Exposure (photography)

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. It is determined by exposure time, lens f-number, and scene luminance. Exposure is measured in units of lux-seconds (symbol lx⋅s), 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.

Definitions

Radiant exposure

Radiant exposure of a surface, denoted He ("e" for "energetic", to avoid confusion with photometric quantities) and measured in J/m2, is given by

H

e

=

E

e

t

,

{\displaystyle H_{\mathrm {e} }=E_{\mathrm {e} }t,}

where

Ee is the irradiance of the surface, measured in watts per square-meter, W/m2;

t is the exposure duration, measured in seconds.

Luminous exposure

Luminous exposure of a surface, denoted Hv ('v' for 'visual', to avoid confusion with radiometric quantities) and measured in lux-seconds, is given by|

H

v

=

E

v

t

,

{\displaystyle H_{\mathrm {v} }=E_{\mathrm {v} }t,}

where

Ev is the illuminance of the surface, measured in lux; and

t is the exposure duration, measured in seconds.

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.