The illusion of night in black & wliite cinematography is obtained by combining contrast filtering with underexposure. Since the sky is light by day and dark by night, it is the principal area of the scene requiring correction. Any of the yellow-orange or red filters may be used. A very popular combination is the light red Wratten 23A plus the green 56. This combination does everything the red filters accomplish — plus it darkens flesh tones, which are rendered too light by the red filters alone. When combining filters, remember that red filters add contrast but green filters flatten; if a greater flattening effect is desired, add a heavier green filter. Since flesh tones are not important in long shots, they are sometimes filmed with heavier red filters, and only the medium and close shots are made with the combination red-green filters. Care must be taken, however, that clothing and background colors do not photograph differently when filters are switched in the same sequence. If in doubt, shoot tests before production filming begins. Remember that only a blue sky can be filtered down. No amount of color filtering will darken a bald white sky. Use graduated neutral densities, or avoid the sky under these adverse conditions. The 23A-56 combination is usually employed with a filter factor of 6, rather than the 20 normally required (5 for the 23A and 4 for the 56, which multiplied equals 20). The factor of 6 automatically underexposes this filter combination approximately 1XA stops and achieves the desired effect without further computation. If a red filter is used alone, bear in mind that it will lighten faces, and use a darker makeup (approximately two shades) on close shots.
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