Quote:
Originally Posted by PhotosbyChuck
Most cameras' reflective metering will give an accurate reading of exposure on an 18% gray surface. That's exposure ... not color balance. However, many people do use 18% gray cards as a way to set white balance -- often with disappointing results since the cards were a way to precisely meter density, not color.
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As long as the card you use is really gray, or actually any neutral color (having equal parts of r, g and b) then it will work just fine for CWB (Custom White Balance). Here are a couple of demo shots to show this. Both images are of a Macbeth card. I used a gray card (Photovisiion Calibration target (middle stripe)) to set the CWB and then took the shot of the Macbeth. On the second shot, exact same thing, except I used the white stripe to set the CWB. As you note the two Macbeth cards are almost identical.
Keep in mind I used the target shown below to create the reference shot that was used to set CWB for each of the above. I just made sure that in one the gray part was in the center area and for the other the white part was in the center area (Canon 20D uses just center area for CWB).
And here is what the target looked like
Cheers,
rfs