must be to get an 
 accurate reproduction of the various tones or brightnesses 
 which occur, keeping each tone in its same relative position 
 in the scale as it occupied in the subject which was photo- 
 graphed. This is, of course, easier to do if the range of 
 brightnesses is small than if it is very great. 
 
 When we make a photograph we do the operation in two 
 separate steps. We first make a negative upon a highly 
 sensitive material and obtain a result in which all the tones 
 of the original are inverted, the brightest part of the sub- 
 ject being represented by a deposit of silver in the negative 
 which lets through the least amount of light, while the dark- 
 er parts of the subject are represented by transparent areas 
 in the negative which let through the most light. This nega- 
 tive is then printed upon a sensitive paper, in which opera- 
 tion the scale of tones is again reversed so that the bright 
 parts of the subject which were represented by heavy de- 
 posits in the negative now appear as the light areas of the 
 print and the dark portions of the subject which were trans- 
 parent in the negative are represented by dark deposits in 
 the print. 
 
 In order to find out how closely the tones of the print 
 follow those of the original subject we must follow the 
 changes of these tones through both steps: we must study 
 first how far the negative reproduces in an inverted form 
 the tones of the subject and then how accurately the 
 printing paper inverts these again to give a representation 
 of the original. 
 
 70
 
 REPRODUCTION OF LIGHT AND SHADE 
 
 Fig. 67. 
 Five Toned Block. 
 
 Any silver deposit in the negative will let through a certain 
 proportion of the light which falls upon it. A very light 
 deposit may let through half the light, a dense deposit one- 
 tenth, a very dense 
 deposit one-hun- 
 dredth or even only 
 one- thousandth. 
 The amount of de- 
 posit through which 
 one can see depends, 
 of course, upon the 
 brightness of the 
 scene at which one 
 is looking, but it is 
 interesting to note 
 that one can see the 
 sun through a de- 
 posit which lets 
 through only about 
 one- twenty-billionth 
 of its light. 
 
 These fractions of 
 the light which are 
 let through are re- 
 ferred to as the "transparency" of the deposit, and the 
 inverse of the transparency is called the "opacity", the 
 
 opacity, therefore, ( 
 
 being the light-stop- 
 ping power of the 
 deposit. A deposit 
 which lets through 
 half the light, for in- 
 stance, is said to 
 have a transparency 
 of YL and an opacity 
 of 2. Similarly, one 
 which lets through 
 one- tenth of the light 
 has a transparency 
 of 1/10 and an opac- 
 ity of 10. 
 
 If the negative is 
 to be the exact in- 
 verse of the scale of Pi g . 
 tones of the subject, Negative of Five Toned Block. 
 
 7 1
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 then the opacities of the different areas must be in propor- 
 tion to the brightnesses of the parts of the subject which 
 produce them. In Fig. 67 we have a subject in which if we 
 take the black background as having a brightness of 1, the 
 brightest portion will have a brightness of 10, and the other 
 portion will be in proportion. Then when we make a nega- 
 tive of this we shall get the picture shown in Fig. 68, and 
 in this, if we measure the opacities of the negative, we ought 
 to find them exactly inverse to those of Fig. 67, so that the 
 transparency of the background, A, would be ten times that 
 of the table, B, or the opacity of the table, B, will be ten 
 
 Fig. 69. 
 Graded Strip of Exposures. 
 
 times that of the background, A. Not only this, but the 
 relative opacity of the deposits in the areas C, D and E 
 should also be the same as the brightnesses of C, D and E 
 in the original subject. 
 
 It will be seen by the foregoing, therefore, that a tech- 
 nically perfect negative will be one in which the opacities 
 of its different gradations are exactly proportional to the 
 light reflected by those portions of the original subject 
 which they represent. 
 
 Let us now consider how far we can fulfill this condition 
 and what must be done to obtain such a perfect negative 
 of any subject. 
 
 Suppose that a photographic plate or film is exposed to 
 a series of known brightnessess; for instance, that we photo- 
 graph a scale made up of steps of different reflecting powers 
 so the brightness of each step is doubled with regard to the 
 next one. 
 
 The result that such a series of exposures will give has 
 already been discussed in Chapter VI , but we must now look 
 into the matter somewhat more carefully. We shall get a 
 negative which will look like Fig. 69. Now if the rendering 
 is technically perfect, the opacities of this negative should 
 be the same as the brightnesses of the different steps of the 
 original ; that is to say, as each step is twice the brightness 
 of the next step, the light let through each step of the 
 negative should be half the amount of the step next to it. 
 72
 
 REPRODUCTION OF LIGHT AND SHADE 
 
 This would be attained if each step in the negative added 
 the same amount of silver to the deposit, so that- if we 
 could represent the silver for each step as altering the 
 thickness of the silver deposit (it does not do this really, 
 of course; it adds to the number of grains in the same 
 layer) and then could cut an imaginary section through 
 
 Fig. 70. 
 
 Heights of Silver Deposits 
 (diagram). 
 
 Fig. 71. 
 
 Heights of Silver Deposits 
 (Line Diagram). 
 
 the negative so as to show the height of the deposit 
 of silver, it should look like Fig. 70; and if we draw a dia- 
 gram in which the amount of silver is represented by the 
 height of a vertical line, the diagram showing the amount 
 of silver for the different steps might look like Fig. 71. 
 
 If we actually try this experiment, however, we shall find 
 that the silver does not rise quite uniformly in this way as 
 the exposure is increased through the entire scale, but that 
 
 instead we get the diagram 
 
 shown in Fig. 72, and this dia- 
 gram, which represents the act- 
 ual relation between the silver 
 deposit in a photographic ma- 
 terial and the increase of ex- 
 posure, requires careful study. 
 
 Starting.at A and proceeding 
 to B we notice that at the be- 
 ginning, in the lower exposures, 
 the steps are marked by a grad- 
 ually increasing rise, and, there- 
 fore, in this part of the exposure 
 scale there will be too great a 
 gain in opacity for each given 
 increase of exposure. A nega- 
 tive, the gradations of which 
 fall in this period, will yield prints in which an increasing 
 contrast is shown between tones of uniform increase of 
 brightness; that is to say, it will appear what we term 
 "under-exposed." From this period at B we pass imper- 
 
 73 
 
 Fig. 72. 
 
 Diagram of Actual Increase and 
 Graded Strip.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 ceptibly into the period where the densities show an equal 
 rise for each equal increase of exposure, and here we have 
 our technically perfect negative, that is, one in which the 
 opacities are exactly proportional to the light intensities 
 of the subject. This is termed the "period of correct ex- 
 posure," and only through this period of the curve where 
 the opacities are directly proportional to the exposures and 
 where the densities show an equal increase each time the 
 exposure is doubled shall we get a perfect rendering of the 
 original subject. From the point C onwards we have a 
 gradually decreasing rise in the steps with increase of expos- 
 ure until, finally, the increase of density with further expos- 
 ure becomes imperceptible. This period is the period of 
 "over-exposure," in which the opacities of the negative fail 
 to respond to increasing amounts of exposure and the 
 correctness of rendering is again lost. It will be seen at 
 once, then, from this curve that only through the period 
 of correct exposure where equal increases of exposure are 
 represented by equal rises in density can tones of the 
 original subject be correctly reproduced in the print. 
 
 If we join all these points to- 
 gether instead of representing 
 them as a staircase effect, as is 
 shown by dotted line in Fig. 72, 
 we get a smooth curve, Fig 73, 
 of which the straight line por- 
 tion (B to C) represents the 
 period of correct exposure, 
 while the more or less curved 
 portions at the beginning and 
 end of the curve correspond to 
 the periods of under-exposure 
 and over-exposure. 
 
 Fig. 73. 
 
 Curve Showing Under, Correct 
 and Over-exposure. 
 
 It must be realized that no ordinary negative can show 
 the whole range of exposures from beginning to end of this 
 curve. This is because the range of brightnesses covered 
 by the whole curve is much greater than that which occurs 
 in ordinary subjects and consequently it is quite possible 
 to represent an ordinary subject entirely in the period of 
 correct exposure, avoiding both the period of under-expos- 
 ure and the period of over-exposure. If, therefore, we wish 
 to obtain a technically perfect negative, we must expose so 
 that the subject which we are photographing falls into this 
 period of correct exposure, when we shall obtain a negative 
 in which there will be no wholly transparent film, since this 
 
 74
 
 REPRODUCTION OF LIGHT AND SHADE 
 
 would mean that we had entered the period of under-ex- 
 posure, and there will be no blocked up masses of silver 
 since this would mean that the negative was over-exposed. 
 The capacity of a photographic material to render the scale 
 of tone values correctly is, therefore, entirely a matter of 
 the length of the straight line portion of the curve, and it 
 is the length of this straight line portion in the case of Kodak 
 film which gives its well-known "quality" to the material. 
 By the use of a material of this kind which has a long 
 straight line portion to the curve, and of an exposure which 
 will place the scale of intensities on that straight line por- 
 tion we can correctly translate the tones of the subject into 
 corresponding opacities in the negative and obtain a tech- 
 nically perfect negative. 
 
 When we come to the second step of the process, however, 
 and make a print from this negative, we find that however 
 carefully we choose our exposure and development perfect 
 reproduction in the print is unobtainable. For a negative 
 material the relation between the silver deposit and the in- 
 crease of exposure is given by a curve similar to that shown 
 in Fig. 73, and in this curve the straight line portion (B to C) 
 represents the period of correct exposure, so that to obtain 
 perfect reproduction in the negative we must expose so that 
 the whole range of brightnesses in the subject falls within 
 this period of correct exposure, none of the tones being 
 represented by densities in the negative which fall on the 
 curved portions at the beginning and end of the curve cor- 
 responding to the periods of under and over-exposure. 
 
