CHAPTER XVII
THE DISTRIBUTION OF LIGHT, SHADE AND COLOR IN THE AERIAL VIEW

The general appearance of the earth as viewed from above has already been described and illustrated (Figs. 10 and 11). It remains to deal with the earth's appearance in a more analytic and quantitative manner, in order to decide upon the characteristics to be sought in our photographic sensitive materials.

Range of Brightness.—The absence of great contrasts so apparent in the view of the earth from a plane is confirmed by photometric observations. These show that the average landscape, as seen from the air, rarely presents a range of brightness of more than seven to one, even when seen without the presence of veiling haze. It is to be remembered that shadows constitute no important part of the aerial landscape. Vertical walls in shadow, which form a substantial part of the surfaces seen by an observer on the ground, are invisible or greatly foreshortened from the air. Moreover, they are never contrasted against the sky, which is photographically often the brightest part of the ordinary picture. To the aviator's eye shadows on the ground are only of any length at early and late daylight hours. Even at these times they cover but a small area, since the number of high vertically projecting objects in a representative landscape is small. Lacking shadows, the brightness range is only that between various kinds of earth, water, and vegetation. Chalk (from freshly dug trenches), reflected sunlight from water, or marble buildings, furnish almost the only extensions to the brightness scale as above given.

Diurnal and seasonal changes. During the winter months on the Western Front photography from the air was only possible for two or three hours around noon, on clear days. This calls attention to another factor of prime importance, namely, the large variation in the intensity of daylight during the course of the day and during the course of the year.

Measurements showing typical variations from morning to night are exhibited in Fig. 101, from which it appears that there is an increase in illumination of four to five times from 8 o'clock—when it would be considered full daylight for purely visual observation—until noon, while there is a corresponding decrease by four o'clock. Fig. 102 shows sets of measurements by two different authorities which give the average intensity of daylight for each month throughout the year. From December to July there is an increase of approximately ten times. From both sets of data it therefore appears that—neglecting the frequent occurrence of clouds which reduce the illumination to a half or a quarter or even less—a variation in illumination of forty or fifty times occurs between mid-day in summer and morning in winter. In the photography of stationary objects on the ground this range of intensities is easily taken care of by selection of lens stop and shutter speed. On the airplane it is quite otherwise, because the shutter speeds called for at the lower illuminations are much slower than the motion of the plane will allow.

Haze.—At low altitudes the brightness range is substantially that which would be obtained by photometric measurements of soil and vegetation made at the earth's surface. At higher altitudes, especially above 2000 meters, this brightness range is materially decreased by atmospheric haze. The significance of this lies in the fact that for safety from anti-aircraft guns, war-time aerial photography must be carried out at very great elevations. Toward the end of the Great War photographic missions traveling at from 5000 to 7000 meters were the rule. At these heights, even in very clear weather, a veil of bluish-white haze reduces the already small contrasts still more. Some means for overcoming the effect of this haze becomes imperative, therefore, in order to secure in the picture even the normal contrast of the object.

Haze is to be sharply distinguished from clouds or fog. Clouds and fog consist of globules of water vapor of large size, opaque to light. Haze, on the contrary, is more opaque to some colors than to others, or is selective in its veiling effect. Its scattering action on light is greatest in the violet and blue of the spectrum, decreasing rapidly through the green, yellow, and red, the exact relation being that the scattering is inversely as the fourth power of the wave-length. It is, consequently, possible to pierce or cut haze by using yellow, orange, or red color screens. It is this possibility which has led to the extensive use of yellow or orange goggles for shooting and for naval lookout work. In aerial photography the equivalent is to be found in color filters, used with color sensitive (orthochromatic or panchromatic) plates, which have been found essential for all high altitude work.

