Exposure value - Definition and Overview

In photography, exposure value (EV) is a value given to all combinations 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 negative logarithmic scale, where an increase of one EV 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>\mathit{EV} = \mathit{AV} + \mathit{TV}<math>

If N is the f-number (such as 2.8, 4.0, 5.6, ...), the Aperture Value is

<math>\mathit{AV} = \log_2 N^2 = 2\,\log_2 N<math>.

If t is the shutter time in seconds, the Time Value is

<math>\mathit{TV} = \log_2 \frac1t = -\log_2 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, the metered light value is identical to the exposure value required for a "proper" exposure. Slower films with ISO films speeds below 100 require a corresponding increase in exposures, i.e. lower exposure values. 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 cameras, like certain Hasselblad models, also have markings for EV.

Typical shutter/aperture combinations and corresponding exposure values are listed in the following table:

External links

• LV and EV explained (http://www.kenrockwell.com/tech/ev.htm)