Advanced Mode: Luminosity Masking
Capturing a 24-bit image doesn’t leave much room for error. Higher-end cameras bump the typical eight bits per channel up to 12, employing proprietary RAW formats to capture a wider density range. That extra information comes in handy when making color and tonal corrections.
But even an ordinary 24-bit capture can hide (rather than lose) detail in the highlights or the shadows of an image. Fortunately that detail can be revealed quickly with a simple image-editing trick.
Previously we’ve published masking tricks for evening out flash exposure (where the color adjustment is a gradient) and building a contrast mask (where you want to work on both ends of the curve at the same time). This time we’re modifying just the highlight or shadows of your image.
If your image editor lets you create a selection based on the brightness or luminosity of the image and offers layers with blending modes, follow along in the software of your choice. And all the better if your software lets you automate the process. We’ll describe the process in Photoshop but we’ll use plain English so you can adapt the concepts to your own image editor.
When you make a luminosity mask in Photoshop, pixels that are pure white are selected while pure black ones are masked out. But the mask doesn’t stop there. Instead it masks pixels in between white and black according to how white or black they are. The lighter the pixel, the less is it masked.
With that in mind, the first step is to bias the selection to the part of the image containing the tones you want to modify — either the highlights or shadows.
With your image open, select the highlights of your image with the keyboard command Command-Option-~ [M] or Control-Option-~ [W]. Whites and lighter tones are selected, while darker tones are masked.
If the problem is muddy shadows, invert the selection with Shift-Command-I [M] or Shift-Control-I [W]. That selects the shadows of the image. Blacks and darker grays are selected, while lighter tones are masked.
The next step is to move that selection to a layer of its own where you can use it like a mask to alter just the selected tonal values of the original image.
So create a new layer consisting solely of the selection by pressing Command-J [M] or Control-J [W].
Now we want to blend the mask with the original image. The blending mode we need depends on whether we want to burn in the highlights or dodge the shadows.
If you’re working on the shadows, change the selection layer’s blending mode to Screen to lighten the darkest parts of your image. If you are working on the highlights, change it Multiply to burn in the lightest parts of your image.
Finally, the mask itself can be modified to accommodate more difficult subjects, either by changing its opacity, drawing directly on it or using Levels or Curves to modify the tones it affects. Keep both layers visible as you modify the selection layer to see the effect of your changes in real time.
With just a few keystrokes you can instantly bring out the blue in your sky or the green in your grass, extending the range of your 24-bit image.