Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
Shop cameras & lenses ▾
New MemberPosts: 22
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


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.

Reply   Reply with quote   Complain
Post (hide subjects)Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark post MMy threads
Color scheme? Blue / Yellow