Third-party raw photo processing applications need custom input profiles to make nice colour from camera RGB. For a long time now, I have making my own camera profiles with Argyll CMS. For this we need a profiling target (a test chart). All targets are not equal, as you will find in this review.
Profiling targets examined
I have evaluated the following reflective targets which are supported by Argyll CMS. I found reference data on the manufacturers websites for most charts. I don’t quote batch numbers for reference data because I believe the manufacturing is precise.
|Summary of camera profiling targets examined|
Modern digital SLR (DSLR) cameras have very wide colour gamuts. A wide gamut test chart is preferred, to exercise the camera’s response.
Below are two-dimensional gamut plots for seven targets (including two from HutchColor). The calculation of xyY is very simple from XYZ reference data. For the X-Rite charts I could only get Lab data and conversion to XYZ and then xyY is more involved (equations on Bruce Lindbloom’s site).
All targets are weak towards green. The HutchColor targets have the widest gamut. The ColorChecker Digital SG and LaserSoft DCPro targets have similar gamuts. The IT8.7/2 target provides satisfactory coverage with 288 patches. Surprisingly, the CMP Digital Target 4, with 570 patches, has a small gamut and is weak in blues and reds. Is the CMP Digital Target 4 simply a home-printed target?
Dynamic range and white point
Modern DSLR cameras have very high dynamic range and a high contrast test chart is preferred. The white patch should be bright and neutral. The dark patch is less important because it is difficult to avoid glare.
The graphs below summarise brightness, contrast and white point for the seven targets. The ColorChecker Digital SG seems best overall, with a bright (L* = 96.5), neutral (Delta-ab = 1.1) white patch and not the worst contrast ratio (White/Black = 15).
Also Observe that the medium makes a difference: the HutchColor HCT target on Kodak paper (Delta-ab = 0.7) is more neutral than Fuji paper (Delta-ab = 2.3). The Wolf Faust IT8.7/2 C1 is also printed on Kodak paper (perhaps different to HutchColor) and the white patch is slightly bluish (Delta-ab = 2.2).
Camera profiling targets are commonly photographed in sunlight, which includes ultraviolet wavelengths (UV). Photographic papers can contain “fluorescent whitener additives” (or “optical brightening agents”) which makes these papers appear more blue. These papers and printed camera profiling targets can show colour shifts towards blue when there is ultraviolet in the light source.
Metamerism is another problem for photographic and printed targets. The processes are optimised to produce colour, usually from just three of four colourants, that look natural to the human eye but actually might be composed of quite different spectra. This is a problem if the camera spectral sensitivities are different to the human eye. ColorChecker targets are made using multiple different pigments, giving reflective spectra that are more representative of the real world.
Modern DSLR cameras surpass the gamut and dynamic range of any camera profiling target. For real world photography, forget about buying the “biggest and best” camera profiling target and consider more general matrix profile models.
I have found the simple and low-cost ColorChecker Classic 24 to be satisfactory for matrix profiles. The bigger ColorChecker Digital SG is expensive, but the increased number of patches may be useful for more detailed modelling (which I am yet to attempt).
ColorChecker charts are made using multiple different pigments, giving reflective spectra that are more representative of the real world. They are not printed and I don’t expect there are any fluorescent whitener additives in the pigments. I sold my Coloraid IT8.7/2 target because the Kodak photographic paper has fluorescent brightening additives.