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films seem to build up to a definite maximum density and then stop, no matter how greatly exposure increases.
The II. and D. C urve
The H. & 1). curve which owes its name to two English experimenters. Hurter and Driffield, is simply a picture of this. If you plot this relation as a curve, with increases in exposure indicated by the distance to the right of a common starting-point, and increases in density as the elevation of the curve above that starting-point, you will find, in the low-exposurelow-density region, that your line curves upward very slowly. In the normal region, wdiere exposure and densitx increase about proportionally, you will have a practically straight, upward-slanting line. In the extreme high-exposure-maximum-density r e gion. your curve will flatten off, moving to the right, to indicate increased exposure, but not climbing much, since there is little or no increase in density.
Technically, the bottom of this curve is naturally called the “toe,” and the flattened top. the “shoulder.” The slanting middle portion is logically called the “straight-line portion”.
Now if wre plot these curves for tw'o tvpes of film, one very contrasty, the other very flat, we’ll get, in one case, a line that slants up at a very sharp angle, and in the other, one that slants at a much flatter angle. In the same way, hard and soft development of the same film, giving contrasty or soft results, will give us similiarly steep or flat inclines. We can quite accurately compare the contrast of the results by mentioning the angle of these slants. That, though expressed as the result of a more in
volved mathematical formula, is the simple meaning of “Gamma.”
G amnia
I he now familiar sensitometric or “gamma" strips are the means by which we get the facts for plotting these curves. One end of the strip gets very little exposure; the other end. an extremely high exposure. The rest of the strip gets varied intermediate exposures, ranging by progressive and accurately known steps from high to low. So the strip itself is a graduated range of densities from virtually clear film at one end to virtually opaque film at the other.
Clearly, if we cut down the total exposure of the strip, several of the light-exposure gradations are going to be underexposed, and will merge into each other, while the high-exposure end will fall short of getting full exposure. If. on the other hand, we give too much exposure, the normally light end of the strip is going to pick up more exposure than it should, while several of the steps at the top end of the stri p will all be overexposed to the maximum the film permits, and will crowd together in a single, heavy density.
What we’re doing is simply this: in a normally exposed strip, we’re using not oidy the full length of the strip and the full gradational scale of the film, b u t we re using the whole straight-line portion of the film’s curve.
If we underexpose were lowering the gradational scale into the toe of the curve; if we overexpose, we’re ignoring the toe, losing much of the lower straight-line portion, and causing the gradational scale to be crowded
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