This calculator is intended for using an SLR camera as a luminance meter. Just frame the scene you want to measure and read the suggested shutter time and aperture.
If the exposure meter is saturating (too much or not enough light), try using a different film speed.
Of course the reading will depend on how the exposure meter measures the light and what kind of measure is required. If it's in "spot meter" mode, the reading will correspond to the measured spot, if used in "average" mode the reading will be the average of the framed scene. Using it in "center weighted" mode doesn't make much sense.
This calculator assumes that the exposure meter of the camera is calibrated and reliable, which is almost never the case, but still this procedure can be used to get a rough idea of the luminance of a scene or an object. Losses in the lens are not taken into account.
Unfortunately many different definitions of film sensitivity and exposure exist and the calculated luminance can be very different. Different commercial exposure meters give also very different results. This calculator assumes that a uniform luminance of 1 fL (foot-Lambert) on the whole frame with a film speed of 3.125 ISO will be correctly exposed with an exposure of 0 IL.
If you wonder from where the constant 0.9121 ISO s cd/m2 comes from, it's just 1 s multiplied by 3.125 ISO multiplied by 1 fL multiplied by 3.14159 cd/(ft2 fL) divided by 10.763910 m2/ft2.
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