Color.—Visual observation from the airplane is aided in no inconsiderable degree by the differences of color that exist between various objects of nearly the same brightness. This means of distinguishing differences of character fails in the photographic plate, which is color-blind; that is, it reproduces all objects as grays of varying brightness. It is color-blind in another sense as well, in that it evaluates colors as to brightness differently from the way the eye does, overrating blues and violets and underrating yellows and reds. This first kind of color-blindness is a positive disadvantage, for it leaves available for differentiating objects only their brightness differences. The second kind of color-blindness may on occasion actually be an advantage. For it may happen by accident, or by design (through the skilful use of color filters), that objects appearing nearly the same to the eye appear different in the plate. More will be said about this in connection with the use of filters for the detection of camouflage.

The range of hues seen in the aerial landscape is not large. Greens (grass and foliage) predominate, followed by browns (earth), neither color being bright or saturated. Over towns or cities we find that grays (roads) and redder browns (brick) are conspicuous. Blues are practically never seen, although it is to be noted that a fair share of the illumination of the ground is by blue sky light and that the haze itself is bluish. Consequently, the general tone of a landscape is much bluer than one would be apt to imagine it from consideration of the general green and brown character of the constituent objects. A color photograph from the air would greatly resemble a pastel in its low range of tones and the absence of bright colors.

The Photographic Requirements Dictated by Brightness and Color Considerations.—Considering only the demands made by the character of the view presented to the airplane camera, and leaving out of account other limitations to photographic operations in the plane, certain requirements as to sensitized materials may be outlined. First of all, the photographic process must not reduce, but should rather be capable of exaggerating, the range of brightness of the object. Preferably the seven-to-one range of the object photographed should be lengthened out to the full range of the printing paper, which may be two to three times this. With such an increase of range, those minute differences of brightness are accentuated, on which the detection of many objects depends.

Next, the plate or film must be sensitive to the portion of the spectrum transmitted by a yellow or orange filter which will cut out the effect of haze. This calls for orthochromatic or panchromatic plates, depending on the depth of filter required. Next, if the objects to be photographed differ little in brightness but are different in color composition, we may have to rely on color filters of peculiar transmissions, capable of translating these color differences into brightness differences. These will, in general, call for fully color sensitive, or panchromatic plates.

In conclusion it may be pointed out that the endeavor in ordinary orthochromatic photography—to reproduce the visual brightness of colors in the photographic print—has no real justification in aerial work. Neither in respect to color values nor in respect to brightness range is it the object of aerial photography, especially for war purposes, to present a truthful tone reproduction. Its aim is rather the adequate differentiation of detail, by whatever means necessary.

The purely photographic problem in aerial photography, as distinct from the instrumental one, is the selection of photo sensitive materials which will yield useful results under the conditions peculiar to exposure from the air. After such materials have been found by extensive field tests, it is preeminently desirable to determine their characteristics in such terms that the kind of plate or film may thereafter be specified and selected on the basis of purely laboratory tests. Specification must be made in terms of the ordinary sensitometric constants of the photographic emulsion—its speed, contrast, fog, development factor, its color sensitiveness, its ability to render fine detail, and its grosser physical properties such as hardness and shrinkage.

Sensitometry.—The most generally used system of sensitometry is that of Hurter and Driffield, commonly referred to as the “H & D.” By this system, in order to determine the characteristics of a given photographic plate, it is necessary to take a series of graduated exposures, a standard illumination of the plate being varied in known amount by a rapidly rotating disc cut to a series of different openings, or by some other suitable means. The negative thus obtained is developed in a standard developer for a definite time, at a fixed temperature, and is then measured for transmission on a photometer. The following terms are defined and used in plotting the results:

Hurter and Driffield pointed out that a negative would give a true representation of the differences in the light and shade of the object if it reproduced these differences by equivalent differences in opacity. This is equivalent to stating that if the densities are plotted against the logarithms of the corresponding exposures, a straight line should be obtained at 45 degrees to the axis of exposure times. If the line is at another angle the opacities of the negative will be proportional to the brightness of the object photographed, but the contrast will be different.

