The Practical Zone System for Film and Digital Photography, 6th Edition

Appendix R Filter Factors, the Reciprocity Effect, and Bellows Extension Factors

Filter factors, the reciprocity effect, and bellows extension factors greatly affect your exposure.

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 you need 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 ...