Luminosity Masks
I often see in the various forums here a shorthand term such as "a luminosity mask at 30%" or some other percentage. So a question and clarification, if I may.
The first step in creating luminosity masks is selecting the areas in an image whose luminosity is > 50% on the histogram. (Cmd-click on the RGB channel in the Channels palette.) And from there one can select the > 50% light tones of THAT selection, and successively narrow down up to lighter and lighter tones. And conversely one can inverse the first selection to encompass the darker than 50% areas, and work down from there. The first step gives a very feathered selection and subsequent narrowing down gives tighter selections.
But once a luminosity selection is in effect there are a couple of things one can do with it regarding darkening an image. It could be used to mask a Curves (or Levels) adjustment layer, or it could be used to copy those selected pixels from the BG layer (or any pixel layer in the stack) to a new layer above it. In that latter case a common approach is to darken that layer by putting it in a darkening mode (Multiply or others) and then lowering the opacity of the layer if necessary to mitigate the darkening effect.
But neither approach would be considered standard over the other, and no one can know which approach is used if it's not specified. Just looking for clarification. Something seems to be an accepted standard here, and I suspect it’s the latter.
In fact neither of these approaches is preferable to getting the Exposure, Contrast, Shadow and Highlight detail correct in RAW conversion, where you have many more tonal levels to push around.
The use of luminosity masks goes back to the days when we could only scan slides and try to improve the resulting limited tonalities in PS. Although it can be a quick fix, I suspect many people who are relatively new to digital darkroom processing might not be aware of the limitations of this technique.
The tonal adjustments in RAW conversion are similar to the initial "50%" luminosity mask in that they also span a widely feathered tonal range. The use of the narrowed-down luminosity ranges in Photoshop can go beyond what you can do in RAW adjustments, and is thus more useful.
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