Editing Exposure value (section)

==Relationship of EV to lighting conditions==

"Correct" exposure is obtained when the f-number and exposure time match those "recommended" for given lighting conditions
and ISO speed; the relationship is given by the exposure equation prescribed by [[ISO 2720|ISO 2720:1974]]:
:<math>
\frac {N^2} {t} = \frac {L S} {K} \,,
</math>

where<ref>Symbols for the quantities in the exposure equation have varied over time; the symbols used in this article reflect current practice for many authors, such as [[#CITEREFRay2000|Ray (2000)]].</ref>

* ''N'' is the relative [[aperture]] ([[f-number]])
* ''t'' is the exposure time ("[[shutter speed]]") in seconds<ref name="no_units"/>
* ''L'' is the average scene [[luminance]]
* ''S'' is the ISO arithmetic [[film speed|speed]]
* ''K'' is the reflected-light [[Light meter#Calibration constants|meter calibration constant]]

Applied to the right-hand side of the exposure equation, exposure value is

:<math>\mathrm {EV} = \log_2 {\frac {L S} {K} } \,.</math>

If the common value of {{nowrap|1=''K'' = 12.5 (unit: cd s/m<sup style="font-size:70%">2</sup> ISO)}} is used, an EV of zero (e.g., an aperture of {{f/|1}} and a shutter time of 1 sec) for {{nowrap|1=ISO = 100}} corresponds to a luminance of {{nowrap|1=0.125 cd/m<sup style="font-size:70%">2</sup>}} ({{nowrap|1=0.01 cd/ft<sup style="font-size:70%">2</sup>}}). At EV = 15 (the "[[sunny 16 rule|sunny sixteen]]" amount of light) the luminance is {{nowrap|1=4096 cd/m<sup style="font-size:70%">2</sup>}} ({{nowrap|1=380 cd/ft<sup style="font-size:70%">2</sup>}}).

Camera settings also can be determined from incident-light measurements, for which the exposure equation is

:<math>
\frac {N^2} {t} = \frac {E S} {C} \,,
</math>

where

* ''E'' is the [[illuminance]] in lux or lumens/m²
* ''C'' is the incident-light meter calibration constant

In terms of exposure value, the right-hand side becomes

:<math>\mathrm {EV} = \log_2 {\frac {E S} {C} } \,.</math>

When applied to the left-hand side of the exposure equation, EV denotes actual combinations of camera settings; when applied to the right-hand side, EV denotes combinations of camera settings required to give the nominally "correct" exposure. The formal relationship of EV to luminance or illuminance has limitations. Although it usually works well for typical outdoor scenes in daylight, it is less applicable to scenes with highly atypical luminance distributions, such as city skylines at night. In such situations, the EV that will result in the best picture often is better determined by subjective evaluation of photographs than by formal consideration of luminance or illuminance.

For a given luminance and film speed, a greater EV results in less exposure, and for fixed exposure (i.e., fixed camera settings),
a greater EV corresponds to greater luminance or illuminance.

Illuminance is measured using a flat sensor; if the common value of {{nowrap|1=''C'' = 250 (unit: lux s ISO=lm s/m<sup style="font-size:70%">2</sup> ISO)}} is used, an EV of zero (e.g., an aperture of {{f/|1}} and a shutter time of 1 sec) for {{nowrap|1=ISO = 100}} corresponds to an illuminance of  {{nowrap|2.5 lux}} ({{nowrap|0.23 fc}}). At EV = 15 (the "sunny sixteen" amount of light) the illuminance is {{nowrap|82,000 lux}} ({{nowrap|7600 fc}}). For general photography, incident-light measurements are usually taken with a hemispherical sensor; the readings cannot be meaningfully related to illuminance.