A recent article posted here appears to assume that the purpose of calibrating ACR, (and by extension, Lightroom), using scripts such as the Thomas Fors script, is to cater for differences between cameras of the same model. I disagree!
I believe that the primary purpose of calibrating ACR or Lightroom using such a script is that the default Adobe method of calibration gives results that are unacceptable to many people, while the use of such a script gives results that many of those people prefer. To be very simplistic indeed, these scripts correct the Adobe tendency to give reds that are too orange, and grass that is too yellow.
Have a look at the many results of using such scripts for many camera models that have been published in various forums. They nearly always have a significantly negative Red Hue, and a significantly positive Red Saturation. (The values in the ACTUAL cameras that Adobe used originally would also give such results!article are respectively: -9; 20). This tendency is obviously systematic, and not to do with within-model variations which would give values either way. I suspect that using such a script on the
These are not results from anti-Adobe people, or people with no knowledge of colour management. These are typically results from people who own a GretagMacbeth ColorChecker, who care enough about their colour balance to use the ColorChecker and a suitable script to try to get things right, and who then share their results so that others can benefit. We don't have all the answers, but surely we can raise serious questions!
Most of us agree that the purpose of calibration with the Fors script or using Bruce Fraser's manual method is to cause the pixel values of the patches to be as close to the published nominal values as possible. The above methods adjust for only the primary colors of the red, blue, and green patches, and one hopes that the values for the other patches will also fall into line.
I used the most recent Fors script (ver 1.0) and ACR 4.0 to calibrate my Nikon D200 and got the following values: tint -1, red hue -20, red sat 21, green hue -4, green sat 8, blue hue 6, and blue sat 2. The red values are in the range Barry has documented. If the purpose of the calibration is merely to compensate for variations from camera to camera in the same model, the values of many calibrations with different cameras should have a bell shaped distribution with a mean of zero. Pending further data, it would seem to me that Barry is on to something.
The standard way to evaluate color error is with ΔE*ab, which is the Euclidian distance between the measured and ideal (reference) values in the a*b* plane of the CIELAB color space as explained by [a href=\"http://www.imatest.com/docs/tour_colorcheck.html]Norman Koren[/url] on his Imatest web site. His Imatest program can plot the ΔEs for the Macbeth Color Checker. Results for many cameras are given on the Imaging Resource
web site, and a link to results for the Canon EOS 5D is shown. Readers should refer to the Imatest web site and the Imaging Resource web site for information on the interpretation of these plots. Color error can be with Chroma (saturation), in which case ΔE lies on a radial line extending from the white point, or with hue, where ΔE is not on this radial line. Increased saturation (positive chroma error) is not necessarily bad: many people prefer saturated blue skies and green grass. However, hue error in the form of purple skies or yellowish grass is usually not desired.
Here are plots of my recent calibration for discussion. Exposure was in direct sunlight with the checker mounted on a black background and care was exercised to that no colored objects that could reflect colored light were near the target and exposure was made according to the Imatest suggestions. The red, blue, and green patches are 13, 14, and 15 on the plots.
ACR default. Note the reds are in the yellow direction as Barry noted.
ACR calibration with brightness 36 and contrast -34 as determined by the script for the low contrast target:
ACR calibration with brightness and contrast at their default ACR values of +50 and +25:
Nikon Capture NX for comparison:
The Nikon Capture rendering is not that much different from that of the default ACR. The script calibrated results with the contrast and brightness as set by the script gave a very accurate result, but one does not use these contrast and brightness settings for normal photography. With the ACR default contrast and brightness but with the script calibration settings, the overall saturation is increased as indicated by the chroma of 134% as reported by Imatest. However, the chroma corrected color difference (ΔC) is quite small. The increased saturation could be handled easily in ACR or Photoshop with the saturation control, and the calibration was quite successful.
Comments and additional calibration results are welcome.