Editing Exposure value (section)

==Formal definition<span class="anchor" id="Ev"></span>==

[[File:Spinning into Action.jpg|thumb|Extended exposure time of 26 seconds]]

Exposure value is a base-2 [[logarithm]]ic scale defined by ([[#CITEREFRay2000|Ray 2000, 318]]):

<math display="block">\begin{align}
\mathrm {EV} &= \log_2 {\frac {N^2} {t} } \\
&= 2\log_2{N} - \log_2{t} \,,
\end{align}</math>

where

* {{mvar|N}} is the [[f-number]]; and
* {{mvar|t}} is the exposure time ("[[shutter speed]]") in seconds<ref name="no_units">In a mathematical expression involving physical quantities, it is common practice to require that the argument to a [[transcendental function]] (such as the [[logarithm]]) be [[dimensionless]]. The definition of EV ignores the units in the denominator and uses only the [[ISO 31-0#Quantities and units|numerical value]] of the exposure time in seconds; EV is not the expression of a physical law, but simply a number for encoding combinations of camera settings.</ref><!-- TBD: I guess this can be improved to have a mathmatically correct formula here. -->

The second line is just applying the [[Logarithm#Product, quotient, power, and root|quotient identity]] of logarithms to the first line.

EV&nbsp;0 corresponds to an exposure time of 1&nbsp;[[second|s]] and an 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&nbsp;1.

<!-- TBD. The following sentence apparently still causes confusion amongst readers and should be further improved for clarification. -->
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 lower [[film speed|ISO speed]]s.