Gray cards : the 12% - 18% controversy

Started Apr 13, 2013 | Questions thread
xpatUSA Forum Pro • Posts: 13,476
Re: Gray cards : the 12% - 18% controversy

Roland Karlsson wrote:

xpatUSA wrote:

Roland Karlsson wrote:

xpatUSA wrote:

Not only that, ISO gives what they call "latitude" so, when you set ISO 100 on the camera, it could be anything between about 75-125. A good indication might be to shoot a gray card metered to zero plus half a stop and check the histogram (with as little processing as possible, esp. brightness and saturation) - it should show a narrow distribution in middle of the horizontal axis.

The middle? How do you define that?

A trick question, I suspect

I define the middle of the histogram's horizontal axis (assuming 0-255) as 127.

Of course, it could also be defined as 118 depending how gray you like your mid-gray . . .

Yes, it is s trick question

So - you assume JPEG with a gamma curve?

Eek! Another trap? I should have clarified further:

If I shoot on my raw-only SD10 I would be referring to the in-camera JPEG thumbnail, which is not adjustable by the User. It reflects the exposure, without all those stupid scene modes or choices of color space.

If I convert RAW to RGB linear the gray distribution would be crammed up on the extreme left and would be of no use to man or beast.

If I convert to TIFF ProPhoto, gamma 1/1.8, D50, it might well depend on the viewer.

If I converter to JPEG (8-bit sRGB, gamma 1/2.4 linear 12.95, D65) it might well turn out that mid gray is 117/118 or close to, assuming no editing other than conversion.

Anybody done the test yet?

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