The law of reciprocity states that the exposure required for a photographic image to be correctly formed is equal to the intensity of the light to which it is subjected multiplied by the time for which the light is applied. Most films are designed to be exposed to image-forming light of practical brightness for a period between 1 second and 1/1000 second, and have a more-or-less linear relationship over this range. When much longer or shorter exposure times are used, a film may react in a non-linear manner. Consequently, doubling the exposure time has less effect than opening the aperture by one stop. This is known as reciprocity failure. The phenomenon occurs because the effect of exposure on photographic emulsions is dependent upon the rate at which energy is supplied.

As film is exposed to light, it becomes less and less sensitive. Consequently, to achieve the equivalent an exposure of 10 seconds predicted by a light meter it might, with some films, be necessary to expose for up to one minute. The effects of reciprocity failure usually become evident with very low shutter speeds, such as those used for low-light night scenes. The loss of effective film speed is probably the most significant effect, but contrast changes may also occur. Colour shift which cause images to look less natural may also become apparent.

In general, check a film's specification before using exposures longer than a few seconds. All films are affected to some extent, but some are particularly designed to have good reciprocity characteristics. Black-and-white films may also need reduced or increased development times, and some colour films may need colour correction.

In practical terms, digital sensors do not suffer from reciprocity failure.