Airplane Photography

Exposure data charts. Fig. 109 shows a chart, prepared in the French service, indicating aerial exposures for all hours of the day throughout the year. These are for clear sunlight, for a lens of aperture F/5.6 and for “ortho” plates without a filter. They are based on what is probably an over-estimate of the actual speeds given by the French shutters. For “light” clouds the exposures are to be doubled, for “heavy” clouds quadrupled, and for forests and dark ground “lengthened.” Charts of this form should be extremely useful, but they were actually not of great service because of the prevalent lack of knowledge of true shutter speeds.

Exposure meters. Aerial photography offers an excellent opportunity for the use of exposure meters, particularly those of the type in which a sensitive surface is exposed to the light for a measured time sufficient to darken a predetermined amount. The sensitive paper of the meter may either be exposed from the ground to the direct light of sun and sky, or from the plane to the light reflected from the ground. The first method will give figures subject to some correction for the character of the ground to be photographed—whether fields, forests, or snow. The second method is to be preferred where the shutter speed can be adjusted in the air, according to the indications of the meter, or where the filter can be selected and put in place during flight. Trials with a commercial Wynne exposure meter, used in the latter manner, give as a working figure an exposure of .001 second for each 4½ seconds taken to darken the sensitometer strip to match the darker comparison patch. This relation applies to a lens of aperture F/4.5, on Cramer Commercial Isonon plates without filter.

Skilled photographers can examine a negative and can interpret its renderings with practically as much satisfaction as they get from a print, whereby a considerable amount of time can be saved in an emergency. The original glass negative should always be used when accurate measurements are to be made. These and a few other cases constitute the only use of a negative apart from its normal one, namely, for producing positive prints, usually in large numbers. The commonest form of print is on paper, although the most satisfactory print from the photographic standpoint is the transparency on glass or celluloid film.

Transparencies.—Transparencies are made by the regular photographic processes of exposure and development, on glass plates or films placed in contact with the negative, or in the appropriate position in an enlarging camera. The sensitometry and the terms used to describe the qualities of plate or film for this purpose are those already given in connection with the general discussion of plates and films. But the kind of emulsion to be selected is quite different from the aerial negative emulsion. There is here no practical limitation to the speed, contrast or latitude. Consequently, we can choose a positive emulsion on which the exposure through the aerial negative falls entirely on the straight line portion of the characteristic curve, thus reproducing all of its tones, and the contrast of the negative may be increased to any desired extent. The possibilities of positive emulsion are indeed rather greater than the usual aerial negative can utilize. A range of clearly graduated opacities of two or three hundred to one is possible, so that not only can detail be well rendered in the high-lights, but also equally well in dark shadows where, indeed, an increase of illumination is necessary for it to be made easy to examine. This range is to be contrasted with the 1-to-7 range in the aerial landscape, which may be doubled by a contrasty plate. In resolving power, the positive emulsion, which is slow, exceeds the negative emulsion. It easily bears examination through a magnifying glass, thus making any enlargement unnecessary in the printing process.

Glass transparencies are of course impractical for general distribution, on account of their fragility. Heavy film transparencies are not open to this objection, and, especially in the form of stereos, constitute the most beautiful form of aerial photographic print.

Paper Prints.—Prints on paper suffer by comparison with transparencies, in the range of tones which they exhibit. This lies between the white of the paper, which never has more than 80 per cent. reflecting power, and its darkest black, which differs with the kind of paper. In dull or mat papers the blacks will reflect as much as 5 per cent.; in glossy papers, ordinarily used for aerial negatives, the reflection from the black may be as low as one per cent., but in order to get the benefit of this the paper must be so held as not to reflect any bright object to the eyes. This deficiency in the range of paper gradations is not so serious with aerial negatives as with ordinary properly exposed negatives because of the small range of brightness in the aerial view.

The sensitometry of papers is similar to that of plates, with the difference that reflecting powers take the place of transparency. As in the case of transparency emulsions there is in papers no dominating requirement for extreme speed, to which other characteristics must be subordinated. Yet speed is of sufficient importance in handling large quantities of prints so that aerial negative printing for military purposes has been done almost entirely on the rapid enlarging papers, rather than on the true contact printing papers, which are slower.

