Reciprocity (photography)

In photography and holography, reciprocity refers to the relationship between the intensity of the light and duration of the exposure that result in identical exposure. Within the normal range for intensity and time for the film, the reciprocity law states that exposure = intensity × time. Outside the normal range the reciprocity law breaks down, which is known as reciprocity failure.

Photography

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 linear 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 1/250 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 due to the fact that light is in fact composed of quanta known as photons, and film in turn in composed of discrete grains. The grain of the film, usually crystals of a silver halide, require to be hit by a certain number of photons within a certain timeframe in order for the light-driven reaction to occur and the latent image to form. In particular, if the surface of the silver halide crystal or film grain has approximately four or more ionised silver atoms, corresponding to an absorption of four or more photons, it is rendered developable.

This breakdown in the linear relationship between aperture and shutter speed is known as reciprocity failure. Most film manufacturers publish data on the necessary 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. Reciprocity failure generally occurs at exposures of above 1s for film, and above 30s for paper.

Reciprocity effects can also occur within the tonal range of a photographic scenen when at the limit of exposure, resulting in burnt highlights while losing detail in the shadows. The composition of the film stock used, and in particular the relative amounts of silver bromide, silver chloride and silver iodide, can adjust this tonal response for the desired effect.

The linear relationship of exposure also breaksdown at extremely high levels of illumination, but this is generally of more concern to physicists than photographers, as such short (<1ms) exposures are only required for such subjects as exploding atomic bombs and particle physics.

Holography

A similar problem exists in holography. The total energy required when exposing holographic film using a continuous wave laser (i.e. for several seconds) is significantly less than the total energy required when exposing holographic film using a pulsed laser (i.e. around 20–40 nanoseconds) due to a reciprocity failure. It can also be caused by very long or very short exposures with a continuous wave laser. To try to offset the reduced brightness of the film due to reciprocity failure, a method called latensification can be used. This is usually done directly after the holographic exposure and using an incoherent light source (such as a 25–40W light bulb). Exposing the holographic film to the light for a few seconds can increase the brightness of the hologram by an order of magnitude.

References

• http://www.holography.ru/tech3eng.htm Retrieved March 8, 2005.