Here's the essential idea:
- Use Luminosity mode when adjusting light/dark tones (or for sharpening)
- Use Color mode for color-specific edits
Actually LAB mode has native capabilities that essentially do the same thing (since L is a separate lightness channel, and 'a', and 'b' are the color channels). I've been using the RGB mode primarily though (so far), and for this color mode (RGB: red, green, blue), both the lightness and color information is mixed together, so adjustments intended only to effect light or dark values, might tweak the colors as well. These artifacts might be subtle, but over several adjustment layers, I found myself noticing the impression of my image to be increasingly 'off' from what I was expecting. Being very specific about whether the adjustment layer is intended for the light/dark (luminosity) or color and setting the Photoshop layer modes accordingly has helped me to create adjustments that function as I originally intended.
If your processing technique is only intended to effect the light or dark values, then try changing the layer mode from 'Normal' to 'Luminosity' in order to prevent side effects of your adjustment from unexpectedly effecting color tones. Adjustment layers where you might find this useful are:
- Curves or levels adjustments to brighten or darken the midtones, or overall dynamic range
- Sharpening techniques (since it is usually achieved by increasing contrast around edges)
Sometimes, I like when the 'Multiply' layer mode adds some color saturation as it darkens, but sometimes I don't (which is when I would use this Luminosity clipping technique). If you want to use photoshop layer modes like Multiply (darkening) or Screen (lightening), you can also 'mask' these effects with the luminosity mode by adding a new 'dummy' adjustment layer (I usually use levels) and "clip" the light-related adjustment adjustment layers to it. (alt/option+click the line separating layers to clip the top layer to the bottom) Then set the dummy layer mode to 'luminosity', and any layer clipped to it (you can stack them) will be restricted to luminosity-related changes only.
Likewise, if your processing adjustment layer is only intended to effect the color, I would try changing the adjustment layer mode from 'Normal' to 'Color' which would prevent unexpected light/dark tone artifacts. I find this most useful for:
- Local Color adjustments (fixing lens flare artifacts, toning down a distracting element, ...)
- Vibrance/Saturation adjustments
So, that's the basic idea. I've been finding it very helpful in all my recent processing, and hope you find it useful too!
Blog Post By Laura A Knauth