Gray cards : the 12% - 18% controversy

Started Apr 13, 2013 | Questions thread
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xpatUSA
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Re: Gray cards : the 12% - 18% controversy
In reply to cquarksnow, Apr 14, 2013

cquarksnow wrote:

http://dpanswers.com/content/tech_kfactor.php

Given spot-metering with a Merrill based camera, what percentage would seem appropriate for a gray card ?

Thanks,

Chris

This is a transcript from one of the links by Doug Kerr that I posted earlier:

"Now, where can we get such a gray card of 12.8% reflectance? It isn’t easy. But we can easily acquire a gray card whose reflectance is 18%—all photo supply houses sell them. If we follow the scenario above with such a card as our “metering target”, the exposure recommended or set by the metering system will be 1/2 stop less than [that] needed to produce the effect we seek. (The greater illuminance seen from the 18% card will cause the metering system to feel that a lesser exposure is required than we [actually] need, by 1/2 stop). So, after we see the exposure that the metering system recommends (or actually sets) when the meter is regarding the 18% gray card, we [could] manually force the camera to use a 1/2 stop greater exposure and use that for the shot."

So, assuming the 'Merrill-based camera' to be compliant with the ISO standard, the answer to the actual question is . . .12.8%.

Having said all that, what's half a stop between friends?

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

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