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I just got a grey card which comes with a 11 levels grey scale :

L 18.9 | Ev −2 | Ev −1.5 | Ev −1 | Ev −0.5 | L 50 Ev 0 | Ev +0.5 | Ev +1 | Ev +1.5 | Ev +2 | L 98.2

How do I take advantage of this chart to calibrate exposure in post using RawTherapee ? Should I set L* to 18.9 % on the black end and 92.2 % on the white end ? And are "black" and "exposure compensation" the right settings to adjust ?

This answer to Unclear whether grey card can be used for exposure in post production? provides hints, but RawTherapee's highlights & shadows sliders don't seem to provide the expected effect, and most online resources focus on white balance only.

  • A gray card is not for white balance calibration. It is for exposure. The ranges are probably for dynamic range. White balance is only if the grays are really neutral gray. – Rafael Sep 10 at 15:27
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    @Rafael And it’s off-topic anyway, question edited. – Skippy le Grand Gourou Sep 10 at 15:36
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After some further reading I feel a reasonable workflow would be

  1. set white balance using the pipette on the 18 % grey area ;
  2. 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 ;
  3. adjust the white point with the Exposure compensation slider ;
  4. adjust the black point with the Black slider ;
  5. 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.

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