EXPOSURE 316. Time of Exposure. For the majority of subjects there is no single correct time of exposure, but a more or less extended range of times each of which is equally admissible. The minimum exposure is that which will record the shadows as a faint density just distinguishable from the chemical fog, while the maximum exposure is that above which the density recorded in the lightest half-tones becomes indistinguishable from the density given by the perfectly white parts.
In the case of a very contrasty subject taken on an emulsion poor in silver salts, it is quite possible that no time of exposure will satisfy at the same time the two conditions stated above, but such a case is exceptionally rare. Iii such an extreme case (see Chapter III), which is always avoidable in technical photography, there is no necessity to expect the photographic emulsion to record variations in luminosity which are imperceptible to the eye (§ 17).
In all other cases, the range of satisfactory times of exposure (measured by the ratio of the maximum and minimum times) is known as the tolerance or latitude of exposure. This latitude becomes greater firstly with decreasing contrast in the subject to be represented, and, secondly, as the range over which the emulsion is capable of rendering intensities increasing in geometrical progression by densities in arithmetical pro gression is extended, or in other words, as the straight-line portion of the characteristic curve is extended (§§ 201 and 202, Fig. 139c). This condition is usually best fulfilled with emulsions very rich in silver salts. 2 The range of which can be ren dered by good emulsions often exceeds 180 : 1, whereas very few photographic subjects possess a contrast exceeding 30 : I It will be seen from these values that the second is contained six 18o times in the first ( 30 = 6), so that the maximum and minimum exposures are in the ratio of 6 to I.
Success in photography would be exceedingly rare if it were not for this latitude in exposure. Fortunately, exposures outside this range still give results which, although less correct, are nevertheless often quite satisfactory. If this
were not the case, the photography of rapidly moving objects would be practically impossible.
327. Results of Errors in Exposure. Negatives which have been given varying times of exposure within the limits defined in the previous para graph and then developed for the same time in the same bath will only differ from one another in their mean density, and not in contrast. They will give, therefore, identical prints on the same paper, but the printing will take longer in the case of the denser negatives. If, for example, one of them was exposed for five times as long as another, then, at a rough approximation, the first will take five times as long as the second one to print, or if equal times of exposure are given in the printing, the first negative will require five times the intensity of light. These two negatives, while very different in appearance when compared in the same illumination, would appear identical if they were illuminated respectively by a 5o and a 20 candle-power lamp. They would appear equal under the same illumination if the lesser exposed negative were combined with a lightly-smoked piece of glass or a uniformly-fogged and developed plate, transmitting about one-fifth of the incident light-intensity.
Of these two negatives, yielding identical results, the professional photographer will al ways consider the thinner one to be correctly exposed and the denser one over-exposed.. Retouching, a necessary operation in profes sional photography, is in fact much more easily done on a thin than on a dense negative. He u ould probably have stopped the development of the more exposed negative some time before the lesser-exposed one, in order to obtain greater transparency. The negative developed for the shorter time would then be much less contrasty and would no longer, therefore, yield on the same paper a print identical with that from the other negative, but each could he made to give excellent prints by the choice of suitable papers with characteristics appropriate to the separate requirements of the two negatives.