[argyllcms] colormouse vs. digital camera

  • From: Stephan Bourgeois <strangelv@xxxxxxxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Wed, 19 Oct 2005 19:37:49 +0100

Hello everybody,
this is a sequel to my previous posts about "a Digital Camera as a poor man's colorimeter". I have purchased a Colormouse (LED based colorimeter) and did the following:
a. create a 128 patch target and produce an RGB profile for the Stylus Pro XL under gimp-print
b. create a 220 patch target using the previous profile as a "training" gamut and re-profiled
I have also had access to a Canon EOS-300D (aka. "Digital Rebel") and produced a profile with my usual method: shoot the IT8 and a 700 patch target under a rostrum with xenon strobes as a light source. On this occasion I set the camera on AdobeRGB to prevent clipping on the IT8 ;-)

These are the results so far:
1. much lower errors reported on the IT8 with the EOS-300D than with an sRGB digital camera. That was expected, it accounts for the absence of clipping in the IT8.
2. there is only a minor difference between the profiles generated with the EOS-300D and the Powershot A80 !
3. the profile generated with the Colormouse has a slight red/magenta cast that can be compensated by the following gamma correction [R=0.95 G=1.05 B=1.0]
4. there is only a subtle difference between the profile generated with the colormouse and the ones from the digital cameras. The difference is in the reproduction of saturated colors. But of course the comparison is flawed by there being a different number of patches.

This is what I don't understand:
1. why is the clipping of IT8 colors when using sRGB digital cameras of almost no incidence on the final profile? Is it because the colors from the target (i.e. the printer) are *not* clipped ?
2. why is there more color cast with the colorimeter than the digital camera method? Hypothesis: the colormouse is more accurate and my screen is wrong. But then I shouldn't have a color cast when I print a gray scale.
3. Is the difference between the digital camera method and the use of a colorimeter only going to show on "prepress" jobs (i.e. trying to reproduce spot colors)?

This makes me wonder if there is a point at-all of using a colorimeter if one's aim is to reproduce photographs that satisfy what I call the "kodak criteria": neutral grays, correct gamma, pleasant skintones, fairly accurate close to the gray-axis, errors on saturated colors.

All your comments will be appreciated. Yours,

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