Fig. 41 is an over-exposed plate with the shadow and high light developed to the same degree as in the previous cases. And now we find that the half-tone is much nearer to the high light. The line becomes convex, a sure representa tion of over-exposure.
When the wrong exposure has placed the half-tone out of its true position it is impossible to put it back with the developer. An incorrect exposure is always an incorrect exposure, however the development has been carried out. The idea that any developer can develop up the shadows and at the same time hold back the high light is a fallacy. To do this the developer would have to produce a nega tive and positive effect at the same time, which is con trary to all chemical laws. Photographers have been taught, and the impression is deeply rooted, that incorrect exposures can be corrected by the developer, whereas the only thing that can be done is to develop for more or less contrast. It is impossible, however, to divert either the concave or convex lines of incorrect exposure into the straight line of correct exposure.
When we come to the conclusion that it is impossible to correct one half-tone in a picture to any great extent, what is the good of trying to correct the numerous half-tones in our negatives ? Contrast.—The only control the photographer has in the development of negatives is the amount of contrast ; and the easiest way to exercise this is simply by altering the time of development—the longer the development the greater the contrast.
Fig. 42 represents the section of a correctly exposed plate, with short, medium, and long development. The difference in the lines A B C, is the shallowness or steepness of the contrast obtained by the varying time of development. The negative having had correct exposure the half-tone is in the centre in each case, although the outer tones vary considerably ; A, B, and C might all he good negatives for different purposes.
A.—A soft negative with little contrast, suitable for enlargements on bromide paper of a large size. B.—A good all-round negative.
C.—A negative with more contrast, suitable for carbon printing.
Altering the strength or the composition of the developer is equivalent to altering the time of development, which would produce exactly the same effect.
The development of either an under-exposure (Fig. 43) or an over-exposure (Fig. 44) proceeds in exactly the same way as a correct exposure ; that is, each tone will develop in the ratio decided by the exposure. If development is continued long enough to double the deposit on the shadow, the deposit on the high light and the half-tone will be doubled also.
Fig. 43 shows three stages in development of an under exposed plate : A, development for a short time; B, the develop ment continued until the deposit in the shadow has been doubled ; C, continued development until the deposit on B has been doubled ; and in each case the deposit on the high light and half-tone has also been doubled. The exposure having placed the half-tone much nearer to the shadow, it remains in that position, whether the negative is flat as in A, or contrasty as in c.
Fig. 44 shows the same three stages of development in an over-exposed plate ; only that in this case the half-tone has been placed by the exposure nearer to the high light. We get practically the same result in both cases, with the exception that the displacement of the half-tone is towards the shadow with under-exposure and towards the high light with over-exposure.
In Figs. 43 and 44 the development has been carried on so that the density of the shadow and high light may correspond to those of correct exposure (Fig. 42) ; but it will be seen that in both cases the half-tone is untruthfully rendered. And when the half-tone has been placed in this false position it remains there in whatever way it may have been developed. The only control the photographer has is to decide how far he will carry development, and by this means he can determine the amount of contrast obtainable in the negative. This degree of contrast should be regulated according to the medium to be used for printing ; some processes require a soft negative, while others require one with more contrast.