Objective exposure and color accuracy with ACR

SS
Posted By
Steve_Snow
Oct 4, 2006
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287
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7
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While I am familiar with the strategies and techniques for using curves to "correct" color in an image, I would like to get even more "objective" color accuracy with RAW images.

I want to shoot a perfect image of a Color Checker (using a handheld light meter for calculation of the precise incident exposure under highly clontrolled and repeatable studio lighting conditions), run it through ACR, and the apply software (like Gretag Macbeth Profile Maker) to create a profile that can be applied for all shots taken under identical lighting circumstances. Exact representational color accuracy is critical for the these images.

Unfortunately, the adjustment tabs for exposure, brightness and contrast are completely arbitrary and seemingly MUST be applied to the image before getting it into CS2. I am completely unclear on what "neutral settings" are. "25" for brightness?? "50" for contrast?? How do I get the CC image out of ACR with minimal alteration while maintaining the perfect lighting for the exposure so it can be accurately and appropriately analyzed by a profiling software? If the target image is altered, then the resulting software analysis and subequent profile are rendered useless.

I need to know what "ground zero" is for ACR adjustment settings. Thanks for your help.

Steve Snow

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Peter_Figen
Oct 4, 2006
ACR is the wrong tool for what you want to do. There is no way to use custom profiles with ACR. It always interpolates between a daylight and tungsten profile that was built the supported camera type. While you can calibrate ACR to your needs with the controls in the advanced section, it’s not exactly the same as what you’re looking for. You want something like CaptureOne or RawDeveloper which can use real custom ICC input profiles, and also have better raw processing capabilities than ACR.
C
chrisjbirchall
Oct 4, 2006
I need to know what "ground zero" is for ACR adjustment

Selecting "Camera Raw Defaults" from the Drop-down is effectively "ground Zero"

I would like to get even more "objective" color accuracy with RAW images

Most times, shooting a grey card and using the eyedropper will do this.

You can use the calibration tab to make fine adjustments for individual cameras.

Try the ACR forum for more in-depth advice on this one.

Chris.
MV
Mathias_Vejerslev
Oct 4, 2006
Most times, shooting a grey card and using the eyedropper will do this.

I have some notable exception images that can’t be corrected in ACR or LR. They are tungsten concert photos; the stage is covered in different color spots, yet any tweaking of temp and tint will only give me a kind of washed out pink look (temp is down to 2000).

ONLY opening and using levels and curves can correct these images somewhat.

I’d prefer to be able to get a more correct tone balancé out of RAW, but it doesnt seem possible without drastic recalibration for this set alone.
SS
Steve_Snow
Oct 4, 2006
Thanks for the ideas.

Selecting "Camera Raw Defaults" from the Drop-down is effectively "ground Zero" . . . .

When I try this, my whtie patch on a Gretag Macbeth 24 patch CC with a metered exposure should be in the neighborhood of 240 RGB level. I find it to be in the neighborhood of 180-190 . . . grossly underexposed. I may be starting at "ground zero" as far as pixel processing goes, but I don’t seem to be starting at ground zero when it comes to exposure accuracy.
C
chrisjbirchall
Oct 4, 2006
but I don’t seem to be starting at ground zero when it comes to exposure accuracy.

Correcting exposure (and colour balance) and getting the exposure correct, are two different things.

The more under exposed your original capture is, the more difficult it will be to obtain a correct tonal – and white – balance throughout the more limited dynamic range of the image.
C
chrisjbirchall
Oct 4, 2006
some notable exception images that can’t be corrected in ACR… [ ] …tungsten concert photos; the stage is covered in different color spots…

Quite right Mathias.

I had a job only yesterday for a manufacturer of car protection products. We had to photograph black PVC covers in a car showroom with daylight from the front and tungsten from above.

Using the eyedropper to set correct white balance for the front (daylight lit) cover, resulted in strong orange reflections on parts lit by tungsten.

To correct this, I opened a Hue/Sat layer and selected Red from the drop-down and then yellow, reducing their relative saturation levels to eliminate the cast.

Then by painting out parts of the layer mask, restored correct colour to the rest of the image.

Time taken – about three minutes. How much longer would it have taken to move the car, mask out the daylight, switch off the tungsten and light the whole thing with a balanced light source.

Thank Thomas for Photoshop, eh! 😉

Chris.
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<http://www.beautyphoto.co.uk/proofs/car.jpg>
MV
Mathias_Vejerslev
Oct 4, 2006
Yes, Photoshop is a wonderful plug-in to ACR.

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