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 “γ,” 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 γ∞, (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 κ.
Fig. 103.—Typical characteristic curves of photographic plate.
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.
