ReciprocityFailure

Reciprocity failure is a phenomenon encountered in photography where the reciprocity law no longer holds true. The reciprocity law states that the density of an exposed photographic material is determined solely by the total amount of light it receives, meaning a brief exposure to intense light will produce the same density as a prolonged exposure to dim light.

Reciprocity failure occurs when exposures are either very short (high intensity) or very long (low intensity). In these extreme conditions, the relationship between exposure time and light intensity breaks down, and the photographic material becomes less sensitive to light. This means that to achieve the same level of density, more light energy is required than predicted by the reciprocity law.

For very long exposures, reciprocity failure is often attributed to the exhaustion of the silver halide developing agents or chemical changes in the emulsion over time. For very short exposures, it can be due to the limited speed at which the silver halide crystals can react to the incoming photons.

The practical consequence of reciprocity failure is that photographers must use longer exposure times than theoretically calculated, or employ techniques like "exposure compensation," to achieve correct exposure in these challenging lighting situations. Different photographic films and papers exhibit reciprocity failure to varying degrees, and understanding a specific material's behavior is crucial for successful photography.