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Subject: Re: ACR Calibration



Les Helgeson wrote:

> … But I've found that saving one's initial tonal calibrations as a new
default (along with color calibration) effectively overcomes this problem.
Indeed, running Rags' (and Tom's)script(s) gave substatntially different
color profiles depending upon the initial exposure and tonal settings.
Clearly, tonal changes do have an effect on color with ACR, especially
saturation.





I now leave shadows, contrast, saturation and curves at their new defaults
(0,15,-9 and linear for my Canon 300D and 17-40 lens, for example) when
applying the new color calibration(s). These values were obtained via
an initial neutral patch calibration process. Occasional tweaks to white
balance, exposure and brightness are now generally all that are needed
prior to conversion using ACR.





It certainly would be beneficial if Adobe would provide more information
so that others might profile their cameras/lenses more easily. <<




Les,

It’s not a contradiction. My friend Simon Tindemans also likes this ‘linear’ approach. You might wish to have a look at his website. He offers a respective Script which runs an advanced ACR calibration after elimination of the default tone curve. Also, there’s a CurveTool-plugin for download which allows to realize tonal settings in PS without distorting hue & saturation again.



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).

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. This underexposure may sound contradictive to the idea of a correct exposure. But in this range the side effect from the tone curve on color saturation is quite stable (minimum delta-deltaHSB-saturation as function of +/-EV).

My second assumption is, that by no means of ACR calibration it will be possible to outperform above results substantially – means to get a significant lower ‘total error’ compared to the best of above results. From my own tests, as well as from some excellent posts by Magne Nilsen, I got convinced that unit-to-unit deviations play a subordinate role.

For some time I was believing that we just have to optimize the mechanics of ACR calibration; multi-patch analysis, etc. Now I think it’s more decisive to have the ‘right’ camera. That’s why I was asking … with less hope that Adobe will release such sensitive information on ‘the best cameras for Camera Raw’ :(.

Kind regards, Peter

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