A typical H & D plot is shown in Fig. 103. It will be noted that two curves are shown. These are obtained with different developments, and illustrate the fact that the contrast or proportionality between exposure differences and opacity differences is a matter of time of development. Each of these curves exhibits certain characteristics which are common to all made in this way. There is primarily a straight line portion, where opacities are proportional to illumination. This is commonly called the region of correct exposure. The slope of this straight line portion—the ratio of density
log exposure—is the development factor, commonly denoted by “?,” a gamma of unity denoting exact tone rendering. Below the region of correct exposure is a “toe,” or region of smaller contrast, called the region of under exposure. Above the correct exposure region is another where the opacity approaches constancy (afterwards decreasing or “reversing”), called the region of over exposure.

The speed of a plate on the H & D scale is given by the intersection of the straight line portion of the characteristic curve when produced, with the exposure axis. This intersection point, called the inertia, is the same irrespective of the time of development, as is shown in Fig. 103. The numerical value of the speed is obtained by dividing 34 by the inertia, when the exposure is plotted in candle-meter-seconds.

If a plate is developed until no more density and contrast can be obtained, its development factor is then ?₈, (gamma infinity), and the larger this is the more a plate can be forced in development. If the plate fogs in its unexposed portions this fog is measured and recorded in density units along with the other constants. The speed of development is represented by the velocity constant, commonly symbolized by ?.

The length of the straight line portion determines the latitude of the plate, or the range of permissible exposures to secure a “perfect negative.” Thus if we assume that an object has a range of brightness of 1 to 30, then a plate with a straight line characteristic extending over a range of 1 to 120 would have a latitude of 120
30 or 4. That is, the exposure could be as much as four times the necessary one, and still give the same result on a sufficiently exposed print. If the latitude of the plate is too small, the shadows will fall in the under exposure region, the high-lights in the over exposure portion of the characteristic curve, with consequent poor rendering of contrasts.

Criteria of Speed.—In airplane photography speed is of paramount importance, but great care must be exercised to insure that all the factors are considered which can contribute toward yielding the desirable pictorial quality in the brief exposure which alone is possible from the moving plane. A “fast” plate on the H & D scale is not necessarily suitable for aerial work, when we remember that accentuation of natural contrast is desirable, particularly under hazy conditions. For, as is shown in Fig. 104, it is a common characteristic of “fast” plates to have comparatively small latitude and low contrast at their maximum development.

It is to be noted that the Hurter and Driffield measure of speed is bound up with the idea of correct tone rendering and with the use of the straight line portion of the characteristic curve. Other criteria of speed exist. For instance, the exposure necessary to produce a just noticeable action (threshold value); and the exposure necessary to give a chosen useful density in the high-lights when development is pushed to the limit set by the growth of fog.

As has already been pointed out, correct tone rendering is not necessary or even indicated as desirable in aerial views. It is, moreover, a matter of experience that the majority of aerial exposures with existing plates fall in the “under exposure” period, where contrasts with normal development are less than in the subject. This being the case, the problem is to select not necessarily a fast plate, by the H & D criterion, but a plate which will develop up workable densities in the under exposure region. A plate of medium speed will sometimes develop to greater densities in the short exposure region, if development is forced, than will a fast plate. The contrast in the normal exposure region will be excessive, but this is of no significance if no exposure falling in this region is present on the plate.

In addition to its capacity for developing density, the plate should have as low a threshold as possible, thus meeting to some extent the requirements of both the alternative criteria of speed given above. At the same time it is true that low threshold and good density for short exposures are not to be found in really slow plates. Consequently, while high speed, as ordinarily understood, is undoubtedly the first requirement, we may expect the complete specification for the best aerial plate to be a rather complicated thing, describing the characteristics of a workable “toe” of the curve, in terms of which several (e.g., contrast and speed) are derived from another and quite different exposure region.