 When we make a print, however, we cannot do this be- 
 cause in a print we are forced to use the whole range of re- 
 flecting power of the printing paper; we must have high- 
 lights which are almost white paper, and shadows which 
 are as black as the silver deposit will give. This is necessary 
 because the total range of tones which can be obtained by 
 reflected light is none too great for the reproduction of 
 natural subjects, while in negatives, where the light is trans- 
 mitted instead of reflected, the available range is enormous 
 and we need make use of only a small portion of it. This 
 is also true in the case of transparent positives such as lan- 
 tern slides and motion picture films, which give the best 
 rendering of any printing material. 
 
 We can try the effect of an increasing series of exposures 
 upon a printing paper in exactly the same way as upon a 
 film, that is, we can give a first exposure just sufficient to 
 get a barely perceptible image after development, then ex- 
 
 75
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Fig. 74. 
 Curve of a Printing Paper. 
 
 pose another portion for twice the time, another for four 
 times, and so on. Now instead of measuring the light 
 transmitted by the various densities, as we did in the case 
 of the film, we must measure the light reflected from them. 
 We get a series of "reflection densities" on paper correspond- 
 to the transmission 
 densities of the 
 film and we can ex- 
 press the result in 
 the form of a curve 
 just as we did in 
 the case of the film. 
 Thus in Fig. 74 
 we see that the 
 densities increase 
 gradually at first, 
 as shown on the 
 lower portion of 
 the curve, then 
 grow in equal steps 
 for equal increases 
 of exposure, as with 
 the film, and then 
 the increase not only grows less, but very soon stops alto- 
 gether, as shown by the upper portion of the curve. This 
 result only occurs with a film with very great exposures 
 indeed, since after a film begins to be over-exposed there 
 is still a considerable range of exposures before the increase 
 of density with exposure actually ceases. Therefore, a paper 
 is seen to differ from a film in that we rapidly reach a point 
 where we have obtained the maximum blackness of deposit 
 which the sensitive emulsion is capable of giving and where 
 no further increase of exposure will enable us to obtain a 
 more intense black. 
 
 The reason for this is that with the paper we are dealing 
 with reflected light, and not with transmitted light, as in 
 the case of the film, and the light is reflected from three 
 surfaces from the surface of the gelatine, from the surface 
 of the silver deposit, and that which is not absorbed in 
 passing through the silver deposit is reflected from the paper 
 beneath. 
 
 The rule for correct rendering of tones on the paper is 
 the same as for the negative; that is, the tones which fall on 
 the straight line portion of the curve are rendered correctly, 
 and those which fall on the top and bottom portions of 
 
 76
 
 REPRODUCTION OF LIGHT AND SHADE 
 
 the curve do not reproduce the tones of the negative in 
 their correct position. As has already been said, how- 
 ever, the difference is that in the negative we can gen- 
 erally confine the scale of the subject to the straight line 
 
 part of the curve, while in 
 printing we are forced to use 
 the whole curve, including 
 those portions which cannot 
 give a perfectly correct ren- 
 dering of the tones of the 
 negative. 
 
 Different papers some- 
 times show very different 
 curves; thus in Fig. 75 we 
 see the way in which two 
 different papers give their 
 scales of tones; both give the 
 same range of tones, both 
 require the same range of 
 
 Fig. 75. 
 
 Curves Showing Good and Poor 
 "Quality" in a Printing Paper. 
 
 exposures to give the entire 
 range of tones, but in the 
 one the deposit grows evenly with the increase of exposure 
 while in the other the curve is scarcely straight at all. The 
 paper showing the even growth of deposit will give a correct 
 rendering of the tones of the negative throughout the 
 greater part of its curve (shown by dotted line in Fig. 75) 
 and it is generally said that such a paper has good "quality" 
 while the paper with the uneven growth (solid line Fig. 75) 
 has poor "quality". For papers, therefore, as well as for 
 negative-making materials, quality depends upon the pro- 
 portion of the curve which is a straight line, and the 
 straighter the curve the better the quality. 
 
 77
 
 CHAPTER IX. 
 
 PRINTING. 
 
 A GREAT number of different processes have been used 
 at one time or another for printing negatives. The 
 earliest printing processes depended upon the fact that 
 silver compounds darken in light, and the first printing 
 paper to be used generally was made by soaking a sheet of 
 paper in a solution of table salt and washing this over with 
 a solution of silver nitrate so as to convert the salt into 
 silver chloride. Paper so prepared was known as "salted" 
 paper on which, after exposure to light behind a negative, 
 a print was obtained which could be toned by the deposition 
 of gold from a solution and then fixed with hypo. A better 
 paper was made by using albumen obtained from the white 
 of eggs. After adding salt to it the albumen was spread over 
 the surface of the paper and then sensitized by treatment 
 with a solution of silver nitrate. 
 
 After the gelatine process for negatives was discovered 
 gelatine emulsions were applied to printing papers. Gela- 
 tine paper was made by emulsifying silver chloride in gela- 
 tine with an excess of silver nitrate and then coating it on 
 paper just as films are coated with the sensitive negative 
 emulsion. The typical gelatine-chloride paper of this type 
 is Solio. 
 
 To use Solio, the negative is put in a printing frame, and 
 the paper is put with its coated side in contact with the 
 emulsion side of the negative and pressed into contact by 
 closing the back of the printing frame. The frame is then 
 exposed to daylight and the image printed on the paper, 
 which darkens to a brownish-red colour. From time to time 
 the depth of the printing is observed by opening the back 
 of the frame. The image must be printed to a somewhat 
 darker colour than will be required in the finished picture. 
 When printed the paper is removed in subdued light and 
 the print is toned by immersing in a solution containing 
 gold so that the metallic gold is deposited on the print, 
 giving it a purple colour. After toning, the print is fixed in 
 a hypo solution and washed. A toning process is necessary 
 
 78
 
 PRINTING 
 
 with all printing-out silver papers, such as Solio, albumen- 
 ized paper, or salted paper, because if the printed-out silver 
 image is fixed without toning, the fixing bath changes it to 
 an ugly yellow color and a very poor-looking print results. 
 The gold toning produces a rich-looking, permanent image 
 which varies in color from brown to purple; these colors, 
 indeed, used to be regarded as the only satisfactory colors 
 for photographs. 
 
 The chief use for printing-out papers at the present time 
 is for the making of photographers' proofs. For this purpose 
 the negatives are printed, but the prints are not toned or 
 fixed, and, while they are satisfactory for examination, they 
 cannot be kept, because they darken in the light, the 
 photographer supplying them only as samples to show the 
 pose and expression, and making permanent prints to order 
 later. 
 
 Quite early in the history of photography it was dis- 
 covered that many substances besides the salts of silver are 
 sensitive to light. One process of printing, the platinum 
 process, is founded upon the sensitiveness to light of iron 
 salts. If paper is coated with ferric oxalate, which is a 
 green soluble salt of iron, and this is exposed to light, the 
 ferric oxalate is changed into another oxalate of iron, ferrous 
 oxalate, which is insoluble, so that a sheet of paper thus 
 prepared and printed will, after washing, give a faint image 
 consisting of ferrous oxalate. If, to the ferric oxalate with 
 which the paper is prepared, a solution of a platinum com- 
 pound is added and then, after printing, the faintly visible 
 image is put into a solution of a soluble oxalate, the ferrous 
 oxalate is dissolved and attacks the platinum salt, which is 
 not affected by the ferric oxalate, precipitating metallic 
 platinum on the paper so that an image is obtained con- 
 sisting of black metallic platinum. 
 
 Another process depends upon the fact that gelatine con- 
 taining bichromate becomes insoluble in water on exposure 
 to light, and this process is known as the "pigment" process 
 or more commonly as the "carbon" process, the name being 
 derived from the fact that the gelatine used in the early 
 days of the process contained finely divided carbon or lamp 
 black to act as a pigment. The printing paper is made by 
 coating the paper stock with a thick gelatine solution con- 
 taining finely divided pigment suspended in it. The pigment 
 is chosen according to the color of the print required. For a 
 black image it may be lamp black, for a red image red ochre 
 or burnt sienna, and for images of other colors any perma- 
 
 79
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 nent and stable pigment of the color desired which can be 
 finely powdered. After the coated gelatine has been dried 
 the paper is immersed in a solution of bichromate of potash 
 or ammonia and again dried. This bichromated gelatine is 
 quite soluble in hot water, but if it is exposed to light it 
 becomes insoluble where the light has acted upon it. The 
 bichromated gelatine is, therefore, printed under the nega- 
 tive in the same way as a Solio print. No visible image is 
 produced, and to get the visible print it is necessary to wash 
 away the soft gelatine. The gelatine, which has been hard- 
 ened by the action of light, is on the surface of the print 
 and the soft gelatine is at the back, so in order to develop 
 the print it is put face down on to another sheet of paper 
 and placed in hot water. After a short time the soluble 
 gelatine begins to ooze out at the edges of the print and 
 the whole of the original paper can be pulled off, leaving the 
 image covered with a sticky mass of partly dissolved gela- 
 tine on the paper to which it has been transferred. This 
 image is then washed in hot water until all the soluble gela- 
 tine has been washed away, leaving a clear image of the 
 pigmented gelatine on the paper. 
 
 All these printing-out processes require a long exposure 
 to strong daylight, and they have become more or less ob- 
 solete owing to the trouble of working them and especially 
 the difficulty of judging the correct exposure with such a 
 variable illuminant as daylight. They have been displaced 
 by printing processes in which the paper used is coated 
 with an emulsion very similar to that used for making the 
 negative, but of considerably less sensitiveness. This paper, 
 known as development paper, is exposed behind the negative 
 and is then developed, in the same way as a negative, to 
 give a visible image. 
 
