Upon exposure, the amount of light energy that reaches the film determines the effect upon the emulsion. If the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor by varying the camera's shutter speed and aperture, so that the energy received is the same, the film will be developed to the same density. This rule is called reciprocity. The systems for determining the sensitivity for an emulsion are possible because reciprocity holds. In practice, reciprocity works reasonably well for normal photographic films for the range of exposures between 1/1000 second to 1/2 second. However, this relationship breaks down outside these limits, a phenomenon known as reciprocity failure.
What happens outside the 1/1000 second to 1/second exposure range?
reciprocity failure