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Subject: Re: raw 3.1 professional custom profiles for highend digital cameras

Geoffrey Hervey wrote:

"As I noted before, the in-camera histogram appeared to show LESS overexposure that the ACR histogram in the three shots that I posted. I will check this out again, but, assuming that this is so, it seems backward to me. Any ideas?"

One instance when this can occur is when the subject contains large amounts of saturated red or especially blue. The histogram on most cameras is a luminance histogram, which is weighted for color according to human perception. Green is weighted the most, followed by red and then blue.

Here is an image with a lot of saturated red with the luminosity histogram shown in PS (the camera histogram appeared similar to luminosity, and it appeared to indicate underexposure). The camera histogram is derived from a jpeg preview image rendered into the working color space (often sRGB).

ACR shows a composite of the individual RGB channels and appears quite different from the luminosity histogram which gives less weight to the red.

As an aside, if you try to render such an image into a narrow color space such as sRBG, channel clipping may occur and this can sometimes be avoided by rendering into a wider gamut space such as ProPhoto RGB. Bruce Fraser covers this well in his ACR book. Here is the preview in ACR using ProPhoto RGB. The high end Nikon and Canon cameras show histograms of the individual channels in order to avoid this problem.


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