Proper Blind Exposure with a Gray Card

Started Nov 12, 2012 | Discussions thread
OP Jack Hogan Veteran Member • Posts: 6,160
Thanks Mack

That's very helpful, and I understand that there were many variables in the old days in part because of the very different shapes of characteristic curves of many types of film available.  Today though DSLR sensors characteristic curves all look like parallel straight lines because modern sensors are linear and therefore all pretty well vary 1:1 with Exposure.  The only difference in this context is where their saturation exposure falls (Hsat - see my image above).

So the only variable left to determine is this: if I spot meter and take the picture of a neutral card where along the scale of raw values, from zero to maximum, should the camera record it?

Please note that the question of card reflectance hasn't come up.

The standards (ISO 2240, 2720, 2721, 12232:2006 and ANSI/NAPM IT3.302-1994) give the camera manufacturers lots of latitude to decide, but include a recommendation for DSLR sensors: recording the spot metered neutral card at 1/7.8 of the maximum raw value.  That's 12.8% of full scale.

Please note that the question of card reflectance hasn't come up.


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