Stephen Johnson Tutorial: Contrast Without Saturation Change
(excerpt from the book Stephen Johnson on Digital Photography unreleased revised electronic version)
Using Adjustment Layers to edit photographs in Photoshop is a wonderfully freeing and powerful way of working in a non-destructive manner. My most common edits involve small contrast changes in a Curves Adjustment Layer.
In color photographs, increasing the contrast will also likely lead to a perceptible increase in saturation when the Adjustment Layer is set for it's default Normal Blend Mode. This can create an unnatural level of saturation when only a contrast change is being sought. The default Blend Mode, normal is just that, the normal blend mode which edits all three grayscale channels making up the RGB file in a way that also pushes the contrast, even if such a side-effect is not desired.
A simple change in the Blend Mode from Normal to Luminosity will eliminate this saturation change, imposing the curve as though the image was currently in the LAB mode where the grayscale brightness values can be edited separately from the color.
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