 The oldest of these development papers is bromide paper. 
 This paper is coated with an emulsion very similar to the 
 ordinary negative emulsions but of somewhat less sensi- 
 tiveness. The paper is very sensitive to light and must be 
 worked by red or orange light only. The exposure for print- 
 ing is, of course, very short and the paper is, in fact, mostly 
 used for enlarging, the image of the negative being thrown 
 upon the sensitive bromide paper by a projection lantern so 
 as to obtain an enlarged picture from the negative. 
 
 About 1894 Velox paper was introduced and was an en- 
 tire novelty, since while it was similar to bromide paper in 
 that it is exposed to an artificial light and then developed 
 and fixed, it is so much less sensitive than bromide paper 
 80
 
 PRINTING 
 
 that it can be worked in a room lighted by a weak artificial 
 light and does not require a special darkroom, from which 
 fact it is known as "gaslight" paper. Since the introduction 
 of Velox other gaslight papers have been made and at 
 present almost all prints made by contact from negatives 
 are made on gaslight papers, though Velox is still the best 
 known of all. Velox is about a thousand times slower than 
 bromide paper so that it can be handled safely in any sub- 
 dued light. It requires an exposure that ranges from about 
 2 seconds to about a minute and a half, depending on the 
 density of the negative and the grade of Velox used, at one 
 foot from a 25-watt Mazda lamp, and it is characterized 
 especially by the extreme rapidity and ease of its develop- 
 ment, from which its name is derived, Contrast and Regular 
 developing fully in 15 to 20 seconds and Special Velox 
 in about 30 seconds. It is consequently possible by using 
 Velox to make prints in comfort and with great rapidity, 
 the old troubles of judging the extent of the printing, and 
 the difficulties with toning baths being entirely absent 
 with this simple and convenient printing medium. 
 
 Fig. 76. 
 Degrees of Light Intensities. 
 
 Velox paper is made in three grades of contrast to fit dif- 
 ferent types of negatives. The paper was originally made 
 in the Regular grade only, but it was found that many 
 negatives were too contrasty to print well on this paper and 
 Special Velox was manufactured for use with such negatives, 
 while recently Contrast Velox has been put on the market 
 for use with negatives so lacking in contrast that they will 
 not give good prints even on the Regular grade. 
 
 If we make three negatives of the same subject in succes- 
 sion, giving each exactly the same exposure, and then dev- 
 elop these for different lengths of time so that the first will 
 be underdeveloped the second correctly developed, and the 
 third over developed, the first negative will have a short 
 range of contrast, the second a medium range, and the third 
 a long range. If we then print the first negative on Contrast 
 Velox, the second on Regular Velox, and the third on Special 
 Velox, we shall get almost identical prints on all three papers 
 81
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Print from Opposite Negative 
 on Regular Velox. 
 
 Hard Negative of Strong Contrast. 
 
 Print from Opposite Negative 
 on Special Velox. 
 
 provided that the contrasts of the negatives just fit the 
 various grades of the paper. This is shown in Fig. 77. 
 
 We might think that Contrast Velox would always give 
 more contrasty prints than Regular Velox; it will if both 
 papers are printed from the same negative, but if the Contrast 
 Velox is printed from a flat negative and the Regular Velox 
 from a normal negative, then the Contrast Velox will com- 
 pensate for the flat negative and give a normal print, just as 
 82
 
 PRINTING 
 
 the Regular Velox gives a normal print from a normal neg- 
 ative, and the Special Velox a normal print from a con- 
 trasty negative. 
 
 Fig. 78. Range of 44 Distinct Tones. 
 
 All the grades of Velox give the same range of reflecting 
 powers in the print provided that they are used with nega- 
 tives which will enable this range to develop. Suppose we 
 take a black wedge which contains all the degrees of light 
 intensities, from absolute opacity at one end to absolute 
 transparency at the other end and make a print of it. We 
 should get the result shown in Fig. 76. This shows the 
 entire range of reflecting power of which the paper is 
 capable, the range varying from white paper at one end to 
 the blackest silver deposit which the paper can give, at 
 the other. 
 
 With any "velvet" surface paper, such as Velvet Velox, 
 we shall find that the white paper will reflect about twenty- 
 five times as much light as the deepest silver deposit. The 
 
 number of distinct tones 
 which are included in this 
 range from white to black 
 depends, of course, on the 
 ability of the eye to dis- 
 tinguish them. The eye 
 can actually see about one 
 hundred distinct tones in 
 such a range. 
 
 In Fig. 78 is shown a 
 range of tones made up, 
 not as a continuous wedge, 
 but of forty-four distinct 
 tones. The number which 
 can be seen in the illustra- 
 tion is less than the num- 
 ber which the eye can dis- 
 tinguish in a print because 
 ^dSf R VeTox ar v^of of th u e limitations imposed 
 Fig. 79. by the process of half-tone
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Fig. 80. 
 Print Showing Empty Highlights. 
 
 Fig. 81. 
 Print Showing Blocked Shadows. 
 
 84
 
 PRINTING 
 
 re- production. If the full one hundred tones which the eye 
 can distinguish in a print were reproduced by the half-tone 
 process the halftone illustration would look like a con- 
 tinuous wedge. 
 
 In Fig. 79 the same wedge has been printed on all three 
 papers, and it will be seen that Contrast Velox has reached 
 its full blackness only a short distance up the wedge, 
 
 Regular Velox 
 has gone further, 
 and Special Velox 
 has gone the far- 
 thest of all, so 
 that while all 
 three papers will 
 give the same 
 range of tones, 
 this range is im- 
 pressed on Con- 
 trast Velox with 
 only a short range 
 of densities in the 
 negative ; f or 
 Regular Velox a 
 longer range is 
 needed, and for 
 Special Velox a 
 p. 82 still longer range. 
 
 Gray, Flat Print. The . ran S e . of 
 
 densities required 
 
 in a negative to just print out the full range of tones on a 
 paper is called the "scale" of the paper and this is measured 
 by trying an increasing series of exposures until the range 
 of exposures which will just give the whole range of tones 
 on the paper is found ; that is, if an exposure of one second 
 to the bare paper with no negative will just give the first 
 perceptible difference from white paper, so as to show the 
 first trace of tint on the paper, and an exposure of twenty 
 seconds will give the deepest black the paper is capable of 
 rendering, so that no increase of exposure will produce any 
 denser black, then we should call the scale of the printing 
 paper 1 to 20. 
 
 Thus the word "scale" applied to a printing paper does 
 not refer at all to the range of tones in the print. It indi- 
 cates the range of contrast in the negative which should be 
 printed on that paper. A paper with a scale of 1 to 20 will 
 
 85
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 require a negative in which the densest part lets through 
 1/20 of the light transmitted by the clearest part, because 
 if this negative is printed on that paper the print will just 
 have the whole range of tones from white to black com- 
 pletely printed out, each tone in the print corresponding 
 to a density in the negative, and there will be no differ- 
 ences of density in the negative unrepresented by differ- 
 ences of tone in the print. 
 
 Special Velox has a scale of about 1 to 20 and is suitable 
 for printing from contrasty negatives. Regular Velox has 
 a scale of about 1 to 10 and is suitable for printing from 
 negatives of moderate contrast, while the very flattest and 
 least contrasty negatives, which are the result either of ex- 
 cessive over-exposure or underdevelopment should be 
 printed on Contrast Velox, which has a scale of about 1 to 5. 
 
 It is important to choose the grade of paper correctly for 
 the negative. If the paper is too contrasty for the negative; 
 if, for instance, we print a hard negative (one that has 
 strong contrast) on Contrast Velox, then we shall have to 
 sacrifice a part of the scale of the negative; either we shall 
 get the highlights empty and white, as shown in Fig. 80, or 
 we shall get the shadows blocked up, as shown in Fig. 81. 
 On the other hand, if the scale of the paper is too long for 
 the negative and we print a soft negative (one that has 
 little contrast) on Special Velox, for instance, when we 
 should have used Regular Velox, then we shall get a gray, 
 flat print, as is shown in Fig. 82. 
 
 With paper, as with film, the density of the picture is 
 controlled by the duration of the exposure and the develop- 
 ment, but whereas with films the contrast is dependent upon 
 the time of development, the contrast increasing as the 
 development is continued, with paper the contrast is fixed 
 by the maker, and after a few seconds the development 
 does not change the contrast of the print at all but only 
 affects the density of the deposit. This is illustrated in 
 Figs. 83 and 84. 
 
 In Fig. 83 we see that with increasing time of develop- 
 ment, a film shows an increase in contrast, while in Fig. 84 
 that by prolonging development it is clear after reaching a 
 certain stage in the development of a print there is only 
 an increase in total density and no increase in contrast. 
 
 If a print is over-exposed, it can be taken out of the dev- 
 eloper before it is fully developed, and if under-exposed, it 
 can similarly be forced in development, though there is 
 some risk of yellow stain if development is continued too 
 86
 
 PRINTING 
 
 long. The best results can, of course, only be obtained by 
 getting the exposure right and giving the normal time of 
 development, which is from 15 to 20 seconds for Contrast 
 and Regular Velox, and about 30 seconds for Special Velox. 
 
 Fig. 83. 
 
 Growth of Contrast with Devel- 
 opment, Eastman N. C. Film. 
 
 Fig. 84. 
 
 Increase of Density with Devel- 
 opment, Velox Paper. 
 
 The matter of greatest importance for getting really first- 
 class prints, therefore, is to give them the right time of 
 exposure. 
 
