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Concept
Exposure (photography)
Summary
In photography, exposure is the amount of light per unit area (the 's illuminance times the exposure time) reaching a frame of photographic film or the surface of an electronic , as determined by shutter speed, lens F-number, and scene luminance. Exposure is measured in lux seconds, and can be computed from exposure value (EV) 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 (Hv) is the same so long as the total exposure time is the same.
Radiant exposure of a surface,
denoted He ("e" for "energetic", to avoid confusion with photometric quantities) and measured in , is given by
where
Ee is the irradiance of the surface, measured in ;
t is the exposure duration, measured in s.
Luminous exposure of a surface, denoted Hv ("v" for "visual", to avoid confusion with radiometric quantities) and measured in , is given by
where
Ev is the illuminance of the surface, measured in lx;
t is the exposure duration, measured in s.
If the measurement is adjusted to account only for light that reacts with the photo-sensitive surface, that is, weighted by the appropriate spectral sensitivity, the exposure is still measured in radiometric units (joules per square meter), rather than photometric units (weighted by the nominal sensitivity of the human eye). Only in this appropriately weighted case does the H measure the effective amount of light falling on the film, such that the characteristic curve will be correct independent of the spectrum of the light.
Many photographic materials are also sensitive to "invisible" light, which can be a nuisance (see UV filter and IR filter), or a benefit (see infrared photography and full-spectrum photography). The use of radiometric units is appropriate to characterize such sensitivity to invisible light.
Official source
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