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Reference this excellent article by Martin Evening in a Photoshop News 9/5/07 posting.

You’ll notice that the first graphic shows a very busy Curves Dialogue Box in which the Channel selected is Composite RGB. That might be a problem.

As we explored in a recent review of Mark D. Segal’s exploration of Curves and Hue Shift, RGB Curves in PS aren’t unadulterated-there are some “behind the scenes” manipulations introduced by the programmers to produce a more “pleasing” result. Additionally, adjustments to the Composite Curve (instead of manipulating the Channels individually) can produce unintentional Channel asymmetries. Lab Curves in Photoshop, on the other hand, are pure, though I personally prefer the capabilities of Curvemeister.

Conversely, Blending Modes (at least, those addressed in the article) are straight math. They aren’t blessed with or cursed with (whatever your persuasion) any compensatory responses to input.

With that said, and realizing that if my opinion is correct a fundamental axiom of the experiment is demonstrably flawed, the article is invaluable. I strongly recommend the article for all members of PAPA, and suggest that if you have any questions on my interpretation, you contact PSN and ask them for some clarification. The point isn’t to avoid hurting Martin’s or my feelings, the point is to determine what is accurate.

I had never even considered repackaging Curves or Blending Modes as different file species, and am very grateful to Geoff for the lead, and to Martin for the article!

One other important point that may serve you well and which relates to this matter is the accurate construction of Gradients. Ensure, if dithering will frustrate your Gradient’s accuracy, that you disable it. This is demonstrated in the Gamma post of 7/21/07 (see the last paragraph in the post).

L