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Subject: Re: ACR Calibration



Les,

Very good post, very clear. Let me try to comment:

> Exposure itself seemed to have no effect on *relative* color balance
(ie proportion of RGB values) or relative tonal values. The tonal adjustments
alternatively had a significant effect on the absolute and relative values
for not only the neutral patches but color patches, as well. <<




Afaik, the Exposure slider in Camera Raw simply executes a so-called linear scaling; it acts as a multiplier on all RGB data like a Levels’-highlights slider. Therefore it should have no influence on the intensity ratios of R:G:B which are decisive for HSB-hue and –saturation.

BUT, big but, this math happens before the tone curve comes into play. It’s like a layer structure with the tone curve on the top. As you say, Shadows, Brightness and Contrast have a significant side effect on color saturation. And – ref. to Adobe’s presets – this side effect is not constant along the tonal scale.
Therefore, moving the Exposure slider left or right places the colors / RGB data beneath different sections of this tone curve. With a well-exposed ColorChecker, color saturation goes form high (at – Exposure setting) to low at (at + Exposure setting, close to clipping). In-between, there’s a quite stable range. Please check.

When you eliminate the tone curve by setting Shadows, Brightness and Contrast to zero, the Exposure slider should work truly linear again as described above.

> Making curves adjustments in luminosity mode does the same thing as
far as I can tell… <<




Nope, I don’t think so. Luminosity blend mode can often be better than plain RGB, e.g. regarding blackpoint setting or left-curved curves. But, just try a one-point brightening curve and color saturation will decrease … Luminosity blend mode refers to a proprietary version of the HLS color model wherein the Luminosity is computed as a weighted average of G, R and B. In many aspects it’s close enough to Lab to share its advantages, but also its downsides. Whereas Simon’s CurveTools truly maintain the ratios of R:G:B.

> Indeed, I could not get the relative values for the RGB patches to get
close enough until I made relative color corrections with the appropriate
tonal adjustments in place. <<




Interesting! This raises a basic question which I haven’t addressed carefully enough so far: Let’s assume for a moment that a camera’s native gamut would be a perfect matrix space. Would it still be possible to describe it as a matrix space based on colors which have already gone through a saturation-changing tone curve? I thought the latter would be Adobe’s strategy, to coordinate the preset tone curve with somewhat correct color. In your case, this seems to fail. That said, it has to be emphasized that such reverse-engineering always bears the risk to bark up the wrong tree. So it seems that I have to get clear on this point by some more “experimentation, mathematical calculation and persistence”. Thanks a lot for making me aware of this!

> …-an example of how Adobe could help out (even though it may be "illogical
fallacy" to think they would be so cooperative). <<




To my knowledge, the only ‘Adobe-official’ paper which mentions ACR calibration is this excellent article by Jeff Schewe. In my eyes, the chapter on ACR calibration has a certain reluctant touch.


Peter

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