Exposure (photography)

In photography, exposure is the amount of light per unit area (the image plane's illuminance times the exposure time) reaching a frame of photographic film or the surface of an electronic image sensor, 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.

A long exposure showing star trails around the south and north celestial poles, viewed from Paranal Observatory. (Credit: ESO)
A photograph of the sea after sunset with an exposure time of 15 seconds. The swell from the waves appears foggy.
A photograph of Fort du Salbert taken by moonlight with an exposure time of ten minutes.

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.

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