The two principal types of rapid enlarging papers, the bromide and the “gas light,” exhibit certain characteristic differences which are important to bear in mind in seeking to obtain any particular quality of print. Bromide papers, of which “Nikko” is a good example, show sensitometric curves rather like those of plates. That is, they increase in contrast with continued development. At the same time, as is shown in Fig. 110, they increase somewhat in speed with development; that is, under exposure can be compensated for to a small degree by protracted development. These characteristics of bromide paper can be utilized to secure prints of a quality quite different from that of the negative. Thus, if the negative has a long range of tones, a flat print can be secured by full exposure and short development. If, as is apt to be the case with aerial negatives, a print of greater contrast than the negative is desired, a short exposure with long development is called for.

The sensitometric curves of a typical gas light paper “Contrast Enlarging Cyco,” are shown in Fig. 111. Here the contrast is a fixed characteristic of the paper, and the only effect of changing development is on the speed; that is, exposure and development are, within limits, interchangeable.

Choosing a printing paper is a matter of deciding on the contrast required for the class of negative, and selecting a paper which will give this contrast with a good range of tones from a clear white to a deep black. The ideal paper would be one which was all straight line in the H & D plot. In such a paper there would occur no loss of contrast in the lighter tones when the high-lights were rendered by the clear white of the paper. Too great contrast with a short straight line portion, results in loss of detail at the ends of the scale. A negative possessing a very great range of tones cannot be correctly represented on one paper print—two printings are required, one for high-lights and one for shadows, but this difficulty is rarely to be faced in aerial views. The greatest demand for aerial printing papers has been for those of considerable contrast, because of the flat character of the negatives.

General Considerations.—Developing, fixing and other chemicals for aerial work differ in no essential respect from those used in ordinary photography. Full discussions of these are to be found in numerous texts and articles. The aerial photographic problem is to select those most suited for the under-exposed flat negatives characteristic of photographs from the air. At the same time selection from among the chemicals of appropriate quality must be governed by considerations of the conditions surrounding work in aerial photographic laboratories. These laboratories, especially in war-time, are apt to be most primitive in their facilities.

Characteristics of Developers for Plates and Films.—From the standpoint of practicability, aerial negative developers should have good keeping power, be slow to exhaust, and work well over a considerable range of temperatures. From the standpoint of the photographic quality desired in the negative, the developer should bring up the maximum amount of under-exposed detail. This means that it should impart the highest possible speed to the plate, with good contrast, and low fog or general reduction of unexposed silver bromide.

There are many characteristics to study in a developer: its effect on inertia or speed, gamma infinity, fog, time of appearance, “Watkins factor,” speed of development, temperature coefficient, dilution coefficient, keeping power, exhaustion, length of rinsing, stain, color coefficient and resolving power. These are defined and described as follows:

Effect on inertia. The meaning of inertia has already been given under the discussion of plate speed. While this is a constant, independent of time of development, for any one developer, it is altered appreciably by change of the latter.

Time-gamma relation. Contrast, symbolized by γ, has likewise been discussed under plate sensitometry. Viewed from the standpoint of the developer, the point of interest is the rate at which γ varies with development, and the maximum contrast which can be reached or γ infinity. Speed of development is commonly defined by the velocity constant, symbolized by κ, which is arrived at mathematically from a consideration of the time of development to produce two different contrast values. High γ infinity is desired for aerial negatives, and for rapid work κ must also be high.

Fog. The opacity due to chemical fog is to be kept at a minimum in aerial negatives, as it is chiefly prejudicial to under exposures.

Time of appearance and Watkins factor. The time of appearance is measured in seconds. The Watkins factor is a practical measure of the speed of development, and is determined by the ratio of the time of development required for a definite contrast, to the time of appearance. It is useful also as a guide to development time.

Temperature coefficient. This is the factor by which the time of development at normal temperature (20 Cent.) must be increased or decreased in order to obtain the same quality negative, for a change of seven degrees either side of normal.

Temperature limits are the temperatures between which development can be carried out with any degree of control or without serious damage to the negative. These factors are of great importance where climatic or seasonal changes have to be endured.

