effet Schwarzschild

OBS: Chaque type de film possède sa propre courbe de sensibilité. D'un écart trop important résulte un tirage sous-exposé ou présentant des dominantes.

CONT: Reciprocity effect. All photographic emulsions are subject to an effect often called reciprocity-law failure. The reciprocity law states that intensity of the illumination falling on a film times the exposure time equals the amount of exposure (E = I x T). This law does apply for most black-and-white films from exposure times of about 1/5 sec. to 1/1000 sec. and for colour films over a narrower range. Thus, where the law holds, an exposure of 1/60 sec. at f/11 is equivalent to an exposure of 1/30 sec. at f/16.