Before the rush

Before the rush
by evan-pak

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The idea is simple:

  1. Take a RAW photo of a real color chart outside to get a white point of D65.
  2. Use that to get an accurate color correction matrix for the camera sensor.
  3. Use the same camera to take a picture of the chart as displayed on the screen.
  4. Find the color correction matrix of the monitor, and load it into an ICC profile

In theory this be a fast way to get the correct color correction matrix of a monitor, with only a DSLR and a chart, and without additional expensive hardware.

So, what aspect of calibration am I missing?

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In theory, this seems fine, but a cheap color calibration device gives you steps 1 and 2 already done, plus 3 done with many, many more samples than a color chart, and 4 done automatically.

I'm willing to bet that inaccuracies in the first steps plus the limited number of samples add up to less-than-ideal results. With a "real" colorimeter costing under $100, I'm not sure it's even worth the time in doing it another way.

Plus, you're inherently limited by the native color space of your DSLR. You couldn't accurately calibrate any aspects of the monitors' colors that extend outside of that. This is a limitation in colorimeters vs. photospectrometers as well, but at least the colorimeters are made for it.

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Considering that the gamut of even entry level DSLR's far exceeds that of adobeRGB, I think the last point is moot. Regarding the first two points - 100$ is still 100$ :) – nbubis Jul 6 '14 at 4:30
@nbubis - so is a reliable calibrated colour chart that's in date (about $100, I mean). You can't just use any old thing if you want better than eyeballing. – user28116 Jul 6 '14 at 4:55
I believe the assumption is "I have a colour chart anyways, can I use it for this purpose as well?", less than having to buy the equipment. – Cornelius Jul 7 '14 at 10:05

Taking an image "Outside" does not guarantee you D65 color temperature, unless measured. Also without having drivers adjusted to provide a D65 simulation on your monitor, the results can not be accurate enough to be called calibration. Even with the cheap calibration devices available it is difficult to sometimes get an accurate match between monitor and print, due to back light and ambient light issues. But even these cheap methods are better and more predictable than what your attempting. Eyeballing a displays color and judging it correct based on on or two images is like a blind man describing an elephant by touching a small part of the animal. You will soon find out that what worked once won't work very often, and your savings in cash is removed by additional adjustments, tweaks and problems.

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No one said anything about "eyeballing". A least squares fit to the color correction matrix is what I proposed. – nbubis Jul 7 '14 at 1:30
D65 - outdoors is eyeballing. – R Hall Jul 8 '14 at 0:47

"Take a RAW photo of a real color chart outside to get a white point of D65"

The simple fact that you have a blue sky will contaminate the real color chart with a blue-ish cast... making the following other steps error-prone.

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