American cinematographer (Jan-Dec 1941)

Frame enlargements from 16mm. film, showing no filter (left), medium-yellow filter (center) and heavy red filter (right) on panchromatic film. PERHAPS the most common and irritating misconception about niters is the habit so many amateurs have )f referring to them as a "2x," a "4x," ind so on. Really there's no such animal! Those numbers simply refer to the num>er of times a normal, unfiltered exposire must be increased to maintain a nornal exposure-level in the filtered scene. \.nd this varies according to the film ised. A color-filter, you see, is simply a bit if colored glass or gelatin placed be ween the film and the subject. Being olored, it holds back light-rays of some ,0]ors — those of a color opposite or com dementary to the color of the filter — nd lets those of the same color as the ilter pass through unhindered. Since he filter holds back part of the light making the picture, without adding any hing to take its place, the light that gets hrough must either be allowed to work anger, by using a slower shutter-ex •osure, or a larger total of light must e admitted, by using a larger lens-open ng, so that the film will still receive a ormal total exposure. Now, say the filter is a deep yellow ne. This will almost completely stop the ijlue and ultra-violet rays from getting hrough to work on the film, but will let 11 the greens, yellows, oranges, reds, tc, go through almost normally. Now ' the film in question is a modern pan hromatic type, sensitive to all colors, >iat blue light which is diverted by the Iter doesn't represent such a big slice f the total exposure-making light. But ' the film is an ortho type, sensitive only , > blue, green, yellow and possibly a ttle orange, that missing blue is a luch bigger proportion of the total use d light. Therefore, the exposure in this t^e will have to be increased more, and le same filter which on the panchromatic lm may be rated as a 2x filter can easily ;come an 8x on an ortho film! The same distinction is true, though ithin narrower limits, according to ifferences in sensitivity between dif >rent types of panchromatic film. For ■cample, the rather heavy red 25A filter as a factor of 5 when used on one ither highly red-sensitive 35mm. film Agfa Ultra-Speed Pan) while on an her less red-sensitive panchromatic lm made by the same company (Su reme) the factor is 8, while the factors Taking the Guesswork Out of Color Filtering By A. L. CILKS, A.S.C. may be entirely different for films of other manufacturers. So — know your film before you start talking filter-factors! If you go into the average, well-stocked photographic dealer's store and ask him to show you some filters, you'll notice a bewildering variety of different types, of different makes and picturesquely differing colors. And each manufacturer has his own pet scheme of naming and numbering his filters. But you'll notice that all of them keep to a pretty definite range of four or five colors, though in many different shades or densities. In a modern store you'll always find yellow filters and red ones, and also usually orange ones. In some of the larger shops, you'll also see some very pretty green ones, and occasionally a blue one. So for the present, we might as well forget names, and consider only colors, for while performance may differ in detail, in principle one maker's light red filter will produce about the same results as another maker's light red one, regardless of names. For all practical purposes, 99% of movie work — professional or amateur — can be done with two or three filters: a medium yellow one, a medium-red one, and an orange one. The yellow filters produce the least spectacular effects. On panchromatic film they'll generally make an exterior shot more pleasing, giving the different colors in a scene a more natural range of relative brightnesses in your black-and-white picture and toning the sky down a trifle so clouds stand out more normally. The orange filters carry this on quite a bit more. They add snap and contrast to the scene, and darken blue skies, water, etc., quite perceptibly. The red filters carry this correction to an extreme. They add a maximum of contrast, and — especially the deeper red ones — tend to turn blue skies, water, etc., almost black. There's a very good rule to remember about using filters. Any filter will darken its complementary color, and lighten objects of its own or closely related colors. So to photograph blue as white, use a blue filter; to photograph it as black, use a red filter; to photograph yellow as white, use a yellow filter; as black, a blue filter. To photograph green as white, use a green filter; as black, a red filter; and to photograph red as white, use a red filter; as black, use a green filter. All of this suggest interesting possibilities— some of them rather embarrassing. For example, there's the time-honored story of the girl in the red dress. In an interior scene, she made her exit through a door apparently wearing an almost black garment. In the exterior scene cut next to it, the same girl wearing the same dress walked out into the nifjht — but the same red filter that (Continued on Page 242) American Cinematographer May, 1941 225