Reciprocity failure occurs when the reciprocity law of photography no longer holds at extreme exposure times. The reciprocity law states that exposure is determined by the combination of light intensity and duration, such that halving the light intensity requires doubling the exposure time to achieve the same result.
Most films behave predictably between 1/2 second and 1/1000 second, but at very long exposures with low light or very short exposures with extremely bright light, this law fails, and the film becomes less sensitive than expected. Photographers must compensate for reciprocity failure by increasing exposure beyond the calculated value. This phenomenon is particularly relevant in long-exposure night photography, astrophotography, and high-speed imaging, though there is no exact formula for correction, so testing and experience are essential.