9

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?

1

There's some good detail in the other answers but I wanted to point out the challenges belied by your approach:

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

How are you measuring the D65 light, to know that it's actually D65? If you've got a spectrophotometer to measure the light, surely that can be used to calibrate the display. I suspect it would be easier to shoot indoors with D65-balanced lights (well, assuming you've measured the lights to know their color output and any correction necessary for them).

  1. Use that to get an accurate color correction matrix for the camera sensor.

If you've got the software to generate a color profile (exactly what a "color correction matrix" is), then you've probably also got a colorimeter or spectophotometer. So I will instead assume that you are thinking of simply adjusting curves to a set of numbers to make the DSLR image match the color chart.

It's worth pointing out that a ColorChecker Classic chart (such as I'd guess you're shooting) has 24 colors, whereas software meant to do this will instead make hundreds of color measurements. Your result will be significantly less accurate.

  1. Use the same camera to take a picture of the chart as displayed on the screen.

The first problem you'll encounter here: how do you know what color temperature you're working with? Sure, the monitor says "6500" or "D65," but how have you measured this to verify it's what it claims?

How are you setting and adjusting the brightness, contrast, and red, green, and blue values of the monitor to maximize what the monitor is actually capable of creating?

When we talk about calibrating a display, there are actually two parts to be done: calibration and profiling. Calibration is the process of setting the monitor to its best values to maximize what the display is capable of, and the colorimeter or spectrophotometer is doing the hard work here of analyzing the color coming off the screen and determining how to eek out just that little bit more, turning a good display into a great one, or even a mediocre display into a good one.

By simply "using the same camera to take a picture of the chart as displayed on the screen" you are completely bypassing this part of the process, which obviously means you're going to get an inferior result.

The second part, profiling, is effectively mapping what the monitor is producing to what the ideal result is. Here, again, a colorimeter or spectrophotometer is going to take hundreds of measurements to determine exactly what the monitor is capable of producing. A comprehensive look at what the monitor can produce creates a profile with a far more comprehensive result.

  1. Find the color correction matrix of the monitor, and load it into an ICC profile

How are you reading your color correction data to build an ICC profile? It's been years since I seriously worked with color management software, but I suspect it's still true that you'll need to spend some big bucks to manipulate and use external data to build a new profile. And though my experience with profile manipulation is minimal, what I found and what other experts recommend is that if the profile isn't providing the results you expect it's because the profile was created incorrectly; going through the process again is far more likely to yield success than trying to fix a bad profile.

In other words, to build a good profile you need a good calibration. To get a good calibration you need a spectrophotometer or colorimeter.

5

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.

  • 1
    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
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    @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
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    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
4

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.

  • 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
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    D65 - outdoors is eyeballing. – R Hall Jul 8 '14 at 0:47
1

"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.

1

The color temperature of the light and the angle it hits the color charts change the way the raw will look.

Take one photo, then rotate the card 90 degrees and take another photo. Then rotate it 90 degrees again. All three will look different. Of course you also have to have a meter to measure the color temperature of the light since it is not constant depending on time to day, humidity, dust in the air, etc.

And you need to calibrate your camera. Different Canons will show colors different in RAW. And cameras that use a Sony sensor will display different also. Although much information is given in some cameras reviews, does that apply to your camera and lens. If you change lens you will have a different color rendering in the raw file. Some lens are more contrasty and more saturated colors. Some are flat. Some are warmer or cooler.

I would hate to deal with all those unknowns when attempting to calibrate a monitor. But if I have a wide gamut monitor, I can calibrate it. Then use it to develop profiles for my camera and each lens. Which is important if you are making prints from your photos. Of course you have to know what light will be used where your prints are displayed as it will change the color of your print (and also your monitor).

If you are not printing, then the calibration does not matter. You will be using the lowest common denominator (sRGB) to be viewed on dispaly devices each of which may be different.

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