Effect of Temperature on Plate Speed.—It has been found by Abney and Dewar that very low temperatures materially decrease the speed of photographic emulsions. This decrease may amount to as much as 50 per cent. in the temperature range from 30 degrees Centigrade above zero to 30 degrees below zero, which is the range over which aerial photographic operations will have to be carried on in war-time. This effect has not been at all fully studied, and it is not known whether it is general or only found in certain kinds of plates. The remedy indicated is to provide means for heating the plates or films when low temperatures are encountered. This is fairly easy in film cameras, or in plate cameras like the deRam, where the entire load of plates is carried in the camera body. Plates carried in magazines present a more difficult problem. The heating coil incorporated in the German cameras is perhaps partly for this purpose.

Color Sensitiveness.—Complete specifications for an aerial plate cannot be made solely on the basis of its speed, contrast, latitude, threshold, and other sensitometric values which have to do only with the intensity of the light acting on it. These in general apply to photography from low altitudes, where the illumination and natural contrast of the subject are the only factors to consider. When higher altitudes are reached the interposition of haze decreases the already deficient contrast, calling either for the development of more contrast in the plate, or for the use of color filters to cut out the action of the blue and violet light predominant in haze. Along the lines discussed in the last section, it is not surprising to find that some plates are better than others for bringing out gradations masked by haze, even though no filters are used and though the plates are similar in color sensitiveness. But the limitations to securing contrast by manipulating the characteristic curve of the plate are soon reached, and it becomes necessary to resort to haze-piercing color filters, used with color sensitive plates.

Roughly, two general types of color sensitive emulsions may be distinguished: first, those in which sensitiveness to green and yellow is added to the natural blue sensitiveness, and second, those sensitive in a useful degree to all colors of the spectrum. The former are called iso- or ortho-chromatic, the latter panchromatic emulsions. Spectrograms exhibiting the distribution of sensitiveness throughout the spectrum for several representative plates are shown in Fig. 105. Orthochromatic plates are adequate for use with light yellow filters and have the slight practical working advantage that they can be handled by red light. Panchromatic plates are necessary for use with dark orange or red filters. They must be handled in total darkness or in an exceedingly faint blue-green light, taking advantage of the common drop in sensibility in that region of the spectrum. Plates can, indeed, be sensitized for the red alone, leaving a gap of almost complete insensibility in the green, as shown in the fourth spectrogram of Fig. 105. When used with a yellow filter these plates behave as do panchromatic plates with a red filter.

A rougher idea of color sensitiveness than is given by spectrograms is furnished by the tri-color ratio, which is the ratio of exposure times necessary with white light to give equal photographic action through a certain set of red, green and blue filters, expressed in terms of the blue exposure as unity. In an excellent panchromatic plate the three exposures would be equal. In an orthochromatic plate the red exposure will be too large to be figured. In interpreting either spectrograms or tri-color ratios care must be taken that the absolute exposures necessary are known. Thus a relatively high red sensitiveness may mean merely low absolute blue sensitiveness.

Two methods are used in imparting color sensitiveness. Either the sensitizing dye is incorporated in the plate emulsion before it is flowed; or the plate is bathed in a dye solution not long before using. The latter method gives higher color sensitiveness but poorer keeping quality, and is not a practical method for field operations. Greatly enhanced sensibility may be given by treatment with ammonia, but this again is a method for laboratory rather than field use.

Resolving Power.—A question which arises in connection with all photography of detail is the size of the grain of the photographic emulsion. Dependent on the size of the grain is the resolving power, or ability to separate images of closely adjacent objects. This varies with the speed, fast plates being of coarser grain than slow ones; with the exposure; and with the method and time of development. In general, it may be said that the resolving power of the plate does not enter practically into aerial work, because the resolving power of all plates so far found usable corresponds to a smaller distance than the size of a point image as limited by the performance of the camera lens and the speed of the plane. Remembering that ? mm. is a fair value for the size of a point image as rendered by the lens, the rÔle of plate-resolving power is shown by consideration of the following table. Resolving powers are given in terms of lines to the millimeter just separable.

