Exposure value

In photography, exposure value (EV) denotes all combinations of a camera's shutter speed and relative aperture that give the same exposure. In an attempt to simplify choosing among combinations of equivalent camera settings, the concept was developed by the German shutter manufacturer Friedrich Deckel (de) in the 1950s (Ray 2000, 318). Exposure value also is used to indicate an interval on the photographic exposure scale, with 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.


Exposure value was originally indicated by the quantity symbol Ev; this symbol continues to be used in ISO standards, but the acronym EV is more common elsewhere.

Although all camera settings with the same exposure value nominally give the same exposure, they do not necessarily give the same picture. The exposure time (“shutter speed”) determines the amount of motion blur, as illustrated by the two images at the right, and the relative aperture determines the depth of field.

Formal definition

Exposure value is a base-2 logarithmic scale defined by (Ray 2000, 318)

where:

N is the relative aperture (f-number)
t is the exposure time (“shutter speed”) in seconds[2]

EV 0 corresponds to an exposure time of 1 s and a relative aperture of f/1.0. If the EV is known, it can be used to select combinations of exposure time and f-number.

Each increment of 1 in exposure value corresponds to a change of one “step” (or, more commonly, one “stop”) in exposure, i.e., half as much exposure, either by halving the exposure time or halving the aperture area, or a combination of such changes. Greater exposure values are appropriate for photography in more brightly lit situations, or for higher ISO speeds.

Source: http://en.wikipedia.org