 Before starting to print a number of negatives they 
 should be classified for contrast so as to choose a suitable 
 grade of paper for printing them; that is to say, put the 
 negatives in three envelopes according to whether they are 
 to be printed on Special, Regular or Contrast Velox. Now 
 take the negatives in each of these envelopes and divide 
 them again into three more classes normal negatives hav- 
 ing average density, thin negatives, and dense negatives. 
 When printing, if we take the exposure for the normal 
 negative as standard, then the thin negatives will require 
 half this standard exposure and the dense negatives will 
 require twice, while sometimes we may possibly meet an 
 exceptional negative very thin or very dense which may 
 require one-fourth or four times the standard exposure. 
 Having classified our negatives in this way, in order to get 
 our exposures right we need know only the exposure on each 
 grade of Velox paper for our standard negatives, and if we 
 print with a 25-watt tungsten lamp at a distance of one 
 foot, we shall find that the exposure for a standard negative 
 will be about 20 seconds for Special Velox, about one 
 
 87
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 minute for Regular, and one and a half minutes forContrast. 
 These figures are to be taken only as a guide, and when a 
 new light or a new package of paper is used for the first 
 time, trial exposures should be made with the standard 
 negative, giving, say, 15, 20 and 30 seconds exposure, so 
 as to select the exposure which develops to the right density 
 with the correct time of development. 
 
 It is best always to use the same standard negative for 
 testing a new paper or a new printing lamp and any other 
 new conditions that may arise in printing, as more useful 
 information will be gained by making tests with one nega- 
 tive only than if a different negative is selected each time 
 a test is to be made. 
 
 If the subject of exposure is dealt with in this way, if the 
 negatives are classified for density before printing, and a 
 test is made on a standard negative, it will be found easy 
 to print a large number of negatives on several grades of 
 Velox paper and get a very high percentage of first-class 
 prints with normal development. 
 
 With regard to development and after-treatment of the 
 print, there is very little to say, since the matter is fully ex- 
 plained in the instruction sheet that accompanies each pack- 
 age of Velox paper. It is best to buy the ready prepared 
 developers such as Velox Liquid Developer or Nepera Solu- 
 tion and to follow the directions given. 
 
 When fixing prints, take care that they do not lie on top 
 of one another in the fixing bath without change so that 
 each print will get its supply of fresh acid hypo. 
 
 ENLARGING. 
 
 While contact prints are satisfactory to show one's 
 friends, a time comes when we want to attempt something 
 more ambitious and to make photographs which we can 
 hang on our walls or submit for exhibition, and then we feel 
 that we want something more than an ordinary print and 
 something more than an enlarged print; we want to make 
 a picture. The difference between a picture and a print is 
 of course, not a matter of size; it is a matter of composition 
 and balance, of judgment in the choice of subject and of 
 the moment of exposure, and of finish and quality in the 
 result. 
 
 The possibility of using a very great degree of enlarge- 
 ment is shown in Fig. 85, where the small image in the cor- 
 ner represents a contact print from the original negative.
 
 PRINTING 
 
 Fig. 85. 
 
 Extreme Enlargement. 
 Original in lower right hand corner. 
 
 In this case the negative was a portion of a motion picture 
 film which was taken to get the utmost sharpness of defini- 
 tion and was then enlarged to about a thousand times its 
 original size, the definition in the finished enlargement 
 being still quite good. Such work as this is rarely wanted, 
 but the great value of enlarging is that parts can be chosen 
 from a negative and enlarged to make very pleasing pic- 
 tures, where the whole negative if printed as a contact print 
 would be by no means satisfactory. The print shown in 
 Fig. 86, for instance, is an enlargement of a film negative. 
 This negative was taken at the seashore as a snapshot ex- 
 posure, the figures being very small and in the corner of 
 the negative so that if the negative were printed as a whole 
 it would be very unsatisfactory. While a contact print 
 trimmed as is shown in the enlargement was not much 
 larger than a postage stamp, an enlargement of the figures 
 in it, however, made a pleasing picture: 
 
 Another illustration of what can be done in enlarging is 
 shown in Fig. 87, where two negatives have been enlarged 
 together to make a combined picture. The lower half of 
 the original scene, of which the church and trees form the
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 upper half, consisted of a plowed field, so that the fore- 
 ground in the original negative was very unsatisfactory. 
 By taking another foreground, however, taking care, of 
 
 course, that the light- 
 ing was the same, and 
 shading the fore- 
 ground of the first 
 negative so that it 
 did not print in en- 
 larging, then chang- 
 ing the negative in 
 enlarging and substi- 
 tuting the foreground 
 negative, the two have 
 been printed into one 
 another with the re- 
 sult ' shown. Some 
 photoghraphers are 
 very clever at making 
 these combined en- 
 largements. 
 
 There are two prac- 
 tical methods of mak- 
 ing enlargements; 
 those involving work- 
 ing in a dark-room, 
 and those in which no 
 dark-room is employ- 
 ed for the enlarging 
 itself. For the latter 
 purpose the Brownie 
 Enlarging Camera is 
 suitable, this being simply a cone-shaped box with a holder 
 for the paper at the large end and a negative holder at the 
 small end. The lens is fitted inside the cone, at just the 
 right distance to insure a sharp focus so that the camera is 
 always focused, and sharp enlargements are certain if the 
 negatives are sharp. This enlarger is exposed to daylight. 
 The disadvantage with this camera is that the degree of 
 enlargement is fixed and that consequently it is not easy 
 to select a small portion of a negative and enlarge it to a 
 considerable extent. 
 
 Another good arrangement is that shown in Fig. 88, where 
 the film or glass negative is put into the negative holder 
 of the Kodak Enlarging Outfit. With this arrangement the 
 90 
 
 Fig. 86. 
 Enlargement of Part of a Snapshot.
 
 PRINTING 
 
 negative is projected on to an easel or wall on which the 
 bromide paper can be pinned, and since the distance of the 
 enlarger from the easel or wall can be regulated, any degree 
 of enlargement can be obtained and a small part of the 
 negative can be selected and enlarged to any required size. 
 
 TONING. 
 
 In the earlier printing processes used by photographers 
 those in which the image was obtained by the continued 
 action of light and which were toned by the deposition 
 of gold from a toning bath the prints obtained were 
 
 in various shades of 
 purple and brown, 
 and these shades be- 
 came so associated 
 with photographs in 
 the minds of the pub- 
 lic, that when the 
 black and white 
 prints made on Velox 
 and bromide papers 
 began to displace the 
 earlier Solio and Aris- 
 totype prints, the gen- 
 eral public would 
 scarcely recognize 
 them as "photo- 
 graphs" at all, and a 
 demand soon arose for 
 some method of ton- 
 ing the black images 
 of bromide and Velox 
 prints to a brown or 
 sepia similar to that 
 of the gold toned 
 printing-out papers. 
 It seems to be char- 
 acteristic of mankind to want what they have not got, and 
 it is interesting to note that with the earlier printing-out 
 processes which easily gave warm tones, chemists were 
 anxiously working to get methods of obtaining black and 
 white prints, while with the developing-out processes, which 
 naturally give good black and white prints, photographers 
 desire to obtain warm sepia and brown tones. 
 
 The processes for obtaining sepia prints from the black , 
 developed-out images all depend on one chemical reaction; 
 
 9 1 
 
 Fig. 87. 
 
 Combined Enlargement from 
 Two Negatives.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 ^namely, that by which silver bromide is converted into 
 silver sulphide. Silver sulphide is a dark coloured, almost 
 black, substance well known to the housekeeper if not by 
 name as the tarnish which appears on silverware after it 
 has been some time in the air, the surface of metallic silver 
 being attacked by sulphur compounds in the air, which 
 generally come from the products of combustion of gas in 
 the cooking range. 
 
 Now, when any chemical substances can be produced by 
 the interaction of two other chemical substances in solution 
 the question as to whether it will be produced depends upon 
 whether it is more or less soluble than the substances which 
 
 Fig. 88. 
 Brownie Enlarging Camera. 
 
 ^can form it. Silver sulphide is less soluble than silver bro- 
 /mide so that when silver bromide is treated with a solution 
 / containing sulphur in a free form it is changed into silver 
 I sulphide and the silver sulphide is deposited in its place. 
 ' On the other hand, metallic silver, such as that which 
 forms the image in a developed print, is less soluble than 
 silver sulphide and consequently we cannot change it into 
 silver sulphide by simply treating it with a solution con- 
 taining free sulphur, but if in this solution we have some 
 substance which will dissolve metallic silver, then we can 
 change the metallic silver itself into silver sulphide. It is on 
 these principles that the sulphur toning processes are based. 
 One toning process depends upon changing the silver 
 image of the print back into silver bromide. Now, we know 
 that silver is obtained from silver bromide by reduction, 
 just as iron is got out of iron ore, and therefore we can get 
 back silver bromide from silver by oxidation, which is the 
 reverse process to reduction. If we use any solution which 
 will oxidize silver and have potassium bromide present in the 
 92
 
 PRINTING 
 
 solution, the silver image will be turned into silver bromide. 
 
 The usual way to do this is to treat the black print after 
 fixing and washing with a solution containing potassium) 
 ferricyanide, which is an oxidizing agent, and potassium'] 
 bromide, and this turns the black silver image into a yellow- 
 ish-white image of silver bromide which is scarcely visible, 
 so that the process is called "bleaching" since the black 
 silver turns into white silver bromide, and then after wash- 
 ing, this silver bromide is treated with a solution of sodium 
 sulphide, which turns it into the brown silver sulphide 
 which gives us our sepia toned print. So, to make a sepia 
 Velox print by this method, we treat it with the "bleaching 
 solution," which turns the silver into silver bromide, and 
 then "redevelop" this, as it is called, in a solution of sul- 
 phide, which converts the silver bromide into silver sulphide 
 and gives us our sepia print. 
 
