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LIGHT FILTERS AND THEIR USE 131
of fact, probably the best night effect scenes shot in the daytime can be obtained using hypersensitized panchromatic film and either the 70 or 72 filter.
Except for the use of neutral gray filters, the above mentioned filters constitute most of those used in present day cinematography. The use of neutral filters has been written of before. The neutral filters have the effect of non-selectively cutting down the light. Any photographer knows that the amount of light striking the film can be reduced by stopping down his lens. It is very often desirable, however, to photograph a scene with the lens relatively wide open. Without the use of neutral filters oftentimes this would produce an over-exposed negative, unless, of course, the shutter opening of the camera was changed. With the use of neutral filters, it is possible to leave the shutter fixed, open the lens to its maximum if desired, and photograph with the proper transmission neutral filter in the system. It has only been during the past year that use has been made of neutral filters when photographing directly into glaring light, that is, such light as might be reflected from white buildings and sidewalks.
It will be observed that of the filters mentioned, practically all of them were designated for exterior uses. It is not necessary, with the present type of lighting equipment, especially incandescent lights, to use any filters on interior scenes, except in the case of color photography. However, this article is not in any respect dealing with the use of filters for color photography. A comparison of scenes photographed under arc lights and also under incandescent lights, individually, shows that the incandescent light effectively produces a negative similar to one which would be produced if arc lights and a K-2 filter were used. It is quite generally agreed that the incandescent light, in terms of the arc light, has a filtering quality of approximately a K-2 filter. This condition arises because of the difference in the quality of the light emitted by the two sources. The arc lights preponderate in blue and the effect of using a K-2 filter with an arc light would be to reduce some of the blue. The incandescent light, on the other hand, has its greatest emission in the red end of the spectrum and again the K-2 filter in conjunction with arc lights, while lowering the effective blue emission, effectively increases the red emission, so that, to repeat again, the arc lights plus a K-2 filter give a rendering on panchromatic negative similar to the incandescent tungsten lamps.
There are, of course, other filters which are used for special work, trick photography employing quite a few, but as this article is intended primarily to review the use of filters as applied to making production motion picture negatives for black and white work, any references to color photography and trick photography have been omitted.
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