Tabulation of Requirements for Aerial Emulsions.—In terms of the sensitometric quantities just discussed the general requirements for aerial plates may be listed as follows:

1. Speed. The speed usually connected with the contrast and density required for the exposure times available is about 150 H & D. Faster plates in general have too low contrast, but the highest speed that will give the necessary contrast is desired.

2. Contrast. The contrast capable of development without fog should be from 1.5 to 2. This contrast should be produced by light of daylight quality, and, in orthochromatic and panchromatic plates, with the yellow or orange filters intended to be used with them. This contrast means a gamma infinity approaching 2.5.

3. Speed of development. A gamma of nearly 2 should be developed in 2½ minutes at 20 degrees C. in the developers recommended below.

4. Fog. Not over .25 for this degree of development, and not over .40 for six minutes development.

5. Color sensitiveness. This should in general be as high as possible. In terms of certain representative filters (described in a subsequent chapter) color sensitiveness should be such that with the white light speed above specified the relative exposures through the filters shall not be greater than as follows:

Relative Behavior of Plates and Films.—The advantages of film from the standpoint of weight and bulk have been discussed in connection with aerial cameras. Were there no other considerations film would unquestionably be the most appropriate medium for aerial photography. There is, however, the question of ease of handling, to be treated in a subsequent chapter, and the question whether the purely photographic characteristics of film are satisfactory. Can the same speed, contrast, and color sensitiveness be obtained on film as on glass? Is the picture so obtained as permanent or reliable as the plate image?

It must be confessed that up to the present emulsions on film have not proved the equal of those on glass. It has been found by emulsion manufacturers that the same emulsion flowed on film and on glass gives better quality on the glass. Emulsions specially prepared for film fall somewhat short of the best plate emulsions. It has also been found harder to color-sensitize film, and to insure good keeping quality in the color sensitized product.

In addition to the question of photographic quality there arises the matter of shrinkage and distortion. These are negligible with plates, but are a more or less unknown quantity in film. Irregular shrinkages of as much as two per cent. are found on experiment. This defect, of course, would be an obstacle only in exact mapping work.

Positype Paper.—The need sometimes arises in military operations to secure prints ready for examination within a few minutes after the receipt of the negatives. Even the 15 or 20 minutes within which a negative can be developed and a wet print taken may be considered too long. While such occasions are probably more apt to occur in popular magazine stories than in actual warfare, it is important to have available methods of producing prints with an absolute minimum of delay. This need is met to some degree by a direct print process, commercially exploited under the name of “Positype.”

In this process the exposure is made directly on a sensitized paper or card, which is developed, the image dissolved out, the residue exposed, and again developed; thus furnishing a positive picture (reversed right and left). The time necessary to develop a print ready for examination need not be more than three minutes. Only a single print is available, but this is all that would be called for under the extreme conditions suggested. If later, copies are desired they may be made by the same process.

Plates and Films Found Satisfactory for Aerial Work.—The following plates and films have been found particularly good for aerial photography. The list is not intended to be complete. Furthermore, it may be expected to be soon superseded, as the efforts of various manufacturers are directed toward developing special aerial photographic plates.

Among orthochromatic plates: The Cramer Commercial Isonon, the Jougla Ortho.

Among panchromatic plates: The Ilford Special Panchromatic, the Cramer Spectrum Process.

Film: Ansco Speedex, Eastman Aero.

The Function of Filters in Aerial Photography.—The use of color screens or filters has been very common in ordinary landscape photography, for the purpose of securing approximately correct renderings of the brightnesses of colored objects. Plates of the non-color-sensitive type have their maximum of sensitiveness in the blue of the spectrum (Fig. 105) and in consequence blue skies photograph as white, while other colors are likewise reproduced on a totally wrong scale. Filters for correct brightness rendering are calculated for a given color sensitive plate so that the resultant reaction to the light of the spectrum copies the sensitiveness of the eye, which is greatest in the yellow-green. Such filters for use with the common orthochromatic plates are of a general yellow color.