 There is another method of obtaining sulphide toned 
 prints which is somewhat simpler. We have seen that we 
 cannot turn silver directly into silver sulphide by a solution' 
 containing free sulphur unless we have a solvent of silver 
 present in the solution. Now, it so happens that hypo is to ; 
 some extent a solvent of silver, and also that with a weak 
 acid, hypo gives free sulphur. Alum behaves chemically like 
 a weak acid and it also has the valuable property of harden- 
 ing the print, so if we put the print which we wish to tone 
 into a solution containing hypo and alum, the silver will 
 slowly be changed into silver sulphide and the print will be 
 toned brown. This change goes on very slowly at ordinary 
 temperatures, but by heating the solution it goes much more 
 rapidly, so that if we heat a bromide or Velox print in a solu- , 
 tion containing hypo and alum, we shall get a good sepia tone 
 at the end of ten or twenty minutes without any further 
 difficulty, the only objection being that the bath, like all 
 baths containing free sulphur, and like the sodium sulphide 
 used for redeveloping in the other toning process, smells 
 rather unpleasantly. 
 
 Equally good results in sepia toning cannot be got with 
 all papers, but a great deal depends on the development of 
 the print. To get good sepias, development should be full; - 
 an underdeveloped print will always give weak, yellowish 
 tones when compared with one in which development has 
 been carried out thoroughly, which will give a strong, pure 
 sepia. It is important to remember this, as two prints 
 which may look alike as black and white prints will tone dif- 
 ferently if they have not been developed to the same extent. 
 
 93
 
 CHAPTER X. 
 
 THE FINISHING OF THE NEGATIVE. 
 
 AFTER development, the undeveloped silver bromide is 
 removed by immersion of the negative or print in what 
 is called the "fixing bath". There are only a few substances 
 which will dissolve silver bromide, and the one which is 
 universally used in modern photography is sodium thio- 
 sulphate, which is known to photographers as hyposulphite 
 of soda, or more usually as hypo, though the name hypo- 
 sulphite of soda is used by chemists for another substance. 
 
 In the process of fixation the silver bromide is dissolved 
 in the hypo by combining with it to form a compound 
 sodium silver thiosulphate. Two of these compound thio- 
 sulphates exist, one of them being almost insoluble in water, 
 while the other is very soluble. As long as the fixing bath 
 has any appreciable fixing power, the soluble compound only 
 is formed. 
 
 Fixing is accomplished by means of hypo only, but 
 materials are usually transferred from the developer to the 
 fixing bath with very little rinsing so that a good deal of 
 developer is carried over into the fixing bath, and this soon 
 oxidizes in the bath, turning it brown, and staining nega- 
 / tives or prints. In order to avoid this the bath has sulphite 
 of soda added to it as a preservative against oxidation, and 
 the preservative action is, of course, greater if the bath is 
 kept in a slightly acid state. In order to prevent the gela- 
 /tine from swelling and softening it is also usual to add some 
 / hardening agent to the fixing bath so that a fixing bath in- 
 stead of containing only hypo will contain in addition sul- 
 phite, acid and hardener. 
 
 Now, if a few drops of acid, such as sulphuric or hydro- 
 chloric acid, are added to a weak solution of hypo, the hypo 
 will be decomposed and the solution will become milky, 
 owing to the precipitation of sulphur. The change of thio- 
 sulphate into sulphite and sulphur is reversible, since, if we 
 boil together sulphite and sulphur we shall get thiosulphate 
 formed, so that while acids free sulphur from the hypo, 
 
 94
 
 THE FINISHING OF THE NEGATIVE 
 
 sulphite combines with the sulphur to form hypo again. 
 Consequently, we can prevent acid decomposing the hypo 
 if we have enough sulphite present, since the sulphite works 
 in the opposite direction to the acid. An acid fixing bath,' 
 therefore, is preserved from decomposition by the sulphite, 
 which also serves to prevent the oxidation of developer 
 carried over into it. 
 
 Since in fixing baths what we require is a large amount 
 of a weak acid, the best acid for the purpose is acetic_acid. t 
 Citric or tartaric acids can also be used. 
 
 In order to make sure that the films are properly fixed 
 they should be left in the fixing bath twice as long as is 
 necessary to clear them from the visible, white silver bro- 
 mide. If considerable work is being done, the best course is. 
 to use two fixing baths, transferring the films or prints to 
 the second clean bath after they have been fixed in the first. 
 Then, when the first bath begins to work slowly, it can be 
 discarded and replaced by the second bath, a fresh solution 
 being used for the second bath. These precautions are 
 necessary because, as has already been said, silver forms 
 two compound thiosulphates, the first of which is almost 
 insoluble in water but is transformed into the second, which 
 is soluble, by longer treatment with hypo. Consequently 
 when a film first clears, it still contains the first insoluble 
 thiosulphate of silver, and if it is taken out of the fixing 
 bath and washed some of the silver will be left behind and 
 not washed out. Then, on keeping, this silver thiosulphate 
 left in the negative will decompose and produce stains. If 
 a negative or print is properly fixed and washed it will be 
 permanent. 
 
 The actual rate of washing may be understood by remem- 
 bering that the amount of hypo remaining in the gelatine 
 is continually halved in the same period of time as the wash- 
 ing proceeds. An average negative, for instance, will give 
 up half its hypo in two minutes, so that at the end of two 
 minutes half the hypo will be remaining in it, after four 
 minutes one-quarter, after six minutes one-eighth, after 
 eight minutes one-sixteenth, ten minutes one- thirty-second, 
 and so on. It will be seen that in a short time the amount 
 of hypo remaining will be infinitesimal. This, however, as- 
 sumes that the negative is continually exposed to fresh 
 water, which is the most important matter in arranging 
 the washing of either negatives or prints. 
 
 If a lot of prints are put in a tray and water allowed to 
 splash on the top of the tray, it is very easy for the water on 
 
 95
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 the top to run off again, and for the prints at the bottom to 
 lie soaking in a pool of fairly strong hypo solution, which 
 is much heavier than water and which will fall to the bottom 
 of the tray. If the object is to get the quickest washing, 
 washing tanks should be arranged so that the water is con- 
 tinuously and completely changed and the prints or nega- 
 tives are subjected to a continuous current of fresh water. 
 If water is of value, and it is desired to economize in its use, 
 then by far the most effective way of washing is to use suc- 
 cessive changes of small quantities of water, putting the 
 prints first in one tray, leaving them there for from two min- 
 utes to five minutes, and then transferring them to an en- 
 tirely fresh lot of water, repeating this until they are washed. 
 The progress of the washing can be followed by adding 
 a little permanganate solution to the wash water after the 
 prints are taken out of it in order to see how much hypo is 
 left in it, the presence of hypo being seen by decoloration 
 of the permanganate. An even simpler test is to taste the 
 prints. Six changes of five minutes each should be sufficient 
 to eliminate the hypo effectively from any ordinary material. 
 
 REDUCTION. 
 
 Sometimes negatives are obtained which are so dense that 
 they are difficult to print. Other negatives are so contrasty 
 that they give harsh prints. In order to improve these 
 negatives recourse may be had to the process called "re- 
 duction," that is, to the removal of some of the silver by 
 treatment with a chemical which dissolves the metallic 
 silver of the image. 
 
 It is unfortunate that the word "reduction" is used in 
 English for this purpose. In other languages the word 
 "weakening" is used and it is undoubtedly a better word 
 because the chemical action involved in the removal of 
 silver from a negative is oxidation, and the use of the word 
 reduction leads to confusion with true chemical reduction 
 such as occurs in development. 
 
 In order to produce the best results it is necessary that 
 the reduction should be suitable for the negative which is 
 , to be treated. Thus, in the case of a negative which is too 
 dense all over it is necessary to remove the density uni- 
 formly, while in the case of one which is too contrasty what 
 is required is not the removal of the silver from highlights 
 and shadows alike, but the lessening of the deposit on the 
 highlights without affecting the shadows. 
 96
 
 THE FINISHING OF THE NEGATIVE 
 
 In Fig. 89 we see a diagram which represents a negative 
 originally dense from which by the removal of an equal 
 amount of silver from shadows, halftones and highlights, 
 
 Fig. 89. 
 Diagram Showing How Cutting Reducer Acts. 
 
 there can be obtained a negative of proper gradation. A 
 reducer which effects this uniform removal of density is 
 generally called a "cutting reducer". The typical "cutting 
 reducer" is that known as Farmer's Reducer, which is made 
 by preparing a strong solution of potassium ferricyanide, 
 otherwise known as Red Prussia te of Potash, and adding a 
 few drops of this to a solution of plain hypo until the latter 
 is yellow. This reducer will not keep when mixed so that 
 the ferricyanide must be added to the hypo only when re- 
 quired for use. It is especially useful for clearing negatives 
 or lantern slides and is often used for local reduction, the 
 solution being applied with a wad of absorbent cotton to 
 the part which is to be lightened. Another cutting reducer, 
 is permanganate, which is supplied under the name of the 
 "Eastman Reducer." Permanganate, however, tends to 
 act more proportionally on the highlights and shadows than 
 is the case with ferricyanide. 
 
 Proportional reducers are those which act on all parts o 
 the negative in proportion to the amount of silver present 
 there. They thus exactly undo the action of development ^ 
 since during development the density of all parts of a 
 negative increase proportionately. A correctly exposed, but , 
 over-developed negative should, therefore, be reduced with 
 a proportional reducer. This effect is shown in Fig. 90 
 where it is seen that the contrast of the negative is far too 
 great owing to over-development, and that by removing 
 the same proportion of the silver from the shadows, half- 
 tones and highlights, a negative of correct contrast can be 
 obtained. 
 
 97
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Fig. 90. 
 Diagram Showing How Proportional Reducer Acts. 
 
 Unfortunately there are no single reducers which are ex- 
 actly proportional in their action but by mixing perman- 
 
 / ganate, which is a slightly cutting reducer, with persulphate, 
 which is a flattening reducer, a proportional reducer may be 
 obtained. 
 
 Flattening reducers are required for negatives which have 
 
 ^ been under-exposed and then over-developed. In these cases 
 the negative is much too contrasty but it is important not to 
 remove any of the deposit from the shadows, since owing to 
 the under-exposure, there is already insufficient deposit in 
 the shadows. 
 
