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Subject: Re: ACR Calibration



The following calibration details will hopefully be useful to some and lead to further experimentation/refinement with color profiling. I am a botanist by training and not a color scientist so this information is striclty observational in nature!

I followed Bruce Fraser's methodology, in part, described in his article on the creativepro site.
Assuming folks are reasonbly familiar with the basic requirments of a properly exposed GretagMacbeth color chart and so forth, one begins by using the exposure slider to match the color values for the brightest patch of your capture with the values of the target. It's a good idea to check white balance first and make necessary corrections using the second lightest patch. See Bruce's article for more detail.

The brightness slider is then used to match the middle grey patch. Bruce continues to instruct how to use the contrast slider to get the second lightest and second darkest patches to match and finally use of the shadows slider to match the darkest patch is described. One word of caution. In the end, don't worry about the brightest patch after initial exposure adjustment. Just nail the other five neutral patches.

At this point I diverged from Bruce's suggestion to use the saturation slider to make the green value of the green patch match the target. For me this resulted in a nearly grey scale image and much frustration! So, I used the global saturation slider to adjust the red value for the red patch, green for green patch and blue for the blue patch so that they were all equally divergent from the target values (started out that red and blue patches were way off relative to green). Rags describes a similar approach on his website, as well.

I then saved everything thus far (except exposure) as the basic default settings for this particular camera.

At this point I calculated the fractional difference(s) between, for example, the R value relative to the G value for the target green patch, same for the B value for this patch. I did this for the red and blue patches, as well. Recognizing I would unlikely get the actual values to exactly match the target values, I surmised that the relative proportions of R,G and B for each patch were perhaps more important to nail down. Turns out this guess was correct and in the end the absolute values came in suprisingly close, as well (but only if I left ACR tonal values at their new defaults).

One can adjust the values of say the R and B values relative to G for the green patch using the hue and saturation sliders in the color calibration tab. The saturation slider (green for the green patch, for example) will move the R and B values for this patch further or closer to/from one another as needed (use the calculation done earlier to determine target values relative to the displayed value of the primary color) whereas the hue slider moves both values up or down equally relative to the G value in this example. Same for the other two color patches. I found it helpful to use the saturation slider first and then "slide" things into place with the hue adjustment.

It took only two iterations to get the "non-primary" values to match proportionately the calculated references from the GM color chart. For example, the ProPhoto version of the chart provides values of 85,123,67 for the green patch. Rather than try to match the absolute numbers, I strived to get the R and B values to be proportionate to the G value of the green patch on my captured file, which was 129 if I recall correctly. Simple calculation yielded 89 (as opposed to the target value of 85) for the green patch R value (85/123 x 129 = new R value = 89) and so forth (use same process for all threee patches). A couple of passes with the saturation slider to get the right "spread" and hue slider to move it all into the right "range" got things very close and also coincidentally moved the red and blue patches much closer in absloute terms than they were previously (couldn't do this when I tried this as my objective, though!).

Well, for what it's worth this is my attempt at reverse engineering Adobe's reverse engineering of the camera vendors stuff for at least one particular camera. Results so far are reasonably consistent and acceptable as long as I don't attempt tone corrections using ACR (after having set the new defaults) beyond exposure and brightness. Generally, any further corrections needed in Photoshop are relatively minor (and non destructive) unless particular creative goals are desired.

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