Reciprocity (photography) - Definition

In photography, reciprocity refers to the relationship between different choices of aperture and shutter speed that result in identical exposure.

In most situations there is an inverse relationship between aperture and shutter speed, with a wider aperture requiring a faster shutter speed for the same exposure. For example, an exposure value of 10 may be achieved with an aperture of f/2.8 and a shutter speed of 1/125 s. The same exposure can also be achieved by doubling the aperture to f/2 and halving the shutter speed to f/250 s; or by halving the aperture to f/4.0 and doubling the shutter speed to 1/60 s.

However, during very long exposures, film responds much more slowly than usual, resulting in underexposure. It is said then, that reciprocity failure has occurred, i.e. the conventional relationship between aperture and shutter speed has broken down. Most film manufacturers publish data on the latitude of their films (how much they can be pushed or pulled beyond their normal exposure range), and also reciprocity corrections. For example, if a light meter indicates a required EV of 5 and the photographer sets the aperture to f/11, then ordinarily a 4 second exposure would be required; a reciprocity correction factor of 1.5 means that the actual exposure must be extended to 6 seconds.