 What is required in this case is shown in Fig. 91, where 
 a large amount of deposit is removed from the highlights, 
 a smaller amount from the halftones, and very little or 
 none from the shadows. This can be accomplished by the 
 
 V use of ammonium persulphate. Ammonium persulphate at- 
 tacks silver deposit with the formation of silver sulphate and 
 this attack is increased by the silver salt which is produced, 
 the rate of attack increasing as the attack goes on. Such 
 
 y f chemical actions are called "auto-catalytic," a "catalyst" 
 .'being a substance which increases the rate of a chemical 
 
 Fig. 91. 
 Diagram Showing How Flattening Reducer Acts.
 
 THE FINISHING OF THE NEGATIVE 
 
 Fig. 92. 
 
 a. Negative too dense all over. 
 
 b. Result of using Farmer's Reducer. 
 
 action without actually taking part in it, and an auto-' 
 catalytic action being one in which the rate of action in- 
 creases of its own accord. Since the action of ammonium 
 persulphate is auto-catalytic it acts most rapidly where the 
 greatest amount of silver is present, and consequently it 
 attacks the highlights far more energetically than it attacks 
 the shadows of the negative and is, therefore, suitable for 
 the reduction of under-exposed, over-developed negatives. 
 (Whether any silver will be removed from the shadows will 
 depend on how long the reducer is allowed to act.) Because, 
 it is auto-catalytic in its action, however, it is very likely 
 to go too far and get out of control so that it is not by any 
 means an easy reducer to handle, and it is not recommended 
 that it be used upon a valuable negative unless the user 
 has had considerable experience of its action. 
 
 For some time after ammonium persulphate was intro- 
 duced as a reducer for negatives its action was very 
 
 99
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Fig. 93. 
 
 . Correctly exposed but over-developed negative. 
 . Result of reducing with a Proportional reducer. 
 
 100
 
 THE FINISHING OF THE NEGATIVE 
 
 Fig. 94. 
 
 c. Too dense in highlights, deep shadows not clear. 
 b. Effect of the Eastman Reducer on such a negative. 
 
 uncertain; some samples would reduce silver while others 
 would not. When this peculiarity in its behavibr was inves- 
 tigated by the Research Laboratory of the Eastman Kodak 
 Company the reason was found to be a chemical difference 
 in some of the samples tested. 
 
 INTENSIFICATION. 
 
 Sometimes we get negatives which are too thin and weak 
 to print even on Contrast Velox; if we developed them 
 in the tray perhaps we were deceived in judging the density 
 101
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Fig. 95. 
 
 a. Negative with dense, blocked up highlights. 
 
 b. Shows that a Flattening Reducer removes much silver from the 
 
 Highlights, less from the Halftones and little or none from the 
 Shadows. 
 
 and we under-developed them, or possibly the subject itself 
 was very flatly lighted, as often happens when the subject 
 is an extremely distant landscape or a view across a large 
 body of water, and from such negatives we cannot get a 
 bright print, even on Contrast Velox. 
 
 Sometimes, also, we may not have Contrast Velox on 
 hand and may wish to use Special or Regular Velox. In
 
 THE FINISHING OF THE NEGATIVE 
 
 Original. 
 
 Strongly Intensified. 
 
 Fig. 96. 
 Showing Effect of Intensification. 
 
 all these cases it is convenient to have a means of increasi 
 ing the contrast of the negative, and the method by which 
 this is done is the chemical process commonly called "in- 
 tensification." 
 
 In order to increase the contrast we must, of course, in- 
 crease all the separate steps of density occurring in the nega- 
 tive, and not only must we increase them but the increase 
 must be proportional to the steps already existing; that is 
 to say, we must multiply them all by the same amount if we 
 are to retain correct gradation. Fig. 97 shows a number of 
 different steps of density before and after intensification, 
 all the densities having been multitiplied by the same 
 amount or increased in the same proportion. 
 
 In order to produce this increase of density we must 
 either deposit some other material on the silver, so as to 
 add something to the image or we must change the color of 
 the image so as to make it more non-actinic and capable of 
 stopping more of the light which affects the printing paper. 
 103 

 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 There are many different substances which can be de- 
 posited upon the image. If, for instance, a negative is 
 treated with a silvering solution suitably adjusted, 
 the silver will be deposited on the image and will increase 
 its density, but this is very difficult to do, and it is more 
 practical to intensify negatives by depositing, not silver, 
 but mercury upon them. 
 
 The Eastman Intensifier 
 is a solution containing mer- 
 cury which will be deposited 
 
 Original 
 
 added Ay intensification 
 
 Fig. 97. 
 Densities Added By Intensification. 
 
 upon the negative immersed in it, and since the deposition 
 is regular, it can be watched and the gain of density ob- 
 served so that the intensification can be stopped at the 
 right time. 
 
 While the mercury method is still the most popular for 
 intensifying negatives it has never been wholly satisfactory, 
 because mercury intensified negatives are apt to undergo 
 changes that affect their quality after a time. 
 
 \ Another method of intensifying a negative is to bleach 
 it in the Velox re-developer and then re-develop the 
 bleached image with the sulphide solution used for obtain- 
 ing sepia-toned prints. By this method the image is changed 
 from silver to silver sulphide, which has a brownish-yellow 
 color and is much more opaque to actinic light than the 
 original silver image, so that a negative treated in this 
 way will show much more contrast than before treatment. 
 This method has proved very satisfactory and it is believed 
 that re-developed negatives will prove as permanent as re- 
 developed Velox prints. 
 
 / It must be understood that intensification is only suitable 
 V for the increase of contrast, it cannot improve a negative 
 which is seriously under-exposed; no amount of intensi- 
 104
 
 THE FINISHING OF THE NEGATIVE 
 
 fication can introduce detail which is not present before the 
 intensification is commenced; but occasionally intensifica- 
 tion will enable us to adjust the scale of contrast of a nega- 
 tive so that better prints can be obtained than are possible 
 without the intensifying treatment.
 
 CHAPTER XI. 
 
 HALATION. 
 
 Sometimes in a photograph there appears to be a blur- 
 ring of the bright parts over the dark parts of the picture, 
 and if lamps or other very bright lights are included they 
 
 may appear in the print 
 as bright spots sur- 
 rounded by a dark ring 
 beyond which is an- 
 other bright ring. This 
 curious effect, which is 
 called "halation" is well 
 illustrated in the photo- 
 graph shown in Fig. 
 98. 
 
 Halation is caused 
 by light which passes 
 completely through the 
 emulsion and also 
 through the glass on 
 
 which the emulsion is coated and is then reflected back 
 into the emulsion from the back of the glass. The sim- 
 plest form of such reflection is shown by the diagram, 
 Fig. 99, where we see a ray of light falling on the 
 emulsion at A. Most of this light is absorbed by the emul- 
 sion but some of it passes through to the glass and is reflect- 
 ed from the back of the glass, so that it reaches th e 
 emulsion again at B. 
 
 But this simple dia- 
 gram does not account 
 for the appearance of 
 the lights in Fig. 98, 
 because if a ray of light 
 had fallen on the plate 
 squarely at right angles 
 and had passed through 
 the emulsion at right 
 angles it would be re- 
 
 Fig. 98. 
 Halation in Print. 
 
 Fig. 99. 
 Simplest Form of Reflection. 
 
 fleeted straight back and the halation would not be spread 
 
 beyond the image, whereas, the halation is just as bad in 
 
 the center of the picture where the light fell squarely on the 
 
 1 06
 
 HALATION 
 
 Grains 
 
 in 
 Emulsion 
 
 Glass 
 
 Fig. 100. 
 Scattered Reflections. 
 
 emulsion as at the edges. Also, it does not account for the 
 
 ring which is shown around the lights. 
 
 As a matter of fact, light falling on a photographic plate 
 
 does not go straight through in this simple way. When 
 
 a narrow ray of light 
 falls on the grains of 
 silver bromide it is re- 
 flected from them and 
 scattered about. 
 
 Silver Bromide So we must imagine 
 that if we could ex- 
 amine a magnified sec- 
 tion through the plate, 
 we should see the light 
 falling on the emulsion 
 scattered in all direc- 
 tions, so that a narrow 
 beam of light is spread 
 out into a kind of blur, 
 the size of the blurring 
 being very minute but 
 still appreciable, Fig. 
 101 ; this effect of the 
 
 light spreading in the film is called irradiation. 
 
 We see then that the light which passes through the 
 
 emulsion of the photographic plate is traveling in all 
 
 directions, whatever may have been its direction before 
 
 it reached the emul- , 
 
 sion, and if we follow 
 
 the light into the glass, 
 
 we shall find that most 
 
 of the rays pass out of 
 
 the glass again into the 
 
 air but that some of 
 
 them are reflected back 
 
 into the emulsion. 
 
 In order to under- 
 stand this we must look 
 
 at the way in which 
 
 different rays of light 
 
 travel through glass. 
 
 (See chapter III.) 
 
 When a ray of light 
 
 passes from air into a 
 
 block of glass, it is bent 
 
 107 
 
 Glass 
 
 Fig. 101. 
 Irradiation.
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 Front 'Surface of 
 
 Back Svrfciceof Glass 
 
 by the glass which is a 
 medium of different 
 density, and when it 
 leaves the glass again 
 it is bent back so as to 
 travel along a path 
 parallel to that along 
 F - 102 which it entered the 
 
 Rays in a Block of Glass. glass, but if a ray leav- 
 
 ing the glass meets the 
 
 surface at too big an angle, it cannot go out and it will 
 
 be totally reflected back again. See Fig. 102. It is these 
 
 totally reflected rays 
 
 which produce the ring 
 
 of halation. 
 
