Editing Exposure value (section)

==EV as a measure of luminance and illuminance==

For a given ISO speed and meter calibration constant, there is a direct relationship between exposure value and luminance (or illuminance). Strictly, EV is not a measure of luminance or illuminance; rather, an EV corresponds to a luminance (or illuminance) for which a camera with a given ISO speed would use the indicated EV to obtain the nominally correct exposure. Nonetheless, it is common practice among photographic equipment manufacturers to express luminance in EV for ISO 100 speed, as when specifying metering range ([[#CITEREFRay2000|Ray 2000]], 318) or autofocus sensitivity. And the practice is long established; ([[#CITEREFRay2002|Ray 2002]], 592) cites [[#CITEREFUlffers1968|Ulffers (1968)]] as an early example. Properly, the meter calibration constant as well as the ISO speed should be stated, but this seldom is done.

Values for the reflected-light calibration constant <var>K</var> vary slightly among manufacturers; a common choice is 12.5 ([[Canon Inc.|Canon]], [[Nikon]], and [[Mamiya|Sekonic]]<ref>Specifications for Sekonic light meters are available on the [http://www.sekonic.com/ Sekonic] web site under "Products".</ref>). Using {{nowrap|1=<var>K</var> = 12.5}}, the relationship between EV at ISO 100 and luminance <var>L</var> is then

:<math>L = 2^{\mathrm {EV} - 3} \,.</math>

Values of luminance at various values of EV based on this relationship are shown in Table 3. Using this relationship, a reflected-light exposure meter that indicates in EV can be used to determine luminance.

{| class="wikitable floatright" style="text-align: center"
|+ Exposure value vs. Luminance (ISO 100, {{nowrap|1= {{mvar|K}} = 12.5}}) and Illuminance (ISO 100, {{nowrap|1={{mvar|C}} = 250}})
|-
! scope="col" rowspan="2" | EV<sub>100</sub>
! scope="col" colspan="2" | Luminance
! scope="col" colspan="2" | Illuminance
|-
! scope="col" | [[Candela per square metre|{{abbr|cd/m<sup>2</sup>|candela per square metre}}]]
! scope="col" | [[Footlambert|{{abbr|fL|foot-Lamberts}}]]
! scope="col" | [[Lux|{{abbr|lx|lux}}]]
! scope="col" | [[Foot-candle|{{abbr|fc|foot-candles}}]]
|-
| scope="row" |  &minus;4 ||    0.008 ||    0.0023 || 0.156 ||  0.015
|-
| scope="row" |  &minus;3 ||    0.016 ||    0.0046 || 0.313 ||  0.029
|-
| scope="row" |  &minus;2 ||    0.031 ||    0.0091 || 0.625 ||  0.058
|-
| scope="row" |  &minus;1 ||    0.063 ||    0.018  || 1.25  ||  0.116
|-
| scope="row" |  0 ||    0.125 ||    0.036 ||       2.5 ||     0.232
|-
| scope="row" |  1 ||    0.25  ||    0.073 ||       5   ||     0.465
|-
| scope="row" |  2 ||    0.5   ||    0.146 ||      10   ||     0.929
|-
| scope="row" |  3 ||    1     ||    0.292 ||      20   ||     1.86
|-
| scope="row" |  4 ||    2     ||    0.584 ||      40   ||     3.72
|-
| scope="row" |  5 ||    4     ||    1.17  ||      80   ||     7.43
|-
| scope="row" |  6 ||    8     ||    2.33  ||     160   ||    14.9
|-
| scope="row" |  7 ||   16     ||    4.67  ||     320   ||    29.7
|-
| scope="row" |  8 ||   32     ||    9.34  ||     640   ||    59.5
|-
| scope="row" |  9 ||   64     ||   18.7   ||    1280   ||   119
|-
| scope="row" | 10 ||  128     ||   37.4   ||    2560   ||   238
|-
| scope="row" | 11 ||  256     ||   74.7   ||    5120   ||   476
|-
| scope="row" | 12 ||  512     ||  149     ||  10,240   ||   951
|-
| scope="row" | 13 || 1024     ||  299     ||  20,480   ||  1903
|-
| scope="row" | 14 || 2048     ||  598     ||  40,960   ||   3805
|-
| scope="row" | 15 || 4096     || 1195     ||  81,920   ||   7611
|-
| scope="row" | 16 || 8192     || 2391     || 163,840   || 15,221
|}

As with luminance, common practice among photographic equipment manufacturers is to express illuminance in EV for ISO 100 speed when specifying metering range.<ref>The metering range for an incident-light meter specified in EV at ISO 100 usually applies to a hemispherical sensor, so strictly speaking, it does not directly relate to illuminance.</ref>

The situation with incident-light meters is more complicated than that for reflected-light meters, because the calibration constant <var>C</var> depends on the sensor type. Two sensor types are common: flat ([[cosine]]-responding) and hemispherical ([[cardioid]]-responding). Illuminance is measured with a flat sensor; a typical value for <var>C</var> is 250 with illuminance in [[lux]]. Using {{nowrap|1=<var>C</var> = 250}}, the relationship between EV at ISO 100 and illuminance <var>E</var> is then

:<math>E = 2.5 \times 2^{\mathrm {EV}} \,.</math>

Values of illuminance at various values of EV based on this relationship are shown in the table to the right. Using this relationship, an incident-light exposure meter that indicates in EV can be used to determine illuminance.

Although illuminance measurements may indicate appropriate exposure for a flat subject, they are less useful for a typical scene in which many elements are not flat and are at various orientations to the camera. For determining practical photographic exposure, a hemispherical sensor has proven more effective. With a hemispherical sensor, typical values for <var>C</var> are between 320 (Minolta) and 340 (Sekonic) with illuminance in lux. If illuminance is interpreted loosely, measurements with a hemispherical sensor indicate "scene illuminance".

[[Light meter#Exposure meter calibration|Exposure meter calibration]] is discussed in detail in the [[Light meter]] article.
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