In our last chapter we stated that most of the advantages formerly attributed to development in the open dish were now proved to be illusory. For a clear understanding of the system which is taking its place with a considerable body of photographers, we shall have to recapitulate to some extent.
The key-note for a successful negative is correct exposure, and the accompaniment must be correct development. The question will then arise, What is correct exposure, and what is correct development? Correct exposure is that which will produce the rendering of the subject that the photographer requires; and the correct development, that which will produce the amount of contrast required by the printing process to be used. Theoretically there is only one correct exposure, but in practice there is a large amount of latitude, or, in other words, the falsification of the tones, which is the result of incorrect exposure, is so slight that it is negligible so long as the variation of exposure is kept within the latitude of the plate or film; and it is by this means that photographers can produce negatives and prints to suit their requirements.
There is no difficulty in obtaining both correct exposure and correct development if the photographer works systematically. Both can be easily calculated. The principal thing necessary is to understand the action of the exposure on the sensitive plate, so as to fully realise the method of developing by time, which is practically developing by calculation. To obtain an absolutely correct rendering of any subject the exposure must be exact, because any other exposure will render the half-tones false. But there is a large amount of latitude before the false tones produced by incorrect exposure become discernible. This latitude is much less on the under side of correct exposure than on the over side - many films and plates to-day allow a latitude of about one-half on the under side, and four times on the over side.
The simplest way of studying the effect of exposure on the half-tones is to work with one half-tone, a shadow, and a high light as follows:
Fig. 38. S = Shadow, H = Half-tone, L = High-light.
The half-tone is exactly half-way between the shadow and high light; that is, the reflective power of the half-tone is to be double that of the shadow and half that of the high light.
If these were exposed on three separate plates in the same ratio as the reflective power, and developed together in the same developer for the same length of time, the result would be three identical negatives, as the difference in the reflective power in each tone would be equalised by the different exposures. On the other hand, if these three tones were exposed on one plate with a single exposure, and that was correct, we should get three tones in exactly the same ratio as the original, whether the plate was developed for either a flat or a contrasty negative. That is, if the density of each tone was measured, the half-tone would be in the centre of the other two, irrespective of whether the shadow and high light were very close together or very wide apart.
Figs. 39-41 represent a section of a developed plate of the three tones. The deposit would of course be considerably greater on L than on S. Fig. 39 is a section of a correctly exposed plate, giving the steps between each tone exactly the same as in the original. The dotted line touching the point of each tone, being straight, denotes that the exposure had been correct.
Fig. 40 is a section of an underexposed plate, developed so that the shadow and high light are the same as in the correctly exposed plate. Now it will be seen that the half-tone has not got the same value, and that instead of being in the centre it is much nearer to the shadow; and the more considerable the under-exposure the nearer will it approach to the shadow. A line drawn touching the points of each tone will no longer be straight, but concave - a concave line represents under-exposure.
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 representation 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 negative and positive effect at the same time, which is contrary 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?
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.