THE RELATION OF ACTINIC GRADATION TO THE TONE STEPS OF THE PICTURE It is necessary now to illustrate the effect of the different actinic planes of a subject on the opacity steps of the negative as brought about by different degrees of exposure. Let a, b, c, d, and e (fig. 8) represent the different areas or parts of a subject, A, which confronts a lens, L, and a', b', c', d', and e' the corresponding image planes on the plate A' on which the sub ject is supposed to be photographed. Suppose the planes a, b, c, d, and e of the subject have, as marked in the figure, 16, 8, 4, 2, and 1 actinos of intensity respectively. Suppose also that the plate used has a latitude of 1 to 16 and will therefore, with normal exposure, reproduce this subject with full tone gradation. In this ex periment it is irrelevant whether or not there results a so-called technically perfect printing negative as described in chapter VIII, as this depends purely on the latitude, i.e., hardness or softness of the medium to be employed in printing the picture from the negative and a correct negative for any medium is obtained simply by developing its most opaque part of the proper density as compared to its least opaque part. What is being studied here is of an entirely different nature and refers solely to the latent chemical preparation of the emulsion to develop into a negative with a predetermined and desired gradation. If the latent image be prepared in an emulsion as just explained it is impossible as well as unnecessary to alter the relation of its density steps (see chapter VIII). As explained, such a latent image may be made into a correct negative for any printing medium by the correct degree of development or after manipulation.
With normal exposure to this subject under the conditions mentioned, the area of greatest intensity would be made to overcome the in ertia of the plate 16 times and the planes b, c, d, and e would do so 8, 4, 2, and 1. times respect ively. Even the area e, of least intensity, by overcoming the inertia once, would be detailed and would be represented by the next to the deepest tone in the subject, since it would still be higher than that of the adjacent background which will be considered as being without actinicity or in darkness. What is to be ob served here is that all the six areas of different intensities (including the background) are re produced and well differentiated in the result ing negative and will be seen in well separated tone steps in the finished photograph, provided of course the latitude of the printing medium and the contrast of the negative agree with each other.
It will now be shown how under-normal ex posure (see chapter VIII) prepares the way for a greater contrast gradient in a negative and in its resulting picture. Since in the foregoing normal exposure the plane e only overcame the inertia of the emulsion at e' once, it is evident that on giving a half normal exposure this plane would be excluded from appearing in the neg ative, by normal development, while the plane d would overcome the inertia but once and would therefore take the lowest tone step above the darker and undetailed backgrounds. The
plane a would create an 8 inertia exposure at a'. Including e' with the background as the lowest tone there are now but five tone steps instead of six in the picture. Now by slightly longer development or by intensification this plate may be carried to the same opacity at a' as the previous one had and the extreme light and dark tones would be the same in each pic ture since the lowest tone is so-called "clear glass" or simply the unexposed and undeveloped gelatin emulsion fixed out. But since there is one less tone step in the picture these steps must of necessity be farther apart than in the previous negative. Again, should the exposure be but i normal so that the plane a would over come the inertia at a' but 4 times, b and c would do so but twice and once respectively and the planes d and e would both disappear into the background and be excluded from appearing in the picture, since the exposure has not been sufficient to allow the intensity of either of them to overcome the inertia of the emulsion even once. This 1 normal exposure, by longer de velopment or intensification, may still enable the emulsion at a' to be carried to the same opacity as in the first negative but its resulting picture would show but 4 tone steps, i.e., those of a, b, and c and the clear gelatin. If the highest and lowest areas are made to have the same opacity contrast as previously, then it is clear that the resulting tone steps must be still farther apart than in the last negative, there being one less step. Continuing the experiment, an exposure of 2 inertias at a' would include only the plane b with it in the negative as those of c, d and e would drop in with the background. This ex posure however would be difficult to develop or intensify to the same opacity as the first one, although it might be carried to sufficient opacity to produce a print on a contrasty printing medium. There would be but 3 tone steps in the resulting picture.
It is seen therefore that too little exposure fails to furnish sufficient chemical base, so to speak, for the building up of the desired opacity and that when carried to extremes it defeats the very end in view. The average emulsion with a latitude of 1 or 16 or 1 to 32 requires at least a 4 inertia exposure to provide sufficient base upon which to procure a practical degree of opacity. On a hard emulsion, one whose latitude is say 1 to 8, a 2 or 4 inertia exposure will produce sufficient chemical change to make a negative of considerable opacity. Such an exposure should not be given except in the case of a subject having a minimum or very little actinic contrast such as will now be explained.