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Archive 2012 · questions regarding using the gretag color checker chart ...
  
 
naypay
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p.1 #1 · p.1 #1 · questions regarding using the gretag color checker chart for image calibration


I have started shooting paintings for artists as a side business. But starting to see the complications arising and decided I need to perfect my technique so that if they question the digital file I can stand behind my work.

I want to make sure the files I give them are as accurate as possible. I'm shooting with a 5d3 50mm lens on tripod with mirror lockup, raw in daylight shade or shadow and taking readings and shooting the color checker card throughout the shoot just incase the lighting may change.

I created a custom camera profile for a recent shoot using the color checker card and the dng profile creator. So I load the color checker raw photo into adobe camera raw using the default settings (photoshop 5.5) Then I go to the camera tab and load my camera profile for that shoot. It looks good on screen but when I mouse over the grey squares they are way blue. Not neutral at all. Ironically when I then use the white balance eyedropper on the gray squares everything looks very yellow. When I delivered the accurate but slightly yellow images to my client she said they looked too yellow. I had to agree with her and luckily she's a friend but I need to refine this process and not sure where to turn. Even though my screen is calibrated I tend to trust numbers more than my screen for accuracy. Also artists want images that look good and even the color checker if correctly exposed looks a little flat.

What is the correct procedure to use this tool and data to get accurate color?

I'm not concerned about how it looks on my monitor--or theirs. I can't count on my clients having a calibrated monitor to view the images, and I can't rely on them to understand the technical issues. The only recourse I have is to know that what I am delivering to them is as accurate as possible.




Sep 01, 2012 at 06:30 PM
Peter Figen
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p.1 #2 · p.1 #2 · questions regarding using the gretag color checker chart for image calibration


Having shot artwork for at least thirty years, I can say that ColorCheckers, Gray cards, Kodak Color Bars, etc. are only of minimal benefit when shooting any kind of art. The reason is that the pigments in the ColorCheckers are not the same as the pigments in the art. Make the ColorChecker look great and everything else goes to hell. Make the painting look great and the CC looks like crap. They can get you in the ballpark, but if you want to do top quality work, be prepared for some hand tuning in Ps after the fact.

What we do at my studio is process the files in CaptureOne, which is much more accurate overall than LR/ACR and then fine tune in Ps using Curves, Hue/Saturation and Selective Color both globally and locally viewing on a high end calibrated monitor using a Solux light for art illumination. Basically match the high end display to the actual art as close as possible, keeping in mind that you can never be perfect.

The big problem, of course, is that you have no control over anyone else's monitor, so what you see on yours has little bearing on your client's or some art dealer across the country. All you can do is know that you started out right and let it go.



Sep 01, 2012 at 07:03 PM
naypay
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p.1 #3 · p.1 #3 · questions regarding using the gretag color checker chart for image calibration


Thanks Peter,
This is good information. I do know that all of the professional photographers I've used rely on capture one more than photoshop. I know a decent amount about digital color, and a huge amount about cmyk being a graphic designer for 30+ years, and I've been using photoshop since the day it came out. And I know a bit about photography. But I need to re-evaluate what I'm doing.

This photographing artwork started out as a friend of a friend who is an artist showing me digital reproductions of her paintings and they looked like crap and when I heard how much she paid I was floored. I said I could do it better and cheaper and it snowballed from there. Recently I've gather a few new clients and I've been trying to fine tune my process and it's getting messed up rather than better because I'm relying on tools more than trusting my eyes because I want to deliver an accurate product. But I don't have the luxury of the paintings in front of me or controlled lighting situations because the paintings at times are quite large.

The bigger issue is that I don't do this enough or make enough money to invest in the equipment you are talking about. I have about 2 shoots a month. So I guess I need to either turn down the work or manage expectations. My latest client after being unhappy with what I delivered sent me an iphone photograph of one of her paintings and asked me to match that. I explained that the image was way underexposed, why it's not a good idea, and I take no responsibility for how it will reproduce but I did it anyway. She probably didn't read my email because she sent me a reply thanking me saying I nailed it. But I don't feel good and it's making me rethink if I should be doing this.



Sep 02, 2012 at 12:22 AM
Peter Figen
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p.1 #4 · p.1 #4 · questions regarding using the gretag color checker chart for image calibration


It's a lot harder if you don't have access to the art afterward. You have to go by memory. The only way you could do it by the numbers, so to speak, would be to paint a profiling target with the same paints/pigments that were used in the painting, measure that on a spectrophotometer to get a data file, photograph that, then generate a new camera profile based on the pigments/lighting that you were actually using. Even then, it might not be perfect, but it would be close. But, it's also a hell of a lot of work for the return.

