Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
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JDinOregon Junior Member • Posts: 46
Re: steephill, sjgcit, gl2k

By "Blind Exposure" I take the situation as similar to doing a portrait without the benifit of an incident light meter. The photographer does not want to guess at the reflectance of the subjects skin (although I remember something like one stop brighter than middle gray as typical for caucasions). So instead an 18% gray card is used as a standard.

The meter constant for a Seconic Incident meter is 250 and the constant for Nikon and Canon reflected meters is 12.5. With exposure defined by

EV= log2(L*s/k) = log2(E*s/c)

This implies reflectance from a uniform perfect diffuser is 15.7%. From

L=E*R/pi

I think this means that the gray card will indicate an exposure about 0.2 stops off that of the incident meter.  Not so much for the reason that 18% is the wrong reflectance, but rather that an average scene is not a flat plane. (L not equal to E*R/pi).

It would be interesting to know what the model of an average scene was and also how that is interpreted by matrix metering in the different camera brands.

Post (hide subjects) Posted by
tko
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