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.
CHAPTER XXII
PHOTOGRAPHIC CHEMICALS
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.