Filter Factors

Colored filters are useful for increasing the contrast between objects that are different colors. A filter will transmit its own color and absorb its complementary color. Thus a red filter will lighten a red apple when used with black-and-white film and darken a blue sky. Filters can produce important effects, but it is necessary to remember that the filter is blocking some of the light that would otherwise be exposing the film. Unless you compensate for this lost light, the negative will be underexposed. Every filter is associated with a number called a filter factor. This number indicates how much exposure to add for each filter. For example, if the factor for a given filter is 2, the exposure must be increased by two times to get the proper exposure. Since each f/stop doubles the exposure as it opens up, a filter factor of 2 requires opening up one stop. A filter factor of 4 would require a two-stop adjustment. Because daylight has a bluish cast and tungsten light is reddish, the filter factors for daylight are slightly higher. (The one exception is a dark blue filter, #47, which has a higher tungsten filter factor number.) Filter factors are listed in the Kodak data guides for filters and black-and-white films.

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