In film it refers to the normal relationship between exposure time and intensity of light. 1
Expressed by (H) Et, where E is the light intensity, and t is time, When E or t are varied to the extreme, an unsatisfactory exposure can result (Kodak) Called the Schwarzschild effect.
At very short and very long exposure, the reciprocity law ceases to hold true, and an extra exposure is needed. With colour film, the three layers suffer differently, causing a colour cast. Reciprocity failure differs from emulsion to emulsion.
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston