Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
JDinOregon
New MemberPosts: 5
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Re: steephill, sjgcit, gl2k
In reply to Jack Hogan, Nov 12, 2012

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.

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