When I have pieces that I can't refer directly back to, I just have to go by feel, and see what the clients think. And then there was a project I did last year for the Wende Museum here in L.A. where I photographed 28 Russian paintings and made vinyl prints for an outdoor Artwalk L.A. presentation. When I "matched" the paintings as close as possible, they looked pretty dingy, so we all agreed to spice them up a bit, pretending that the new versions would be closer to that the art would have looked like after cleaning. Because the display was outdoors at night lit by street lighting, it didn't seem to hurt too much taking a little license.

A lot of what you're doing is influenced by the expectations of your client, I would assume.



Sep 02, 2012 at 01:32 AM
 

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Alan321
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p.1 #5 · p.1 #5 · questions regarding using the gretag color checker chart for image calibration


I have not tried using a colorchecker to make a profile but I would expect that a grey patch on the card should come out as a numerically grey patch in your image file - even if it looks like a different colour on the screen. The fact that you are not getting that numerically neutral grey patch in the file suggests a failing in the profiling process because that card is totally independent of whatever materials and colours are used on the paintings.

I would also expect that your screen calibration and profile should not affect the numeric values or neutrality of the grey patches in your colorchecker image, so what else could be doing it ? Has the colorchecker faded ? Was the card facing the camera sensor when you photographed it ? Was it parallel to the painting and was it located where the painting was ? If not, then it might have been exposed to reflected light that gave the impression of a different coloured light source. e.g. point it at the sky and will look more blue than the painting, or point it at the grass and it will look more green than the painting.

For general scenery we used to point grey cards at the camera to get exposure readings, but maybe for colour profiling suited to flat objects it is more important that it be parallel with the object than aimed at the camera. I don't know the correct answer but it's something you can ask about.

It might pay to regenerate the colorchecker profile to see if you get a different result. And try again with the colorchecker card in direct sunlight or some other lighting source and see if those profiles come up with neutral greys. At least you'll have more info to work with and then you could approach Adobe or the people who make the card for an explanation.

- Alan



Sep 06, 2012 at 04:25 PM
theSuede
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p.1 #6 · p.1 #6 · questions regarding using the gretag color checker chart for image calibration


If you mean the Adobe DNG Profile Editor - which in itself is a much better tool than the X-rite PP software - you have to be aware that the software is geared for the older (and better) Gretag CC24. The pigments are similar - but not identical - in the passport.

Another thing to mind is that the software in no way compensates for uneven lighting - any small imperfection in field uniformity (even +/- a few tenth of an Ev) over the PP surface WILL have an effect on the generated profile.

Generally you can say (as Peter mentioned before) that the PP aand the CC24 are generally not populated with a sufficient amount of unique pigments enough to enable an accurate profile for a digital camera. Most CFA's - color filers on image sensors - are slightly ragged in their responses, they're not what you would call "smooth" response curves. This means that small deviations in light spectras can effect the profile in negative ways. This is mostly regarding CFL's and energy saving bulbs- but even studio lights or sunlight is affected, but in a way that's much less detrimental to the end result.

When colors are obviously tinted (like if covered by a color filter) even when you have a perfect gray balance, that's a clear indicator that the profile has been mangled in some way.

Often I find that this kind of error arises when you have
a) placed the reference card near a very strongly saturated color surface, and get narrow-band contamination on the patches
b) have uneven exposure over the card surface
c) get a partial shadow or darker area on the card surface.

There are other products out there more suited for art-repro work, if you want a high color accuracy. Do note that high color accuracy is only a small part of good art-repro though... :-)

When you go all out, you order a complete set of ceram tiles, but then just the refence boards (that you'll have to assemble yourself...) will cost several thousands of dollars. I use it just because I have access to it.



Sep 06, 2012 at 11:54 PM
theSuede
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p.1 #7 · p.1 #7 · questions regarding using the gretag color checker chart for image calibration


I can recognize the problem with client responses, as many (but definitely not all!) clients often view the pre-delivery proofing results on very bad monitors indeed.

My usual workflow for "managing clients" is to assess their competence, and if they're then deemed to be lower than average, maybe even down on PR-department levels :-) then I always make sure to include smaller sRGB-optimized, slightly to warm WB, slightly to low contrast sighting images in a pre delivery discussion.

They will then look at those images on a crappy MacBook Air or Pro screen and go "wooo!" :-)

Then include a large ARGB or ProPhoto tiff library in the final delivery, that they can use to send to printers, use as a reference library or whatever they like.
When you deliver the sRGB images, make sure to do gamut-checks before converting, by having soft-proofing set to sRGB and gamut-warning on in photoshop. If needed, do local adjustments to the conversion in a separate layer, and name it "sRGB sat adjustments". If they ever try to print the sRGB version, they will then at least have a full contrast material, without (to much) material pinned or clipped to 255 or 0. This often looks awful in print after another colorspace conversion, much worse than on the screen.



Sep 07, 2012 at 12:07 AM





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