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EXTERIOR LIGHTING
scales are attached for very short or very long exposures. The principle of construction is that the logarithms of the times of exposure are proportional to the thickness of the colored prisms. "G. Heyde's ActinoPhotometer" is on a somewhat similar principal, and consists of a circular metal box with dark violet glass viewing screens in the center of both sides, with obscuring iris inside the case worked by revolving the back of the box. On the front of the instrument exposure tables are given for plates of every rapidity, and for diaphragm apertures from f/3 to f/45. Exposure meters of this type are specially applicable for openair work where there is sufficient light for ready measurement.
Practically all of the commercially sold meters give the exposure in a manner suitable for still camera work, which is seldom convenient for the cinematographer.
The following table gives the diaphragm number and shutter opening graduated from the exposures usually given for still camera work. Where longer exposures are recorded for still cameras it is not possible to get full exposure with the motion camera. It is understood that the calculation originally made with the meter is for a still camera using plates of the same relative speed as cine emulsion, which is as fast as the fastest plates ordinarily used in stand cameras, the only exception being the ultra-fast plates sometimes used for Graflex work.
Table of Comparative Exposures for Still and Motion Cameras.
sec. sec. sec. sec. sec. sec sec.
Still camera at fl6 1 1/2 1/4 1/8 1/16 1/32 1/48
Motion camera :
5^ opening shutter F3.5 F4 F5.6 F8 Fll FI6 F22
^ opening shutter F3.5 F4 F5.6 FS Fll FI6
H opening shutter F3.5 F4 F 5.6 FS Fll
for ys opening shutter the diaphragm should be set half way between the reading for the ,J^ opening and the reading for J4 opening.
With a little calculation almost any reliable exposure tables may be usfed for the motion picture camera. As the shutter revolves sixteen times per second it requires one-sixteenth second for the shutter to turn once ; if it has an opening which is onehalf of the circumference the exposure given is one-half of
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