Hi,
A few photographers have recently tested profiles made for the IQ-250 with other sensors like the D800 and Canon and seems to have achieved more satisfactory results.
I was for some time thinking about camera profiles, and I have a few questions:
1) How are they made? Are they using spectral sensivity data to calculate response for different illuminants?
Hi Erik,
Since you are referring to the new PhaseOne IQ250 ICC profile, as used in CaptureOne, I think the DNG profile info you already got is not going to answer your questions fully. Do note that the DNG profiles used in LR/ACR are something different from ICC profiles.
Although they may use a different starting point (e.g. a monochromator), I think PhaseOne starts out by shooting an image (more likely a series at different exposure levels) of a
Digital ColorChecker SG chart, which uses 140 color patches instead of the 24 of a common, or Passport, Colorchecker. They could also use an IT8 standard chart, e.g. one from Wolfgang Faust (
http://www.targets.coloraid.de/) which are usually of high quality.
Shooting such a target requires a
very neutral shooting environment/studio/lab, because we do not want any ambient light influences/reflections to be mixed in with the main illuminant's spectral output. Also surface reflections from the illuminants must be minimized as much as possible (as in a good repro setup), since that will reduce the saturation of the patches and produce over-saturated/compensated results, and light-fall off must be miniimized.
The chart comes with an accompanying reference data set (usually as a file that comes with the profiling software, or one can read the actual chart colors with a spectrophotometer), and the profiling software will then compare the camera's recorded colors after White Balancing and demosaicing with the target colors, adjust the input and create look-up tables that convert the Raw demosaiced data into the desired/expected input data. That calibrated data can then be converted with a Profile to the Working space and finally to the Output colorspace. Such profiles are Scene referred profiles, and as such well suited for transformations through the imaging chain when the other components are also well profiled. The only drawback is that they are most accurate for the exact same lighting conditions as existed during shooting of the target. The more the actual scene lighting conditions differ, the more inaccuracies can develop (but then we usually do not want accurate colors but rather pleasing colors anyway, and our eyes do not respond exactly the same as RGB trichromatic sensors either).
The software that creates the (calibration) lookup tables and ICC profile, can influence/adjust several things in the creation of that profile, which will e.g. result in more (or less) contrast and saturation. But it also allows to shift the accuracy of interpolation for certain color ranges or even the response for those colors which can shift their colors, but at the same time compress or expand other colors. It is also possible to adjust the colors of the Raw demosaiced input file for the profiling software, which will then try to compensate in the opposite direction.
2) Do camera profiles convert raw data in CIE X,Y,Z or into working space like Prophoto RGB? Converting to X, Y, Z makes a lot of sense to me.
Camera profiles (ICC type) just translate from one set of coordinates to an other set of coordinates, and there are several almost lossless conversions between coordinates systems possible. XYZ or L*a*b are common device independent coordinate systems, and it is possible to convert calibrated RGB colors into and from those coordinates.
4) How much of this is solid math and how much black magic?
I think a lot of it is solid math, although the math is adjustable to allow and deal with imperfect (trichromatic) input and match with spectrally accurate data, which is already a bit of magic (some profilers produce better results than others, so it's not straightforward calculus). But the result will not be perfect, and the tweaking that will then be done could be coined as black ( or white?) magic.
A look at some tutorials (
http://xritephoto.com/ph_learning.aspx,
http://xritephoto.com/ph_learning.aspx?action=guide) for X-Rite's i1Profiler, or at the descriptions that come with the
Argyll Color Management suite, may also help to understand better what happens in the process of profile generation.
Cheers,
Bart