Filters for aerial work are meant to serve quite a different purpose. Correct tone or color rendering is of quite secondary importance to another use of filters, namely, to cut or pierce aerial haze. It is quite a matter of accident that the same general color of filter is called for both to give correct color rendering and to pierce aerial haze, namely, yellow. Yet on closer analysis it is found that quite different types of yellow filter are demanded, spectroscopically considered.

Figure 106 (K₁ and K₂) shows the spectral transmission curves of the Wratten K₁ and K₂ filters, intended for correct color rendering with orthochromatic plates. The absorption increases gradually toward the blue. In the same figure is shown on an arbitrary scale the spectroscopic character of typical haze illumination, increasing in brightness inversely as the fourth power of the wave-length, that is, with great rapidity in the blue and violet. It is evident from this that a much more abrupt absorption than that of the K₁ or K₂ filter is desirable, because in the green of the spectrum the haze light is comparatively weak, and more will be lost by any absorption in this region through decreasing useful photographic action than will be gained by cutting out the haze. This latter consideration is important. The use of any filter means an increase of exposure; the use of yellow filters multiplies it several times. Careful experiment has shown that no filter of depth less than K 1½, to use the Wratten filters as a basis for discussion, are of real value in haze piercing. The filter ratio, or ratio of exposures with and without filter, is 4.7 for the K 1½ with the Cramer Isonon plate—a figure which shows the importance of securing the necessary haze-piercing character with the minimum absorption of useful photographic light.

Practical Filters.—Since the character of the absorption of the “K” filters is not all that could be desired, new filters, both of dyed gelatin and of glass, have been produced. The glass, a Corning product having a very sharp-cut absorption, has not yet been produced on a commercial scale with the high transparency in green, yellow and red that selected samples have shown. The United States Air Service has adopted filters of a new dye, called the EK, from the name of the company in whose laboratory it was produced. These filters are standardized in two depths of staining, called the “Aero No. 1” and “Aero No. 2.” Their spectral transmission curves appear in Fig. 106, along with those of certain darker filters useful only with panchromatic plates for exceptionally heavy haze. The characteristic of these Aero filters is their great transparency through all the spectrum except the blue, whereby the greatest haze-cutting action is attained together with a low filter factor. The filter factors of the Aero No. 1 and No. 2 with Cramer Isonon plates are 3 and 5, respectively.

Effects Secured by the Use of Filters.—The efficiency of yellow filters for haze-cutting is best shown by photographs taken at high altitudes with filters and without. Such illustrations are given in Figs. 107 and 108, where the first photograph is one taken at 10,000 feet without a filter, the second taken at the same altitude under the same conditions, but with an orange filter. Both are on panchromatic plates, and it will be seen that even with these plates the filter makes all the difference between a useless and a useful picture. But it must be clearly understood that the difference here lies between a plate sensitive chiefly in the blue and violet, and a plate affected only by the yellow, orange and red. The difference is not between what the eye sees and what a plate with a filter sees, as is sometimes supposed. As shown in Fig. 108, a filter enables the plate to photograph through the haze between clouds, but not through the clouds themselves. In general, no filter and plate combination which is feasible for aerial exposures is capable of showing more than the eye can see if yellow or orange goggles are worn. To do this it would be necessary for the photographic action to take place by deep red or infra-red light, which would demand exposures now out of the question.

Filters are almost always necessary in photographing from high altitudes or in making distant obliques. At times, particularly after a heavy rain, the air is clear enough so that filters may be dispensed with. Clearing weather was therefore chosen whenever possible for making obliques of the battle front.

