# Gray cards : the 12% - 18% controversy

Started Apr 13, 2013 | Questions thread
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 Re: The grey card is a reference for print making In reply to Roland Karlsson, Apr 17, 2013

I was wondering why it says "Merrill-based" in the OP, so I actually went and read the link. To my great delight, it's author used an SD10 to do a test like mine above with similar results. Bearing in mind the variations in ISO performance mentioned by both myself and Roland earlier, we can only expect +/-25% accuracy of exposure (say). So, am I correct in thinking the camera can be off by about 1/3 stop? Any camera, that is.

As has already been said, 1/3 of a stop is peanuts compared to the exposure compensation needed for most scenes, unless you shoot gray blankets or English skies for a living

Back to the OP, I lean toward the following view in the afore-mentioned link:

"But should middle grey be rendered as 127.5 (the mean value between 0 and 255)?

A slightly more sophisticated argument says that 18 % reflectance should be rendered as 117. This number is computed from this formula:

ζ^1/γ x 255

where ζ (zeta) is the reflectance and equal to 0.18 (18 %) and γ is the gamma and is equal to 2.2 (which it is supposed to be these days)."

That would be for the perfect camera (e.g. an SD10) shooting a perfect "18%" gray card at exactly the correct angle on a perfectly gray day . . .

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Regards,
Ted http://kronometric.org
SD9, SD10, EF-500, GH1.

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