How can I use a gray scale card to set exposure and black/white points in RawTherapee?

Asked 4/7/2020

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2 answers

I have a gray card that includes an 11-step grayscale, with patches labeled from roughly L* 18.9 at the dark end to L* 98.2 at the bright end, and 18% gray / L* 50 in the middle. I’d like to use it in RawTherapee to correct exposure in post, not just white balance.

What is the right way to use this chart? Should I target the dark and bright patches with the Black and Exposure Compensation controls, and use the 18% gray patch for white balance?

white-balance post-processing rawtherapee gray-card exposure

Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

6y ago

2 Answers

After some further reading I feel a reasonable workflow would be

set white balance using the pipette on the 18 % grey area ;
add a color picker (I wonder if there is a better tool ?) in the black (L 18.9) area and another in the white (L 98.2) area ;
adjust the white point with the Exposure compensation slider ;
adjust the black point with the Black slider ;
go back to step 3 if necessary until the color pickers values are as close as 18.9 % and 98.2 %.

It seems to give reasonable results, though I wonder if this is the best workflow.

Originally by user43103. Source · Licensed CC BY-SA 4.0

user43103

6y ago

AI Answer

Generated from our catalog & community — verify before relying on it.

A practical RawTherapee workflow is:

Use the white-balance picker on the 18% gray / L* 50 patch.
Place readouts or color pickers on the darkest patch (about L* 18.9) and brightest patch (about L* 98.2).
Adjust Exposure Compensation to bring the bright patch close to its target.
Adjust the Black control to bring the dark patch close to its target.
Re-check and iterate, since changing one can affect the other.

That matches the intended use better than relying on highlight/shadow sliders, which are more for tonal shaping than setting reference points.

So yes: using the mid-gray patch for white balance, Exposure Compensation for the white end, and Black for the dark end is a reasonable approach. The key is to use the grayscale as a reference and fine-tune until the sampled values are close to the chart values.

Just keep in mind this is a practical correction workflow, not a formal camera calibration process.

How can I use a gray scale card to set exposure and black/white points in RawTherapee?

2 Answers

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