This section is from the "Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917" book, by Aristo Motto. Also see Amazon: Studio Light Incorporating The Aristo Eagle - The Artura Bulletin 1917.
Now, if we join all these points together instead of representing them as a staircase effect, as is shown by dotted line in Fig. 4, we get a smooth curve, Fig. 5, of which the straight line portion (B to C) represents the period of correct exposure, while the more or less curved portions at the beginning and end of the curve correspond to the periods of under- and over-exposure.
It must be realized that no ordinary negative can show the whole range of exposures from beginning to end of this curve. The ten exposures given correspond to a range of light intensity of from 1 to 512, whereas ordinary subjects have contrasts of about 1 to 32, or six points on the curve.
Suppose we take a subject with a range of contrast of from 1 to 32, then if the exposure 1 is clear glass, the points 1, 2 and 4 will all be on the "under-exposure" part of the curve, only 8, 16 and 32 being on the straight line and the relative gradations of these points 1, 2 and 4 will be incorrect. See Fig. 6.
By giving more exposure, four times as much for example, we get the six points 1 to 32 represented by the points 4-8-16-32-64-128 on the curve. All but 4 and 128 are on the straight line portion and these are very nearly on it, so the reproduction will be almost perfect. See Fig. 7.
If more exposure is given more of the points will fall in the overexposed part of the curve and reproduction will again be less perfect.
A scale of tone-values will therefore be correctly represented in the negative in so far, and only in so far, as it falls within the straight line portion, the length of the straight line portion being technically called the "latitude" of the material.
This and the illustration on the following page show the portions of the curve occupied by the contrasts of an under-exposed and a correctly exposed negative of a subject with a range of contrast of from 1 to 32. The underexposure is flat because half of its contrasts are on the flat portion of the curve. The correct exposure gives correct reproduction because it is on the straight line of the curve.
Allowance must be made for the inaccuracy of our illustrations because of the inability of the half tone process to give an exact reproduction, also for the fact that we are using prints instead of negatives to illustrate our point. The illustrations show but roughly the difference in quality, which is really great when a subject with a long scale of contrast is reproduced on a long scale plate in comparison with a short scale plate.
The capacity of a given material to render a scale of tone-values correctly is therefore entirely a matter of the length of the straight line portion of the curve and it is in this respect that negative materials differ in quality. Plates or films made for portrait work are adjusted to have the longest possible straight line portion. The Seed Gilt Edge 30 Plate and Eastman Portrait Film excel particularly in this respect, the Seed 30 Plate having a range of 1 to 256 on its straight-line portion, so that the whole range of light intensities in the longest scale subject can be correctly translated into opacities in a negative made on this plate. Plates made for other purposes, such as commercial and process work, where great contrast is essential, do not have this very long scale straight-line portion to the curve, with its accompanying power of exactly reproducing long scales of gradation. We may say therefore that provided:
(1) our negative material is made so as to have a long straight line portion to its curve,
(2) we expose so as to place the scale of intensities of our subject on that straight line portion, then, we can correctly translate the tones of the subject into corresponding opacities in the negative. As we shall see in a later article, it is more difficult to translate correctly the opacities of the negative into the tones of the print.
Portrait by Benjamin Paris, France.
(To be continued.)