page_title: Exposure (photography)
section_title: Reciprocity
hierarchical_section_title: Exposure (photography) / Reciprocity
context_page_description: In photography, exposure is the amount of light per unit area reaching a frame of photographic film or the surface of an electronic image sensor, as determined by shutter speed, lens aperture, and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance in a specified region.
  An "exposure" is a single shutter cycle. For example, a long exposure refers to a single, long shutter cycle to gather enough dim light, whereas a multiple exposure involves a series of shutter cycles, effectively layering a series of photographs in one image. The accumulated photometric exposure is the same so long as the total exposure time is the same.
context_section_description: An important principle of exposure is reciprocity.  If one exposes the film or sensor for a longer period, a reciprocally smaller aperture is required to reduce the amount of light hitting the film to obtain the same exposure.  For example, the photographer may prefer to make his sunny-16 shot at an aperture of f/5.6 (to obtain a shallow depth of field). As f/5.6 is 3 stops "faster" than f/16, with each stop meaning double the amount of light, a new shutter speed of (1/125)/(2·2·2) = 1/1000 s is needed.  Once the photographer has determined the exposure, aperture stops can be traded for halvings or doublings of speed, within limits.
  The true characteristic of most photographic emulsions is not actually linear (see sensitometry), but it is close enough over the exposure range of about 1 second to 1/1000 of a second.  Outside of this range, it becomes necessary to increase the exposure from the calculated value to account for this characteristic of the emulsion. This characteristic is known as reciprocity failure.  The film manufacturer's data sheets should be consulted to arrive at the correction required, as different emulsions have different characteristics.
  Digital camera image sensors can also be subject to a form of reciprocity failure.
caption: A demonstration of the effect of exposure in night photography. Longer shutter speeds result in increased exposure.
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