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Subject: Re: ACR Calibration



I have tried an experiment. I would appreciate the opinion of the experts here about the value of this experiment.

I've just recalibrated my camera in ACR 3.3, using Thomas Fors' latest script. I wondered what the result would be if I printed it, so I did, (with an extra method noted below). Then I wondered whether a "round trip" method might be a good way of fine-tuning the calibration.

I have just tried the following, using the results of this latest calibration:

1. Photograph GretagMacbeth Color Checker, convert with ACR 3.3, and print as "Print-1".

2. Photograph "Print-1", convert with ACR 3.3, and print as "Print-2".

3. Photograph "Print-2", convert with ACR 3.3, and print as "Print-3".

My assumption is that errors will amplify or accumulate, (as a sort of positive feedback), and not self-correct, (as a sort of negative feedback). So problems clearly visible in Print-3 will be indicative of problems already subtly present in Print-1. (I know of an error in this logic, see below).

Does this make sense?

Results:

Print-1 was a much more credible copy of the GretagMacbeth Color Checker than I expected. It wasn't just a "representation". It appeared at first sight to be a credible substitute. The initial impression was perhaps that the Blue patch (R3C1) was slightly dark. And, as result of the later prints, it was perhaps already apparent that "Light Skin" (R1C2) was slightly too "pink". (In fact, I think it was actually slightly too "magenta").

Print-2 exaggerated the problems with Print-1. And now it was becoming clear that "Moderate Red" (R2C3) was too magenta. And Neutral 3.5, the dark grey (R4 C5) next to the Black, was becoming a bit brown. (I suspect this is the result of the printer profile error).

Print-3 was not very good! There was no doubt what it was a print of, but the above problems were even more apparent. Some of the patches were still credible: "Blue Sky" (R1C3), "Foliage" (R1C4), "Blue Flower" (R1C5), and even "Bluish Green" (R1C6), were still plausible. So were some on R2, and even "Green", "Red", "Yellow", and "Magenta", (R3C2 to R3C5), were plausible. But they wouldn't fool an expert for a second!

By Print-3, the RGB values were typically way off, except for the R4 White/Grey/Black patches. But this demonstrated that some colours are subjectively more resilient to RGB errors than others. The numerical error in "Light Skin" (R1C2) in Print-1 was less than that in some other patches, but the result was more obvious. We appear to be sensitive to skin colours in a way that we are not to foliage colours.

"Extra method":

For this to work, I needed to get the tonal balance of each print right. (The calibration tries to get the relative colours right, not the tonal balance). So I downloaded "Macbeth_ColorChecker_LAB.tif", (which I think was developed by Bruce Fraser), and converted it to Adobe RGB 1998, my normal working profile. This gave me target RGB values for the 6 White/Grey/Black patches on R4 of the GretagMacbeth Color Checker.

Each time round, I used control-click on the 6 R4 patches to make 6 points in the ACR "Curve" tab, and used the up & down arrow keys to make the ACR "color samples" of these patches match those target RGB values. In other words, I should each time have a conversion into Photoshop that would match R4 of Bruce Fraser's "Macbeth_ColorChecker_LAB.tif". (If I have the theory right!)

"Error in this logic":

My printer profile for the paper & ink I used is slightly "warm". It might be supposed that this error would accumulate each time round. But with each raw conversion, I first set the white balance, using the typical light grey (R4C2) patch, and this therefore corrected for any warmth in that patch. So this appears to be one error that doesn't accumulate with this method.

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