 When the image of 
 the lamp falls on the 
 emulsion and enters it, 
 the rays are spread out 
 by irradiation, so that 
 we get a small spot at 
 the center of the lamp, 
 then this scattered light 
 passes into the glass of 
 the plate, and the rays 
 which are near the cen- 
 ter pass out into the air from the glass and we get a dark 
 ring, but when suddenly the angle of the rays to the surface 
 of the glass gets too big to get out they are reflected back 
 and produce a sharp ring of halation around the center of 
 the image, and then as they go farther and farther from 
 
 the image the light gets 
 weaker and the hala- 
 tion fades away again. 
 Thus we can account 
 completely for the rings 
 of light shown in the 
 picture. 
 
 If we coat the back 
 of the glass with some 
 substance into which 
 the rays would pass di- 
 10 4. rectly from the glass 
 
 Complete Diagram of Halation. and which would com- 
 
 108 
 
 Fig. 103. 
 Rays in Photographic Plate. 
 
 i
 
 HALATION 
 
 pletely absorb them, we should wholly prevent the hala- 
 tion and if we choose this "backing", as it is called, so that 
 it is of the right kind and almost completely absorbs the 
 
 light, allowing very lit- 
 tle of it to be reflected, 
 then it will be quite 
 effective in reducing 
 halation, but in prac- 
 tice it is not altogether 
 easy to get a satisfac- 
 tory backing and to 
 apply it correctly. The 
 photographer tried a 
 "backed" plate, but al- 
 
 Fig. 105. 
 Print from Backed Plate Negative. 
 
 though he got rid of the 
 sharp rings of halation 
 his lights are still ob- 
 
 scured by irregular blotches of light reflected from the 
 
 back of the glass. (Fig. 105). 
 
 The best way of avoiding halation is not to have any 
 
 glass at all. If we take the photograph on film, the sup- 
 
 port is sothin that the light has very little room to spread 
 
 and we get only a very small spreading of the light rays. 
 
 This spreading in fact is no greater than that necessary 
 
 to give a correct representation of the effect of the 
 
 light on the eye since 
 
 there really is a spread- 
 
 ing of the light in the 
 
 eye and we do not ac- 
 
 tually see a bright light 
 
 on a dark night as per- 
 
 fectly sharp, but as 
 
 having a small amount 
 
 of blur around it. So 
 
 that in Fig. 106, which 
 
 was taken on Kodak 
 
 film, we get a result 
 
 which gives a very good 
 
 idea of the scene as it 
 
 appeared. 
 
 Print From 
 
 Film . 
 
 109
 
 CHAPTER XII. 
 
 ORTHOCHROMATIC PHOTOGRAPHY. 
 
 IF we take a piece of blue cloth and put an orange on it 
 and then photograph the combination we shall find that 
 instead of the orange being lighter than the cloth, as it looks 
 to the eye, the photograph (Fig. 107) shows it as being dark- 
 er. This difficulty in photographing colored objects so that 
 they appear in the print in their correct tone values, as they 
 are seen by the eye, has been well known to photographers 
 from the earliest days of the art. 
 
 In order to understand the cause of it we must consider 
 the nature of color itself. When we speak of a colored object 
 
 
 Fig. 107. 
 Picture of an Orange on Blue Cloth.
 
 ORTHOCHROMATIC PHOTOGRAPHY 
 
 we mean one which produces a distinct sensation, which we 
 call the sensation of color. This, of course, is due to a 
 change in the nature of the light which enters the eye and 
 causes the sensation of sight, and this change is produced 
 in the light by the colored object so that the light after 
 reflection from the colored object is different in composition 
 from the beam of light before reflection. 
 
 In Chapter II we have seen that light consists of waves, 
 and that these waves are of various lengths, the color of 
 the light depending upon the wavelength. 
 
 BLUE VI 
 
 OLET 
 
 BL 
 CB 
 
 uc 
 
 CM 
 
 GRI 
 
 EN 
 
 OB4 
 YtL 
 
 net 
 
 jMI 
 
 RED 
 
 400 450 500 550 600 700 
 
 Fig. 108. 
 Divisions of Spectrum. 
 
 In white light there are waves of all lengths and if white 
 light is passed through a spectroscope it is spread out into 
 a band of various colors which is called a spectrum. The 
 various colors of the spectrum correspond to definite 
 lengths of light waves and if we measure their length in 
 the very small units which are used for measuring waves 
 of light we shall find that the red waves are 700 millionths 
 of a millimeter, the yellow ones are 600, the green 550, the 
 blue-green 500, the blue 450, and the violet waves, the 
 shortest which we can see, are 400 millionths of a milli- 
 meter long (Fig. 108). Thus, we can scale the spectrum 
 by the length of the light waves of which it is composed 
 (Fig. 109). 
 
 Fig. 109. 
 
 Pink filter passing violet, blue, yellow, orange and red rays but 
 absorbing green. 
 
 If we take a piece of colored glass or gelatine, say pink 
 gelatine, and hold it in front of the spectrum, we shall find 
 that the pink gelatine will not let some of the waves of 
 in
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 light through; it will stop them completely, while it will 
 let the other waves through without any difficulty. The 
 pink gelatine, in fact, cuts out or absorbs the green light 
 (Fig. 109). This is because of its pinkness; that is, it has 
 the property of absorbing green light from the white light 
 and of letting through the other light which is not green, 
 that is to say, to a less degree this pink film sorts out the 
 light just as the spectroscope does, but instead of separating 
 the waves of different lengths it stops some of them and 
 lets the others go on, and the eye, missing those which are 
 stopped, records the absence as a sensation of colour. 
 
 If, instead of having a transparent substance like film, 
 we have an opaque colored object, like a sheet of orange 
 paper, and let the spectrum fall on it, we shall find that the 
 
 
 Fig. 110. 
 
 Purple filter passing violet and red, but absorbing the blue, green, 
 yellow and orange. 
 
 orange paper will reflect the red and yellow and green light 
 but will refuse to reflect the blue light; it absorbs it, and its 
 orangeness is due to the fact that it absorbs the blue light 
 and refuses to reflect it. All objects which are colored 
 are colored because they have some selective absorption for 
 some of the waves of light; they do not treat them all alike 
 but reflect some and absorb others, and the modified light 
 which reaches the eye we call "color." Any object which 
 treats all the waves of light alike, which absorbs them all 
 or absorbs them equally or reflects them all in equal pro- 
 portion, is not colored. If it absorbs them all it will be 
 
 Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of 
 Ultra-Violet Visibility Green Green Red Visibility 
 
 Fig. 111. 
 112
 
 ORTHOCHROMATIC PHOTOGRAPHY 
 
 dead black since it will reflect no light. If it absorbs them 
 to a small extent, but equally, it will be gray; if it reflects 
 them all it will be white, but if it absorbs some of the wave 
 lengths and not others, it will be colored. 
 
 If we try a series of experiments in our spectrum we shall 
 find that things which absorb red light are colored blue, 
 and those which absorb green light are colored pink or 
 magenta, or if they absorb a great deal of the light, purple 
 (Fig. 110). Those that absorb blue-green light are orange, 
 and those that absorb blue-violet light are yellow. We see, 
 then, that to each color there corresponds a region of the 
 spectrum which is absorbed. 
 
 If we look at a spectrum we shall see that the brightest 
 part of it is the yellow-green and yellow (the position of 
 the yellow in the spectrum being between the yellow-green 
 and the orange) so that the eye is most sensitive to the 
 yellow, yellow-green and red rays and least sensitive to the 
 blue and violet rays. (Fig. 111.) But if, instead of looking 
 at the spectrum, we use a piece of bromide paper so that the 
 
 Invisible Limit of Violet Blue Blue- Green Yellmv- Orange Red Deep- Limit of 
 Ultra-Violet Visibility ' Grew Green J^^l/-^/^ Red Visibility 
 
 light of the spectrum may fall on it, and then make a posi- 
 tive print from this negative image, we shall find that the 
 photographic action on the print is not produced in the 
 region that is bright to the eye, but in the region which the 
 eye can scarcely see, and, indeed, there is a strong action 
 in the part of the spectrum beyond the visible spectrum, 
 showing that there are waves which are shorter than the 
 violet waves, which were discovered when the spectrum 
 was first photographed and are called the ultra-violet waves. 
 (Fig. 112.) This explains at once why when we photograph- 
 ed an orange on a blue cloth the orange was dark in the 
 photograph and the blue cloth was bright, which is the op- 
 posite to the way they appear to the eye. The bright orange 
 absorbs the blue light to which the film is sensitive and the
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 tjlue cloth reflects it, so that although the cloth looks dark 
 to the eye, ft TsTBriglft in the photograph, and the orange 
 which reflects very little blue and violet light is dark in the 
 photograph. Fortunately, this defect, for defect it is, of 
 photographic materials can be remedied to a considerable 
 extent. 
 
 If dyes are incorporated with the emulsion the dyes sensi- 
 tize the emulsion for the part of the spectrum which they 
 absorb, so that if we put a pink dye of the right kind in the 
 
 Invisible Limit of Violet Blue Blue- Green Yellow- Orange Red Deep- Limit of 
 Ultra-Violet Visibility Green Green Red Visibility 
 
 Fig. 113. 
 
 emulsion the film will not only be sensitive to the blue light, 
 to which it is naturally sensitive, but will also become 
 sensitive to the yellow-green light, which the pink dye 
 absorbs, and if we take a photograph of the spectrum on 
 this sensitized film we shall get a photograph which appears 
 as is shown in Fig. 113. FJilm made sensitive in this way is 
 called orthochromatic, and in photographing colored objects 
 the use of an orthochromatic film is a great advantage. 
 
 The orthochromatic film is still not sensitive to red, which 
 to the eye is a bright color, and so red objects are still ren- 
 dered too dark in a photograph, but this is not a great dis- 
 advantage for most work, and we have the very great... 
 advantage that the film can be developed in a red light. 
 
