Lunch atop a (Springfield) skyscraper

Lunch atop a (Springfield) skyscraper
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Clearly to achieve reliable results, it is essential to calibrate key parts of the imaging pipeline - particularly the computer display on which images are edited.

Few seem to question the presumption that the only really correct way of calibrating a display is with a dedicated hardware colorimeter and associated (generally proprietary) software. I wonder what kinds of results can be achieved through somewhat alternate means; in particular, use of a (profiled) camera in place of dedicated colorimeter.

There is an open source library called ArgyllCMS which is primarily a free colour management tool designed to be used with dedicated hardware colorimeters; they have collected notes on such instruments and use with their software. *

There must be some reason why they don't even bother to mention the possibility of using a camera? is a cross platform GUI front end for ArgyllCMS, which looks quite promising for linux based workflows or as an alternative to being up-sold to top end versions of products just because one wants to calibrate two screens, for example (although I appreciate the cost of software development, I don't really think colour-management is something particularly complicated or requiring of original thought in developers).

I can see that a device which attaches directly to the screen is going to be able to eliminate the effects of ambient light on the readings and also that some devices have facility for monitoring ambient light and compensating appropriately. Clearly the sensor in a dedicated colorimeter will have different characteristics to a camera; what are the salient features?

I've got an inkling that if results good enough to be acceptable to somewhat serious amateurs were achievable without extra specialised hardware, then it would be more widely known and discussed; however I am also generally cynical about orthodox opinions stating 'you must have x or you'll never be a proper y' - something that I'm afraid internet photography forums in particular have a tendency to epitomise.

Hopefully some clear light can be shed on the matter, I think it could be generally educative to get some more views on the subject here.

A discussion from 2005 on ArgyllCMS mailing list about use of a digital camera for printer calibration

*One thing that is notable here is that Datacolor's business model seems to be particularly oriented around not only differentiating their own Spyder products on the basis of the functionality of provided software, but also obstructing the use of the hardware by third party software.

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Here is what you were searching for: Monitor Color Calibration for free using your DSLR – erik Mar 5 '13 at 6:38
Erik; that looks very promising. If you post it as an answer, I'll mark it as accepted (although I'm a little busy to actually check it out at the moment, hopefully it works well. I believe it should in theory, at least better than my utter-rubbish-capitalist-conspiracy colorimeter). – PeterT Mar 5 '13 at 16:45

The obvious problem (or an obvious problem, anyway) is that relatively few cameras have (even very close to) the degree of accuracy and repeatability of exposures to make it work at all well.

Back when magazine review budgets allowed it, some of them included graphs of camera shutter speed accuracy. Especially in faster shutter speeds, it was pretty routine to see an inaccuracy of 30% or more. Apertures varied a little less, but still quite a bit. Cameras using electronic shutters probably get the exposure time more accurate, but still have the same problem with aperture.

Just for example, almost anybody who's stitched a few panoramas learns very quickly that it's routine to have easily visible "seams" where the separate pictures are stitched together. Some of this may be due to changes in ambient light, but I'd guess most are due to the camera's exposure variation. You could reduce the variation at the same exposure by averaging several shots together, but this wouldn't tell you much about how much the exposure had really changed when you did what was supposed to be (say) a one-stop change in exposure.

Bottom line: I'd be very surprised if you could get even close to the exposure accuracy needed to produce a meaningful profile. I'd bet that a reasonably careful eyeball-based profile (e.g., using Adobe's color widget) would be more accurate than what you could plan on getting from most cameras.