Filters for the Photographic Detection of Camouflage.—In the photographic as in the visual detection of camouflage, the problem is to differentiate colors which ordinarily look alike, but which are actually of different color composition. Particularly important are the differences between natural foliage greens and the paints used to simulate them. If these differ in their reflection spectra, a proper choice of filter will show up the two greens as markedly different. Two kinds of difference may be produced; either the two colors may be changed in relative brightness, or they may be altered in hue. Thus foliage green, due to its possessing a reflection band in the red of the spectrum, which is absent in most pigments, may be made to appear red while the camouflage remains green or turns black. Filters which cause changes of color are of course of no use for photographic detection of camouflage, since the photographic image is colorless. Brightness differences are alone available.

Those same filters which have been worked out primarily for producing brightness differences in visual detection of camouflage could be used photographically, provided the plates employed were color sensitive, and were as well screened to imitate the sensibility of the eye. But the most useful visual filters are those causing color differences to appear; more than this, the visual camouflage detection filters as a class have low light transmissions, so that their usefulness in photography is doubtful. Little work has actually been done with camouflage detection filters for photography. Yet in spite of this photography has been of real service in this form of detective work. Its utility for the purpose comes from the fact that the natural sensitiveness of the plate to blue, violet and invisible ultra-violet acts to extend the range of the spectrum in which differences between identical and merely visually matched colors may be picked up. Consequently the plain unscreened plate has proved a very efficient camouflage detector—so efficient in fact that all camouflage materials have had to be subjected to a photographic test before acceptance. Fig. 171 shows how an ordinary photograph reveals the unnatural character of the camouflage over a battery.

Methods of Mounting and Using Filters.—The most primitive way of mounting a gelatin filter is to cut a disc from a sheet of dyed gelatin and insert it between the components of the lens. For this purpose the gelatin must be perfectly flat, which is insured by its method of preparation and test. One disadvantage of this method is that the filter can be inserted and removed only upon the ground. It is less satisfactory the larger the diameter of the lens, and the wastage of filters due to insertion and removal is apt to be high. The camera should be refocussed after filters of this kind are inserted.

Glass filters, ground optically true, or gelatin filters, mounted between optically flat glass plates, are the most convenient and satisfactory. They may be mounted in circular cells to screw or attach by bayonet catches to the front of the lens. Or they may be mounted in rectangular frames to slide into transverse grooves in the camera body. Fig. 44 shows the mount of this latter form adopted in the larger United States Air Service cameras. This is particularly convenient if it is desired to insert or change the filter while in the air—a practice not generally considered feasible in war work with the photographically inexperienced observer, but likely to be common with the employment of skilled photographers for peace-time aerial photography.

German cameras are reported in which the glass filter is carried behind the lens, on a lever which also carries a clear glass plate of the same thickness, to be thrown in when no filter is needed, thus maintaining the focus. The performance of the lens will be impaired by this scheme, unless it is specially calculated to offset the effect of the glass introduced in the path of the rays behind the lens—optically true glass has no effect on definition if placed in front of the lens. Glass filters may also be placed in close contact with the plate or film, in which case they must be much larger, but do not need to be of as good optical quality.

Self-screening Plates.—Mention must be made of a quite different mode of realizing the filter idea, a method available where the sensitive plate is always to be used with a filter. This is to incorporate a yellow dye in the gelatin of the plate itself. The dye must be one which has no direct chemical effect on the plate, but which acts simply as a coloring agent for the gelatin. “Self-screening” plates, as they are called, have been produced by the use of the dye called “filter yellow” and have found some use in orthochromatic photography. They effect a useful saving of light through the elimination of the reflection losses at the surfaces of glass and gelatin filters. The filtering action of the dye in the plate is somewhat different from its ordinary one, since the deeper portions of the sensitive film are subject to greater action than the surface, and this tends to diminish contrast.

The principal factors governing the length of exposure in the airplane camera have already been discussed under various headings. These are briefly, the nature of the aerial landscape, the practically attainable lens apertures, the form of the camera support, the speed of the plane, and the characteristics of plates, films and filters. It is convenient however, to re-assemble this information in one place, in such form as to apply to the practical problem of determining the exposure to be given in any specific case.

