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Subject: Re: ACR Calibration

The point is that Camera Raw is probably not meant to work that way (at
the side of Adobe). There’s a quite demanding test which can shed some
light on this claim: Set ACR to camera default and whitebal the second
gray, patch #20, of the target capture. Process the Raw-file at different
Exposure settings from –4 EV up to +EV, just before clipping. Repeat the
test with Shadows, Contrast and Brightness set to zero. For *every* processed
image calculate the deltaHSB-Hue and –Saturation compared to the simulated
target (for every single color patch, to build the total amount then).

Interstingly, this is exactly what I've done to verify the effects on the color patches. I also used four different inital camera exposures of the same color chart for even more fun! Some authors have suggested exposure and tonal settings have no effect on calibration but I began to doubt this given my initial lack of success. My findings are that exposure has little impact but the opposite is true for tonal adjustments in ACR. But I've not done this experiment for all supported cameras by any means!

My best guess is that the total ‘error’ over all color patches will reach
a minimum when Shadows, Brightness and Contrast are set to camera default,
and Expose is set to (–2 +/-1) EV.

Peter, I'm not sure what you mean by camera default but with your permission I will presume for now you mean that set by Adobe.

For my camera this didn't turn out to be the case. 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. Obviously, white balance effects the relative values within a given patch so this, too is essential to do first using the second patch as you suggest.

Exposure of course has an effect on the absolute color values but I've found the relative values to be of primary importance. My *adjusted* camera (tonal) defaults work fine under any **reasonable** exposure adjustment but if I apply even a slight ACR curve or tweak shadows, contrast, saturation, etc. while using any given color profile, relative color values diverge significantly. I've confirmed this while keeping the Lindbloom chart open and using ACR and/or opening the adjusted image in PS.

Also, there’s a CurveTool-plugin for download which allows to realize
tonal settings in PS without distorting hue & saturation again.

Making curves adjustments in luminosity mode does the same thing as far as I can tell (perhaps the plug in automates or improves upon this setting?). The plug in might be particularly useful if applied to the curve function in ACR, which significanlty affects color saturation AND relative color values (seems even more so than PS curves in normal mode) - not good.

Now some cameras may profile better with a certain curve set in ACR -an example of how Adobe could help out (even though it may be "illogical fallacy" to think they would be so cooperative). But for my particular camera, only a linear setting along with values previously posted would allow correct tonal calibration with the neutral patches. Your mileage may vary!

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.

I'm actually stunned that the new values have come so close to the Lindbloom chart and I'm certainly not complaining. But I don't pretend to understand why this is so from a technical standpoint. To be honest, I arrived at an acceptable calibration through lots of experimentation, mathematical calculation and persistance!

you can (mostly) thank Ronald Reagen for that.

Nunatak, careful you don't fall off that big horse of yours, pardner! RAW formats are patentable, too. Seems a little cooperation from everyone would be to our mutual benefit.


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