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at a density = 1.0 when exposed to an intensity-scale for 1/50 second. It will be noted that the curves are remarkably different, the Ilford film showing an optimum intensity at about log. 7=1.5 corresponding to a time of exposure of about 1/5 second for a density of 1.0, whereas the Eastman film shows an optimum intensity at about log. 7 = 0.5 corresponding to an exposure time of about 2 seconds for a density of 1 .0. Over the range of intensities corresponding to times of exposure of 1/50 second to 20 seconds for a density of 1.0 the Ilford film shows less reciprocity failure than the Eastman film, but its reciprocity failure becomes much greater than for the Eastman film for lower values of intensity.
Since the Eastman IIB sensitometer, when used for exposing positive film gives a log. 7 = 1.43, we should expect the two films to show greater differences in speed and contrast when exposed to a time-scale under these conditions than when exposed to an intensity-scale for a 1/50 second. This is illustrated in Figs. 6 and 7. When exposed to a time-scale under the conditions obtaining
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Fig. 6. Characteristic Curves of SBW and 1302. Time-scale at log I = 1.43 (//B Sensitometer).
Fig. 7. Characteristic Curves of SBW and 1302. Intensity-scale at I 50 sec. (Printer).
on the IIB sensitometer it will be seen that the Ilford film, in addition to being slower than 1302, has a lower gamma, whereas to an intensity-scale exposure at 1/50 second the two films have the same speed at a density of 1.0, the Ilford film having a slightly higher gamma. When positive film is exposed through a negative on a printer it is exposed to a range of intensities for a constant time of the order of 1/100 second, which is sufficiently close to a 1/50 second for the intensity-scale exposure at 1/50 second to be a close approximation to the characteristics of the films when exposed on a printer.
All photographic emulsions show reciprocity failure and the reciprocity characteristic varies greatly with different emulsions. The exposure intensity at which maximum efficiency occurs is usually about that value for which a medium density is produced in from 1/10 second to 10 seconds, but may be as short as 1/50 second for some fast negative emulsions.