Using the Luminance acording to luminance curve

I would interested in an opinion on using this curve. This photo was WB using the color checker. I wanted to set the white and black point according to taste by using the aforementioned curve. For subjects like this one, is it okay to simply pull the points rather than using a curve?

You need to remind yourself that this curve operates in L*a*b* color space which is designed with the perception of colors in mind. The luminance channel behaves in a non-linear way to compensate for the fact that our eyes don’t perceive luminance changes linearly. Compare this to setting a tone curve in linear RGB space where scaling is a simple matter of, well, scaling the RGB values linearly.

If talking about setting a black and white point, I would recommend one of the simple tone curves, not the Lab luminance. But of course, if you get a nice looking result with the luminance curve, it’s a perfectly acceptable alternative.

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If you don’t mind abrupt changes in gradients at the corners, then you don’t have to make your tone “curve” curved.

I played with your idea and find that the difference between setting b&w points with a simple tone curve vs the Lab luminance in a photo like this are only a very slight color shift in the frieze colors. Honestly, I could go either way. I can’t find any reason not to use a simple tone curve as you suggested.

That leaves me wondering as to what type of photos would benefit from edits in the Lab color space. As far as I can see, this one sees little if any.

Any image is better off being worked on in a method that allows full separation of greyscale and colour adjustments Mike :thinking:

I disagree. While that sounds good in theory, I find Lab adjustments give unpleasant results in practice.


Can you elaborate on that a bit more or is it enough to know that by greyscale you are referring to things like exposure, h&S, etc, and RGB or Lab adjustments for color.

Far better than me paraphrasing here…


Thanks, Andy! Slowly but surely it’s all starting to make sense.

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What I’m finding is that there is noticeable color shift when using the RGB tone curve vs none when using the LAB curves.

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Thus, making it easier to control the chromaticity (saturation) with the LAB sliders or curves.

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Yes that is correct, @stuntflyer. In LAB, you can control color with the A & B channels separate from the luminance (L channel).

That’s the ticket…

Careful - think of chromaticity as colourfulness - saturation can have a luminance component and really should be thought of as an RGB model term.

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Got it!