Today’s Question: I continue to see references to luminosity masks and I am confused. Are there distinct advantages to using luminosity masks [in Adobe Photoshop] versus what can be accomplished readily with other software?
Tim’s Quick Answer: A luminosity mask is simply a layer mask (or selection) that is based on brightness levels in an image. These enable you to target adjustments to only the highlights or shadows in an image, for example. I highly recommend using the Color Range command rather than the various shortcuts that are often what I find most photographers are referring to as luminosity masks.
More Detail: Most often, when I hear photographers talking about luminosity masks, they are referencing techniques where selections are created somewhat automatically based on the brightest or darkest pixels in an image. These approaches are interesting, but I find the Color Range command in Photoshop to be far more flexible for this purpose.
A luminosity mask is most helpful when you want to apply an adjustment to all of the brightest areas of a photo or all of the darkest areas. The Color Range command makes it easy to create a selection of those areas with control over the specific range of tonal values being selected. You can even create a selection based on middle tone values rather than highlights or shadows.
To get started, open an image and choose Select > Color Range from the menu. In the Select popup at the top of the Color Range dialog choose the option that suits the area of the image you want to select, which for luminosity would mean choosing either Highlights, Midtones, or Shadows.
Once you have selected an option you will see a Range control. For Highlights or Shadows this enables you to define the cutoff between what is a highlight versus a shadow for purposes of the luminosity-based selection. For Midtones you will have two sliders that enable you to define the range of tonal values to be selected.
You can also fine-tune the Fuzziness slider to expand or contract the selection relative to similar tonal values. When you’re finished click the OK button to create the selection. That selection can then be used as the basis of a targeted adjustment for specific areas of the photo based on the luminance values.