Editing Light meter (section)

===Exposure equations===

For reflected-light meters, camera settings are related to ISO speed and subject luminance by the reflected-light exposure equation:

<math>
\frac {N^2} {t} = \frac {L S} {K}
</math>

where

* <math>N</math> is the relative [[aperture]] ([[f-number]])
* <math>t</math> is the exposure time ("[[shutter speed]]") in seconds
* <math>L</math> is the average scene [[luminance]]
* <math>S</math> is the ISO arithmetic [[film speed|speed]]
* <math>K</math> is the reflected-light meter calibration constant

For incident-light meters, camera settings are related to ISO speed and subject illuminance by the incident-light exposure equation:

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

where

* <math>E</math> is the [[illuminance]]
* <math>C</math> is the incident-light meter calibration constant