For a negative to be capable of giving good results, all that is necessary is that the duration of development to be included within two wide limits, which limits further vary with the range of light-intensities in the subject photographed. All that is then necessary, once the negative has been completed, is to select an exactly suitable mode of printing.
The amateur, who through laziness confines himself to one method of printing, e.g. contact printing on a certain type of print-out paper, or the professional portraitist, limited to a certain style of picture, and therefore compelled to select one of a few methods of printing, or, finally, the cinematographer, who, owing to a curious anomaly, has at his disposal usually only one type of positive film (positive film by the various makers having nearly the same characteristics)-all these workers are compelled to produce in all cases negatives the extreme opacities of which are, if not exactly in the same proportion (a condition difficult to satisfy),' still in proportions little different from one another, or, what comes to the same thing, negatives having extreme densities differing by nearly the same amount. This condition is all the more difficult to fulfil, because the eye is a very bad judge in this quality, and so the operator is often a long way off the mark, unless the subjects photographed are very similar in character, as is the case particularly in studio portraiture.
Plates or films exposed through the back have their development somewhat slowed in the shadows, if exposure has been short ; with a normal exposure no difference is seen between plates exposed the right way or the wrong way round.
373. In addition to the above circumstances, the duration of development (by which a nega tive of a given subject obtains the desired degree of contrast) depends independently on the following: I. The negative emulsion used. Equivalent times of development may vary as from i to 8 according to the type of negative emulsion. Ultra-rapid plates generally develop more slowly than slow ones, and for an equal degree of sensitivity films develop more slowly than plates. The duration of development may vary as from i to 3 with successive batches of a given brand of plate or tilm, and there is no means of knowing these variations in advance.
2. The composition of the developing solution, its dilution, and its temperature. While it is fairly easy to keep the temperature of a develop ing bath almost constant,' the composition of the solution is less under control (except in commercial establishments where the products used are carefully titrated) because of the varying content of active substance in each of the products employed. Variations of even greater magnitude result from the progressive exhaustion of the developing solution and its grow ng strength in bromide, when a consider able number of plates or films are developed in the same portion of solution.
3. The agitation of the bath (§ 343) In all cases where it is desired to obtain uniformity of development of several negatives developed for the same duration, the agitation of the bath under identical conditions must be seen to.
It will be noticed that the time of exposure is not mentioned among the factors influencing the duration of development. Unless very considerable error exists, the time of exposure does not in fact influence the contrasts of the image to a sufficient degree to enable it to be allowed for before too late. In the case of a very considerable error in the time of exposure, it is the composition of the developing solution that must be modified, and there is then no comparison possible between the durations of development in baths of different compositions.
In general, at least for exposures differing little from the normal, the duration of exposure affects chiefly the density of the image in the shadows, whereas the duration of development affects chiefly the difference between the shadow and high-light densities, or, in other terms, the ratio of the corresponding opacities.
374. Judging the End of Development. There is no sure means for gauging the moment when a negative in course of development reaches the degree of contrast requisite for a given purpose. At most it is possible, by using as a basis previous experience acquired under exactly the same working condit'ons, to arrange a dura tion of development giving an image with the same characters as one previously obtained.