EFFECTS OF FOG.
Perhaps the most important considera tion in this connection is that of fog.
used to soften gradations and to strengthen detail, and as such is a valuable adjunct.
How FOG MAY BE UTILISED.
Suppose two plates have been exposed for precisely the same time, and are under exposed impressions—that is to say, the lesser intensities of light have not acted long enough on the plate. Let one be developed free from fog, either chemical or light, and the shadows will all photo graph alike, as the difference of deposit is too slight to be perceptible. Suppose now the other is allowed to receive a faint trace of white light, the result is that these densities are rendered apparent owing to the extra deposit added to the negative. Let the original gradations be represented There are two kinds of fog—(a) light fog, caused by the plate being impressed by some extraneous light or lights other than that passing through the lens or used to form the image ; and (b) chemical fog, which results from the use of a more energetic reducing agent than the sensi tive salts can stand, which induces a re duction of the silver, independent of any light impression, all over the plate equally. Such fog may also arise through errors in manufacture of the plate. Fog not only alters or destroys the gradations of the negative, but may be used to supply the necesary inertia for the production or an image. Generally speaking, fog is very
detrimental, but in certain cases it may be by the numbers 1, 2, 4, 8, 16, 32 (see Fig. 168) and a density equal to 1 he added to each, they then become 2, 3, 5, 9, 17, 33 (see Fig. 169), which shows that the total effect will be much more on the lesser densities than on the greater. The result is a softening or flattening of the picture. Whether such is or is not desirable will de pend on the subject in hand and the taste of the operator. As a further illustration, let Fig. 170 represent one of the two under exposed impressions of contrasts 1. 2, 4, S, 16, 32, already referred to. This, if measured up, will be found to be .0, 1, 3, 7, 15, 31. If now a fog density equal to 1 is added, the contrasts or gradations required —namely, 1, 2, 4, 8, 16, 32—will he ob tained, and the steps will now appear the same as Fig. 168. ',these must only be taken as imaginary, not as actual cases, the figures being used merely as an illustration, as it was found to be impossible to repro duce the actual effect by half-tone printing. It is in such circumstances that fog be comes a valuable aid, and completes the necessary inertia.