Reciprocity (photography) Guide, Meaning , Facts, Information and Description
In Photography, reciprocity refers to the relationship between aperture and shutter speed. In most situations this is an inverse relationship with a wider aperture requiring a faster shutter speed for the same, "proper" exposure --that is without overexposing or underexposing the film being used.
However, during very long exposures, film responds much more slowly than usual, resulting in underexposure. It is said then, that reciprocity failure has occurred, the 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.
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