Editing Exposure value (section)

==Camera settings vs. luminous exposure==

[[File:Shutter with EV scale.png|thumb|Shutter with EV indicator (item 29) on a ring of EV values (item 34), figure from US patent 2829574, Inventor: K. Gebele, original assignee: Hans Deckel, filing date: Nov 2, 1953, issue date: Apr 8, 1958]]

"Exposure value" indicates combinations of camera settings rather than the [[Photometry (optics)|luminous exposure]] (aka photometric exposure), which is given by ([[#CITEREFRay2000|Ray 2000]], 310)

:<math>H = E t \,,</math>

where

* ''H'' is the [[luminous exposure|luminous]]/[[photometric exposure]] (lux seconds)
* ''E'' is the image-plane [[illuminance]]  (lux or lumens/m²)
* ''t'' is the exposure time ("shutter speed") (seconds)

The illuminance ''E'' is controlled by the f-number but also depends on the scene [[luminance]]. To avoid confusion, some authors ([[#CITEREFRay2000|Ray 2000]], 310) have used '''camera exposure''' to refer to combinations of camera settings. The 1964 ASA standard for automatic exposure controls for cameras, [[#CITEREFASA PH2.15-1964|ASA PH2.15-1964]], took the same approach, and also used the more descriptive term '''camera exposure settings'''.

Common practice among photographers is nonetheless to use "exposure" to refer to camera settings as well as to photometric exposure.

=== Relationship of camera settings to luminous exposure ===

The image-plane illuminance is directly proportional to the area of the aperture, and hence inversely proportional to the square of the lens f-number; thus

:<math>H \propto \frac {t} {N^2}\,;</math>

for constant lighting conditions, the exposure is constant as long as the ratio ''t''/''N''<sup>2</sup> is constant. If, for example, the f-number is changed, an equivalent exposure time can be determined from

:<math>\frac {t_2} {t_1} = \frac {N_2^2} {N_1^2}\,.</math>

Performing this calculation mentally is tedious for most photographers, but the equation is easily solved with a calculator dial on an exposure meter ([[#CITEREFRay2000|Ray 2000]], 318) or a similar dial on a standalone calculator. If the camera controls have detents, constant exposure can be maintained by counting the steps as one control is adjusted and counting an equivalent number of steps when adjusting the other control.

=== 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]]).