Editing Exposure value (section)

=== Representing camera settings: EV ===

[[File:Posographe de Robert Kaufmann - 1922.jpg|thumb|[[Robert Kaufmann]]'s posographe or exposure calculator from 1922]]
The ratio ''t''/''N''<sup>2</sup> could be used to represent equivalent combinations of exposure time and f-number in a single value. But for many such combinations used in general photography, the ratio gives a fractional value with a large denominator; this is notationally inconvenient as well as difficult to remember. Inverting this ratio and taking the base-2 logarithm allows defining a quantity ''E''<sub>v</sub> such that

:<math>E_\mathrm v = \log_2 \frac {N^2} {t}\,,</math>

resulting in a value that progresses in a linear sequence as camera exposure is changed in power-of-2 steps. For example, beginning with 1&nbsp;s and {{f/|1}}, decreasing exposure

gives the simple sequence

:0, 1, 2, 3, ..., 14, 15, ...

The last two values shown frequently apply when using ISO 100 speed imaging media in outdoor photography.

This system provides its greatest benefit when using an exposure meter (or table) calibrated in EV with a camera that allows settings to be made in EV, especially with coupled shutter and aperture; the appropriate exposure is easily set on the camera, and choosing among equivalent settings is made by adjusting one control.

Current cameras do not allow direct setting of EV, and cameras with [[Exposure (photography)#Automatic exposure|automatic exposure control]] generally obviate the need for it. EV can nonetheless be helpful when used to transfer recommended exposure settings from an exposure meter (or [[#Table2|table of recommended exposures]]) to an exposure calculator (or [[#Table1|table of camera settings]]).