 Emulsions can be treated in such a way as to make them 
 panchromatic, that is, sensitive to all colors, but such pan- 
 chromatic materials cannot be handled by the light of an 
 ordinary dark room lamp; they have to be used either in 
 total darkness or by means of a very faint green light. For 
 amateur photography it is therefore better to be content 
 with orthochromatic film unless special subjects are to be 
 photographed. A full account of photography with pan- 
 chromatic plates and of the use of light filters in general 
 is given in "The Photography of Colored Objects," pub- 
 lished by the Company. 
 
 114
 
 ORTHOCHROMATIC PHOTOGRAPHY 
 
 Fig. 1 14. 
 Made Through a Yellow Light Filter. 
 
 Great care is taken to make Kodak NC film as ortho- 
 chromatic as will confer satisfactory color sensitiveness upon 
 it without sensitizing it so far that it will be difficult for the 
 user to handle or that there will be danger of fog when 
 developing it. 
 
 While the sensitizing with dye makes the film sensitive ' 
 to the yellow and green light, it is still much more sensitive 
 to the blue and violet waves, as is shown in Fig. 113, and 
 consequently it will still photograph blue objects much 
 lighter than they appear to the eye. This is a disadvantage 
 in some photography, and especially fri landscape photo- 
 graphy where we have blue sky with white clouds. Whitei 
 clouds are much brighter to the eye than the blue sky, but] 
 if they are photographed on the film in the ordinary way! 
 the blue sky appears too light and the clouds are lost against 
 it. In order to overcome this and to enable orthochromatic 
 film fcT represent most of the colors in their correct tone 
 values light filters are used which absorb the excess of blue 
 light and prevent it from reaching the film. 
 
 These light filters are, of course, yellow in color, since 
 yellow absorbs blue light and thus, by the use of yellow 
 light filters, which are sometimes called color screens, the 
 excess of blue light can be absorbed and a much improved 
 rendering of sky and clouds can be obtained. (Fig. 114.) 
 
 When light filters were first introduced it was thought 
 that any yellow glass would be satisfactory, and light filters 
 
 115
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 were made of brownish yellow glass, which really are of no 
 advantage at all. The reason for this is that they transmit 
 the ultra-violet light, which lies out in the spectrum beyond 
 the violet. This ultra-violet light is quite invisible, but pro- 
 duces a strong impression upon the photographic plate, and 
 in^ordeii_tQ get satisfactory action from a filter it is very im- 
 portant to remove the ultra-violet light as completely as 
 possible. The ultra-violet light is far more easily scattered 
 Bytraccs of mist in the atmosphere than visible light, and 
 since it is this mist which so often makes objects in the dis- 
 tance invisible in photographs that are taken without a 
 filter (Fig. 117a) it is necessary to use a filter that will cut^ 
 out this ultra-violet light in order to show the distance 
 welt. (Fig. I17b.) 
 
 / Modern light filters are made by dyeing gelatine with 
 [carefully chosen dyes and then cementing the dyed gelatine 
 'between optically prepared glasses. 
 
 Some yellow dyes, while removing violet light quite sat- 
 isfactorily, transmit a great deal of the ultra-violet light 
 and only a few dyes cut out the invisible ultra-violet satis- 
 factorily. One of the best of these dyes is the dye used in 
 
 INVISIBLE ULTRA-VIOLET BLUE GREEN RED 
 
 LIMIT OF 
 VISIBILITY 
 
 Fig. 115 
 
 Photograph of the spectrum, through two yellow Filters, which are of almost 
 
 the same color to the eye, showing (A) that the K Filter cuts out 
 
 the ultra-violet, while (B) the other Filter does not. 
 
 the Wratten K filters and the Kodak Color Filters. In 
 Fig. 115 are shown two photographs of the spectrum the 
 one taken through a filter made with a dye of a type often 
 used for filters, but not cutting out the ultra-violet, and the 
 other the same spectrum taken through a K filter. 
 
 The K filters were made with a dye produced in Germany, 
 and during the war the requirements of the aerial photo- 
 graphers in the army made it necessary to prepare a new 
 dye which could be made in America and which would cut 
 the mist even more sharply than the K filters. This pre- 
 sented a problem which was solved in the Kodak Research 
 116
 
 ORTHOCHROMATIC PHOTOGRAPHY 
 
 Laboratory by the discovery of an entirely new dye which 
 was named "Eastman Yellow," with which special filters 
 are prepared for aerial photography. 
 
 Since a yellow light filter removes the ultra-violet and 
 much of the blue- violet light, it necessarily increases the 
 exposure, because if we remove those rays to which the\ 
 film is most sensitive, we must compensate for it by expos- i 
 ing the film for a longer time to the action of the remaining 
 rays, and the amount of this increased exposure will be de- 
 pendent both on the proportion of the violet and the blue 
 rays which are removed by the filter and also on the sensi- 
 tiveness of the film for the remaining rays (green, orange 
 and red) which are not removed by the filter. 
 
 The number of times by which the exposure must be in- 
 creased for a given filter with a given film is called the 
 ^LuItiplying factor" of the filter, and since_ the factor de- 
 pends both upon the depth of the filter and upon the color 
 sensitiveness of the film, it is meaningless to refer to filters 
 as "three times" or ^'sTx times" filters without specifying 
 with what material they are to be used. 
 
 It is always desirable that we should be able to give as 
 short an exposure as possible; what is required in a filter is 
 that it should produce the greatest possible effect with the 
 least possible increase of exposure, so that a filter will be 
 considered most efficient when it produces the maximum 
 result with the minimum multiplying factor. To a certain 
 extent the multiplying factor depends upon the result that 
 is wanted ; thus in order to get exactly the same proportional 
 exposure when using a Kodak Color Filter with Kodak 
 NC Film, as that obtained without it the necessary in- 
 crease of exposure is ten times, 
 but in fact the Color Filter is 
 generally used for distant land- 
 scapes where haze is to be cut 
 out, and for clouds against the 
 sky, and under such conditions 
 an increase of -three times the 
 normal exposure that would be 
 correct for an ordinary land- 
 scape will give the most satis- 
 factory results. 
 
 For many purposes, however, 
 Fig. 116. the Kodak Color Filter is too 
 
 Kodak Sky Filter. strong; the exposure when using 
 
 it is so prolonged that it is not 
 117
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 b 
 
 Fig. 117. 
 
 a. Made without a filter. 
 
 b. Made with a Wratten G filter. 
 
 practical to use the Kodak without a tripod, and to meet 
 these difficulties the Kodak Sky Filter has been intro- 
 duced. (Fig. 116.) 
 
 / In this filter only half the gelatine, which is cemented 
 between the glasses, is stained with the yellow dye, the 
 
 118 

 
 ORTHOCHROMATIC PHOTOGRAPHY 
 
 other half being clear, and the filter is placed on the lens 
 with its stained half on top so that the light from the sky 
 will pass through the stained half and the light from the 
 landscape through the clear half of the filter. In this way 
 the yellow dye reduces the density of the sky in the nega- 
 tive without greatly affecting the exposure of the fore- 
 ground and enables us to get a rendering of clouds in a 
 blue sky by cutting out a part of the very strong light that 
 comes from the sky, while the exposure necessary is in- 
 creased only to a small extent. 
 
 The .sky filter is not suitable for the cutting of haze since 
 its Colored half does not cover the landcsape, which is the 
 part of the field where the haze occurs. Its use is confined 
 to that suggested by its name. 
 
 When it is desired to make blue photograph somewhat 
 darker than can be done with the Kodak Color Filter the 
 Wratten K2 should be used, and for recording still more 
 contrast, which is sometimes wanted in pictures of extremely 
 distant landscapes that are under haze, the Wratten G 
 filter is very valuable. Thus, distant mountains and all 
 other distant landscape scenes (Figs. 117a and 117b) may 
 be photographed through a strong yellow filter by giving 
 the necessary increase of exposure, with a Kodak mounted 
 on a tripod. The K2 will require an increase of exposure of 
 about twenty times and the G of one hundred times on the 
 Kodak Film. 
 
 Fig. 118. 
 
 a. Original Definition. 
 
 b. Definition after screwing up tightly in cell. 
 
 In order that filters may not spoil the definition it is im- 
 portant that the glasses between which they are cemented 
 119
 
 FUNDAMENTALS OF PHOTOGRAPHY 
 
 should be of good optical quality. This is very carefully 
 controlled in the case of the Kodak and Wratten niters, 
 which are all measured by an instrument specially built for 
 the detection of optical errors introduced by filters. The 
 filters have to be mounted in the cells so that they cannot be 
 strained by pressure being put upon them, since if they are 
 squeezed the balsam with which they are cemented to- 
 gether will be displaced and the definition will be spoiled. 
 (Fig. 118.) 
 
 Filters should be treated with care equal to that accorded 
 to lenses.. When not in use they should be kept in their 
 cases and on no account allowed to get damp or dirty. 
 With reasonable care in handling they should never become 
 so dirty as to require other cleaning than can be given by 
 breathing upon them and polishing with a clean, soft piece 
 of linen or cotton cloth. A filter should never be allowed to 
 become wet under any circumstances, because if water 
 comes into contact with the gelatine at the edges of the 
 filters it will cause the gelatine to swell and so separate the 
 glasses, causing air to run in between it and the glass. 
 
 The dyes used for the Kodak and Wratten filters are 
 quite stable to light, and no fear of fading need be felt. 
 The filters, however, should be kept in their cases when not 
 in use in order to protect them. 
 
 FINIS 
 
 2160 
 
 120
 
 UNIVERSITY OF CALIFORNIA LIBRARY 
 
 Los Angeles 
 is DUE on the last date stamped below. 
 
 SEPl5f972 
 
 SFP 
 
 04 198? 
 
 Form L9-Series 444
 
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