One of the easiest ways of enhancing the light in a photograph is by using a Luminosity Mask. Very basically, a Luminosity Mask is the tonal luminance mapping of the lights or the dark portions of an image. In fact, some people also call this a Luminosity Map. When Photoshop constructs a Luminosity Mask it starts with neutral gray tones plus all the tones that are lighter than 50% gray. Once this Light tone map is created you can invert the selection made by Photoshop for the dark tones. As you might guess these are the tone that are darker the 50% gray. These masks are derived from the Luminance (brightness) data and not the Hue and Saturation (color) data.
So let’s move from theory to practice in creating and using a Luminosity Mask. Creating the mask is very simple. You can open the Channels Panel and hold down the Ctrl key on PC or Cmd key on Mac. You can also use the shortcut, Ctrl+2 on PC or Cmd+2 on Mac. That’s pretty much it. But now the fun begins. Just so the file will hold this information for future use, go to Select> Save Selection on the Menu Bar. Name your new channel, Light Tones and click OK. Now, go back to the Menu Bar Select> Load Selection. When the dialog box appears check the Inverse Selection check box, and click OK. This selection represents the dark tone. Again Save the selection and name the new channel Dark Tones. Save your image file but don’t close it. Your image file now has two new alpha channels, Lights Tones and Dark Tones.
Now that we have our light and dark Luminosity Masks, let’s put them to work. First load the Light Tones selection, and then apply the Levels Adjustment Layer command. This adjustment layer will only influence the light areas of the image. In the slideshow example, I moved the mid tone slider to the right to darken the flower petals. This is looking much better but I’ve lowered the overall contrast of the image with this adjustment. So next I load the Dark Tones selection, and again apply the Levels Adjustment command. To increase the contrast in the leaves, I move the mid tone slider to the right, this makes the leaves darker but doesn’t increase the contrast. To increase contrast I move the highlight slider to the left. I’m not concerned with numbers here, I seasoning this image to taste. However, as a rule of thumb, the closer to sliders are to each other the greater the contrast.
The reason I suggest using Adjustment Layers is that Photoshop will maintain my last adjustment and of course, it is non-destructive. I can also experiment with the associated Layer Mask using Levels or Curves and by painting on the mask itself. Plus I can duplicate these adjustment layers and change their order and /or Blending Modes. There are lots of powerful adjustments all found within the tonal properties of the image itself. I heard John Paul Caponigro say something more than ten years ago and it still holds true today, “Let the image make the mask.”