Exposure Value - Formal Definition

Formal Definition

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

where

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

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, as shown in Table 1.

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.