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Interesting, thanks. I wonder how hard shutter speed and aperture accuracy would be to test independently, and why these are less routinely covered in current reviews. – PeterT Apr 10 '11 at 21:45
+1 For introducing the point about exposure accuracy. However, it doesn't immediately rule out @PeterT's idea. Lack of accuracy is not a problem--but poor reproducibility is. E.g., if your camera consistently takes a 1/40 sec. exposure for a nominal 1/30 sec setting, it's inaccurate but it's precise. One can measure precision by taking pictures of the same target under the same conditions at the same settings, again and again. So why not photograph a standard target on screen (no ambient illumination)? Comparing different shots will tell us how well the settings were repeated. – whuber Apr 10 '11 at 22:21
@whuber: I also mentioned repeatability... – Jerry Coffin Apr 11 '11 at 1:09
Yes, you did. But you focused on accuracy. There is some hope: one person did accurate measurements of a crummy camera shutter and found a standard deviation of 0.08 stop ( even though it was over 1/2 stop too rapid. Others report that precision is a challenge at the fastest speeds but perhaps not so much at the slower ones, which makes sense mechanically ( Thus it seems possible, even likely, that a decent SLR will have highly repeatable exposure at slow to moderate shutter speeds. – whuber Apr 11 '11 at 1:38
Whatever the characteristics of accuracy and reliability of my Pentax Kx, I would be extremely surprised if they were worse than either of the colorimeters I've used since originally posting the question. – PeterT Mar 5 '13 at 16:56

There is a very nice step by step tutorial for your idea:

Monitor Color Calibration for free using your DSLR

He adjusts his monitor settings starting with color temperature settings. After that he adjusts the color gain of red, green and blue after taking some more photographs of his monitor showing a color calibration chart.

Everything is done manually. No application was written. The guy who wrote the tutorial uses Mac OS X and a probably expensive RAW image editing software.

You can use Gimp for this, which is free (open) software or RAW-Therapee or UFRaw which are also free (open) software and work with most cameras. They work on all common operating systems.

I am sure the whole process could be improved and refined, but it is a good starting point.

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Thanks again for your input. It seems that there is still quite a bit of work to be done by someone to write a tool capable of generating more detailed colour profiles and better automate the process... I've just given you an up vote rather than accepting the answer, I must admit when I looked at it very superficially before I thought there was something cleverer going on. – PeterT Mar 8 '13 at 8:21

I can't give a technical answer but I did this as an experiment - used my DSLR to measure my monitor's color output, after I got frustrated with the skewed and inconsistent results of my dedicated colorimeter. It's a very pedestrian way of calibrating your monitor, but ultimately provided a more accurate solution than the "professional" device (which might have been broken), at least judging by vision and by its consistency when taking more test images.

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I'm tempted to do some open source work to better automate DSLR based calibration at some point... time permitting of course. :) I'm still disenfranchised by the colorimeter thing and reckon good results could be reached. – PeterT Oct 12 '12 at 16:42

Camera Colorimeter is an app on Google Play which uses the camera on your phone as a colorimeter for calibrating other Android devices.

It must be used with a companion gallery viewer app however to view color calibrated photos.

[Some experimental results]

I get very consistent capture results using the back camera on a Nexus 6. The following are 10 captured RGB values of the same (rather cool) white screen on a Nexus 7 2013 device, and their standard deviations:

       R             G           B
  0.64074441    0.82363862  0.960373769
  0.637419746   0.823843618 0.960422471
  0.635585636   0.823491139 0.961107378
  0.637959867   0.823703707 0.960575674
  0.637286725   0.823480298 0.960555295
  0.636529085   0.826055671 0.963249426
  0.637193203   0.822952933 0.961079831
  0.635713642   0.824445067 0.962552416
  0.637704785   0.82248129  0.961555964
  0.635952103   0.82470173  0.962591767

Mean 0.63720892 0.823879407 0.961406399

Stdev 0.00142383 0.00094581 0.000989068

I don't have another Nexus 6 to try and a different Nexus 7 device will no doubt return slightly different (but self-consistent) results. We're not interested in what the photos look like or how noisy they may be, but rather whether we can get a good RGB reading averaged over many pixels. Since camera phones have electronic shutters there is not a consistency problem associated with mechanical shutters in SLRs discussed by other posters here.

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Why the downvote of this answer? It is a legitmate answer that adds another valid aspect to the issue. – Bogdan Willi Jan 4 at 7:21
Not my down vote, but given the premise of the question, it would be nice to address why this app works (and can be trusted) while in general it is not usually suggested to use cameras in this way. – mattdm Jan 4 at 12:18
no wonder you get consistent results, inconsistent results are called noise and they would be clearly visible in your photos. for more thrill, you might check how another nexus 6 sees the same white screen or what values would you read from someone else's nexus 7! – szulat Jan 14 at 3:07

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