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Fig. 9 — Brightness Ratio Curves for Lenses at Various Apertures.
between the visual brightness of an object and the photographic brightness of an image for that object formed on the plate by the optical system which must be employed in the taking of motion pictures, it is possible now to point out definitely a method by which the exposure required under a given system of conditions may be computed, or, on the other hand, if the exposure time is fixed, it is possible to compute the required illumination. One of the factors which must be considered in this is the speed or sensitivity of the photographic material being used.
In Fig. 10 is given a typical characteristic curve for cine negative film, the abscissae values being in terms of log exposure.
In Fig. 11 is given a similar curve for cine panchromatic negative films. These two materials represent almost completely the types of photographic materials used for making the negative in motion picture work. Before considering further the method of computing either exposure time or the requisite illumination as may be desired, it may be well, for the sake of clearness, to tabulate the factors which must be considered. These factors are as follows and for the sake of convenience are designated by the symbols as indicated.
Bo (min.) = the brightness of the object in the deepest shadcw which it is desired to reproduce.
D (min.) = minimum density desired in the negative.
Log E (min.) = exposure indicated by the characteristic curve of the material which corresponds to Dmin.
t = exposure time.
Wv = Relative photic efficiency of illuminant used.
Nx = (min.) = the illumination on the negative material at the point corresponding to the object brightness Bo (min.)
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