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Subject: Re: ACR Calibration



Gialandra,

>>> I have heard that the best way to find the right exposure on white patch for the calibration is using DCraw (with options -3 -m -k 0)

The dcraw –m option says not to convert the camera RGB space. Thus, unless you know the characteristics of the camera RGB space, you don’t know the precise meaning of the values.

In addition, setting exposure based on the white patch is not very reliable. The “neutral 5” patch is photographic middle gray (18%). This works out to 127 for Adobe RGB or sRGB or to 110 for ProPhoto or ColorMatch RGB. Middle gray is the basis of photographic exposure metering and ISO settings. Neither black nor white is specifically defined in terms of density on the CC chart.

The camera color space (after ADC) is typically gamma 1 (a so-called linear space). Thus the values would have lower numbers than in a 1.8 or 2.2 gamma color space. The image is darker.

Maybe you should consider posing your question wherever you saw this suggestion.

Cheers, Rags :-)

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