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Exposure value

In photography, the term exposure value, usually abbreviated EV, is a value given to all combination of camera shutter speed and aperture that gives the same exposure. By definition, EV 0 corresponds to a shutter speed of 1 s and an aperture of f/1.0. It is a base 2 logarithmic scale, and an increase of one corresponds to half the amount of light admitted.

By the Additive Photographic Exposure System, the exposure value is the sum of the Aperture Value and the Time Value:

<math> E_V = A_V + T_V </math>

When <math>N</math> is the f-number:

<math> A_V = lg2 N^2 </math>

When <math>t</math> is the shutter time in seconds:

<math> T_V = lg2 \frac {1} {t} </math>

A light meter will read the amount of light from a scene giving a light value, and this will have to be translated to an EV depending on the film speed used, usually expressed as an ISO rating[?]. For ISO 100 film, there is a one to one relationship. In practice, most light meters for photography will have a setting for film speed, and give exposure value or a list of suitable aperture/shutter combinations directly.

Some examples of EV to shutter/aperture combinations are:

0 1 sf/1.0
1 1 sf/1.4
11 1/60 sf/5.6
12 1/60 sf/8
121/125 sf/5.6
131/125 sf/8
131/250 sf/5.6
141/250 sf/8
191/2000 sf/16

Some cameras, like the Hasselblad, have markings for EV.

See also: light value

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