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98 AMATEUR MOVIE MAKING
As modern lights reach as high an efficiency as one thousand candlepower per ampere of current consumed, we will assume that such is the case. Then the 5,000 c.p. light would consume 5 amperes, and the 20,000 c.p. would consume 20 amperes. The usual house current is equipped with 25 ampere fuses, so that we could safely use 4 of the 5 ampere lights, or 2 ten ampere lights, while we could safely use only one of the 20 ampere lights. Thus we see that there is one decided advantage in using fast lenses and lower intensity of illumination.
Arc Amperage. — Experience has shown us that the practical minimum amperages for arc lights to be used for cinematographic purposes is eight amperes for primary lights and 3 to 4 amperes for secondary lights. With a pair of 8 or 10 ampere lights, cinematography of average home groups in average interiors is possible at normal speed using a lens aperture of f 3.5.
For the fullest possible control, two eight or ten ampere lights, which can be safely used on the usual house line, should be used. In such case, balancing is achieved by variation of the distance between the individual lights and the subject. As a rule then we may assume that for reasons of economy and convenience the combination of low intensity lights with high aperture lenses is preferable to extremely high intensity lights and small aperture lenses.
There are also pictorial reasons for this. The usual interior shot is made with one or more persons as the center of interest. In the case of close-ups and semi close-ups, the high aperture allows us to diffuse the background while retaining abundant detail in the subject focussed upon. At the same time, in full shots, the background is so near the subject that full frame detail may be secured with f 2. Any diffusion which persists in such cases is not objectionable.
If any one lens aperture can be said to be preferable to another, under all conditions, we may say that the f 2 is the most generally useful aperture for interior work with the lens of from 20 to 25 millimeter focus (one inch). It is possible to make use of this stop under most conditions by the proper manipulation of the lights.