Limitations to Exposure.—In the ordinary photography of stationary objects, exposure is a variable entirely at the operator's command. Plates of any speed may be selected, so that attention may be focussed on latitude, color sensitiveness, and other tone rendering characteristics. The exposure may be made of a length sufficient to insure all the useful photographic action lying in the “correct exposure” portion of the sensitometric curve. The exposure ratio of any filter it is desired to use is a matter of indifference—its effect on color rendering need alone be considered.

Airplane photography is sharply distinguished from ground “still” photography by its severe limitations as to the amount of the exposure. The actual duration is definitely restricted by the high speed of the plane. In peace work this can be offset in part by using slower planes or by flying against the wind. The practical limitation to 1
100 second, set by war-time requirements as to definition of fine detail, may be increased to 1
50 of a second, or even more, where mapping of grosser features is the object. A common, but entirely avoidable limitation, is that due to vibration of the camera. By proper mounting this may be entirely overcome, leaving the ground speed of the plane the only source of exposure-limiting movement. The amount of light reaching the plate constitutes a primary factor in exposure, and this is a matter of lens aperture. Generally, lens aperture is smaller the larger the plate required to be covered, and the greater the focal length. Because of their larger aperture, the short-focus lenses which will be favored for peace-time large-area mapping will permit more and longer working days than have been the rule in long-focus war photography. The necessary use of filters, particularly at the high altitudes which would be chosen in mapping, in order to economize in the number of flights needed to cover a given area, introduces an inevitable decrease in the amount of light available at the plate, as compared with surface photography under the same illuminations.

Broadly speaking, it may be said that all the demands made in reference to aerial photographic exposure work are to decrease the amount of light reaching the plate. Any surplus offered, as by the midsummer noon-day sun, must be immediately snapped up, either by decreasing the exposure to get greater sharpness, or by introducing filters to get greater photographic contrast. The absolute exposure of the plate tends to be kept at the irreducible minimum. As already stated, it lies, with present photographic materials, on the “toe” of the “H & D” curve, just reaching up into the straight line portion.

Estimation of Exposure.—According to the foregoing argument the problem of estimating an aerial exposure resolves itself largely into one of deciding how short this may be made. Or, if the light is strong, whether it is sufficient so that a filter may be introduced without demanding more than the 1
100 second or thereabouts which is dictated by the motion of the plane.

Deciding upon exposures in the field has been largely a matter of experience and judgment. A majority of the cameras in use during the war were not furnished with shutters calibrated in definite speeds. Consequently, the sergeant upon whom the decision usually devolved became a storehouse of knowledge as to the slit widths and tensions appropriate to each individual camera. This knowledge had to be acquired from the results of actual photographic reconnaissances, or from special test flights, both of them wasteful methods. But the chief objection to this state of affairs lies in the fact that the knowledge thus acquired is of no use to anyone else, nor is it applicable to other types of camera.

The first essential to placing exposure estimation upon a sound basis is therefore an accurate knowledge of shutter performances. Either the shutter speeds should be placed upon the camera by the manufacturer and periodically checked, or a regular practice should be followed of calibrating shutters, either at a base laboratory or even in the field.

Assuming that the speeds of all shutters are accurately known, the process of estimating the requisite exposure becomes less a matter of mere guesswork and more nearly a matter of precision. For this purpose data on the variation of light intensity during the day and during the year (Figs. 101 and 102) should be taken as a guide. These data refer of course to visual and not to photographic light, but since it is always necessary to use color filters, which make the active light of approximately visual quality, this is no valid objection. The effects of clouds and mist must of course be learned largely by experience, but with the above daylight data at hand, anyone in possession of definite information on the correct exposure with a given plate for a known day and hour need not go far wrong in estimating exposures at any other time in definite fractions of a second.

Fig. 109.—Chart showing aerial exposures for all times of the day and year. Data on basis of F/5.6 lens, Jougla orthochromatic plate, and clear sunlight, no filter. Exposures to be doubled and tripled for overcast and cloudy weather.