Dilution coefficient. This is the factor by which the development time is increased in order to maintain a given quality negative in different dilutions of the developer. It is useful in tank development.

Keeping power. The keeping power of a developer, mixed ready for use, is determined by its ability to resist aerial oxidation. A developer of poor keeping power, which must be made up immediately before use, causes delay and waste of time whenever emergency work has to be done, whereas a developer of good keeping power may be left in its tank ready for instant use.

Exhaustion of a developer is the rate at which it becomes useless for developing, due both to aerial oxidation and to the using up of its reducing power by the work done in developing plates. It is conveniently measured by the area of plate surface developable before the solution must be renewed.

Length of rinsing. The time required for rinsing between development and fixing bath plays a not unimportant part in total development time. Dichroic fog is caused with some developers if, due to insufficient rinsing, any of the caustic alkali is carried over to the fixing bath. Stains develop also if the fixing bath is old, or if light falls on the unfixed plate while any developer remains in the film.

Color coefficient. The function of the sulphite, which forms a constituent of all developing solutions, is two-fold. It acts partly as a preservative, and partly to prevent the occurrence of a yellow color in the deposit. The yellow color, if present, increases the photographic contrast. This phenomenon has been purposely utilized, particularly in the British service, to give “stain” to negatives which otherwise would show insufficient printing density. The color index or coefficient of a negative (with a given printing medium) is the ratio of photographic to visual density. If we take a pyro developer containing five parts of pyro per thousand and ten parts of sodium carbonate, and then vary the amount of sulphite from none to fifty parts per thousand, the color index varies as follows:

The color index is somewhat different with various kinds of printing media.

This staining effect is a variable one, depending upon length of development, dilution of the developer, length of rinsing, temperature, the fixing bath used (plain hypo being necessary for a maximum effect), the length of washing after fixation and the properties of the water used. Standardization of these conditions in the field is difficult; hence any developer which will give the same effective contrast without resorting to stain is to be preferred.

Resolving power. Some developing processes and conditions will introduce bad grain into the negative. Hence the resolving power which a developer brings up must be investigated among its other characteristics.

Practical Developers for Aerial Negatives.—In the English service a pyro metol developer was generally used, producing stained negatives. The French, American and Italian practice was to use metol-hydrochinon, without staining. A special chlor-hydrochinon developer, worked out by the Eastman Research Laboratory for the United States Air Service, has probably the greatest merit of any yet tried. A comparison, given below, between it and a pyro metol formula used on a representative plate, illustrates the use of the various bases of study given above.

Owing to the difficulty of securing pure chlor-hydrochinon a metol hydrochinon of very similar properties has been worked out. Its composition is

To keep the ingredients in solution in cold weather, 50 c.c. of alcohol should be included in every litre of solution. All things considered this is probably the most practical and satisfactory developer for aerial negatives.

Developers for Papers.—The following formula has been found very satisfactory for papers:

Fixing Baths.—For plates the following fixing and hardening bath is recommended:

During hot weather, the above quantities of chrome alum and bisulphite are doubled.

For papers the following:

The acid hardener is constituted as follows:

Intensification and Reduction.—These processes have been little employed in air work. Reduction is rarely necessary, for obvious reasons. Intensification would often be of value, but the common practice, which saves some time, is to use printing paper of strong contrast for those negatives which are deficient in density and contrast. When intensification is desirable or permissible, either the ordinary mercury or uranium intensifier may be used.

Water.—In the field it is found necessary in many cases to purify the water that is to be used in mixing up chemicals. Water may contain suspended matter or dirt, dissolved salts, and slime. It is important to remove the suspended matter, as it may cause spots on the plates and papers, while any slime would coagulate, forming a sludge in the developer which would also tend to settle on the plates and cause marks during development. The dissolved salts may or may not cause trouble. Two methods of purification are possible:

(a) Filter the water through a cloth into a barrel, add about one gram of alum for every four litres of water, and allow to settle over night. Draw off the clear liquid from a plug in the side as required.

(b) Boil the water and allow it to cool over night. If the water contains dissolved lime, boiling will often cause this to come out of solution.