Reciprocity (photography)
In photography, reciprocity is the inverse relationship between the intensity and duration of light that determines the reaction of light-sensitive material.
Therefore, the same response (for example, the optical density of the developed film) can result from reducing duration and increasing light intensity, and vice versa.
The reciprocal relationship is assumed in most sensitometry, for example when measuring a Hurter and Driffield curve (optical density versus logarithm of total exposure) for a photographic emulsion.
[1][2][3] Deviations from the reciprocity law were reported by Captain William de Wiveleslie Abney in 1893,[4] and extensively studied by Karl Schwarzschild in 1899.
Each grain must absorb a certain number of photons in order for the light-driven reaction to occur and the latent image to form.
This is a concern for scientific and technical photography, but rarely to general photographers, as exposures significantly shorter than a millisecond are only required for subjects such as explosions and in particle physics, or when taking high-speed motion pictures with very high shutter speeds (1/10,000 sec or faster).
[16][17] However, a constant value for p remains elusive, and has not replaced the need for more realistic models or empirical sensitometric data in critical applications.
It has already been observed that such extreme sub-latent image is unstable, and it is postulated that ineffiency is caused by many isolated atoms of silver losing their acquired electrons during the period of instability.
Combined, these parameters make these targets extremely difficult to capture with film; exposures from 30 minutes to well over an hour are typical.
When a telescope is tracking an object, every minute is difficult; therefore, reciprocity failure is one of the biggest motivations for astronomers to switch to digital imaging.
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.
To try to offset the reduced brightness of the film due to reciprocity failure, a method called latensification can be used.
Note that the times are approximate, and vary between different films and ASA numbers, but the table shows in general how the exposure time is adjusted.