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I'm trying to get accurate colors for a digital-photographed painting. The photo includes a color card ("Kodak Gray Scale", with cyan "C", yellow "Y", magenta "M", and a 19-step gray scale going from A(0) to M(7) to B(16) - similar to this). The color card makes it clear that the photo is not accurately representing the colors... the goal's to fix that.

I've talked with museum professionals who say "use Raw! Photoshop's too hard" but then they aren't Photoshop geeks. The workflow they described to me (and I don't know Raw, so I'm just repeating what I was told by a digital imaging novice) is to "eye dropper" the #2 gray "to equalize R/G/B", and then adjust "one of the sliders" (exposure?) to bring R, G, and B to 200.

Anyone have a good Photoshop workflow for this?

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3 Answers

The hard part of this task is to define what "accurate colors" means. For different lighting conditions "accurate colors" means something different. So is the accurate representation the one of how visitors of an exhibition see the painting, or the one where the photo has been taken under laboratory conditions? You need a clear vision what the final picture should look like and then search for means to get there. Maybe you have to find the preferred look together with your client ...

Scott provided several good sources. Here are some keywords you can search for additionally: "post processing white balance", "post processing gray card".

If the picture needs to get printed make sure to use "View" > "Proof Colors" in Photoshop.

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I want to second the lighting advice: I actually do exactly what described in the question quite often. If you evaluate all your proofs in "lab lighting" or 6500K, ALL of your proofs will look wrong in museum or gallery conditions. If you are doing a catalog of paintings for a retail operation, your catalog will be judged most unfavorably when the catalog is held up to the painting on the wall--something the gallery owner and their clients will judge you on. –  horatio Jun 4 '13 at 17:10
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This is a huge topic. Each image is different. There will almost never be one workflow or setting which is going to work for all images. You need to treat each image individually. Sure equalizing RGB and then bringing them back to 200 may be sufficient, but that's like buying the same size clothes for all three of your children - might work for one, but not fit the others.

Rather than recite what has already been written so many times in great detail, I'll simply provide some sources to learn from.

With Levels: http://www.photoshopessentials.com/photo-editing/tone-color/

With Curves: http://psd.tutsplus.com/tutorials/photo-effects-tutorials/quick-tip-color-correction-in-photoshop-with-the-curves-adjustment-tool/

A video tut: http://layersmagazine.com/photoshop-color-correcting.html

Books: 1 2 3 4

(Note I'm not endorsing any of the above links personally, merely providing them for quick access)

The primary difference between all these links and your question is that you have a specific target to shoot for whereas many tutorials and books cover general guidelines. However, all the methods work when aiming at a specify tone as well.

Note that if you shoot in RAW, Photoshop does have the Camera RAW plug in which will allow alterations there.

Adobe Camera RAW Help

PeachPit Press Camera Raw tutorial

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Here is how I work with paintings for print:

Calibrate your monitor and then compare your calibrated monitor to (professional) test proofs so that you can be sure that you are not wasting your time.

Use Remote control software: I have a camera which can be operated from a computer remotely via USB. The software provides a histogram and full-size preview and allows me to name the files as I take them, and save them to a network or local disk. The histogram is especially useful as you can be sure you aren't clipping bright areas (overexposure). In most cases it is better to choose an underexposed image because there is still detail to work with. Overexposed areas have no detail to salvage.

Polarizing filters on lens and lights: Most paintings have glare. Polarization removes this but mess with red and yellow saturation.

Shoot RAW: I have a consistent photo setup so every now and again, I manually develop the best basic exposure curve and store it as a preset which I automatically apply to all shots I make in that setup. I can do this because the camera setup and lighting is all consistent and the only variable is the painting being photographed.

Archive the RAW: RAW can be a thought of as a negative. People rarely exhibit negatives

If you cannot work with RAW: take a picture of a photo-neutral white object in the lighting you are working with and then use that photo as the "custom white-point" in your camera. Set the file type to TIFF or uncompressed (not jpeg). (if possible, check your manual for both).

Open a working copy with the curve (mentioned above) applied. Convert to CMYK. Save as lossless TIFF.

Use "select color range" tool to select the grey most neutral (in the painting, not the photo) and closest to around "75% grey value" (dark). The color range set to maximum fuzzyness. This is really the only time the Kodak card is useful. I have access to file drawers full of 4x5 transparencies with cards, and about 0% of the time is sampling or setting a magic point on them effective.

While still selected, hide selection marquee so it does not interfere with your perception. If needed, tweak levels.

While still selected, hide selection. Adjustment: Selective Color. Choose "Neutrals", (adjust neutrals ONLY--don't mess with anything else at this time) make minor adjustments. (Note that it if you think "too much yellow," you might want to add cyan and magenta rather than remove yellow.)

If you still think it needs overall adjustment, repeat the previous step but select a light non-white area and adjust "whites" instead of neutrals.

Don't make sharpness adjustments until after color correction.

There are many pigments which simply cannot be seen by a camera. Certain blues will look purple. You will need to select these areas using the same technique above and make specific adjustments on a case-by-case basis.

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