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It's All About the Light — Using Photoshop's Luminosity Mask
Article by Jeanette Smith

We thought we would share some of our Photoshop and image processing tips on the blog, so for this entry, I'm going to show you how to create a "Luminosity Mask" in Photoshop CS4.

First, you should know what luminance is; it's the amount of light passing through a particular area of an image. Measured in a range from zero to 255 in an 8-bit image, it is expressed as red, green and blue channels of light.

Adjusting the luminance, by taking a selection from the image itself, is great because it creates a natural gradation, based on the intrinsic brightness of the image.

Why? Because every pixel is affected in exact proportion to its native brightness. This technique works nicely, for example, when the sky is overexposed or the foreground underexposed.

Load the luminosity as a selection in Photoshop CS4 in one of two ways:

  • Option+Command+2, or
  • Command+Click the RGB thumbnail in the Channels palette.

You may also inverse the selection, Select > Inverse, or type on your keyboard, Command+Shift+I, to apply an adjustment to the opposite areas of the image. Then travel to the Layers Palette and apply a Levels or Curves Adjustment Layer to the layer that needs correcting.

You can use this technique for color adjustments, too.

  1. Load the luminosity as a selection;
  2. Copy the selection to a new layer in the Layers Palette, Layer > New Layer via Copy, or type on your keyboard Command+J;
  3. Then apply a Blend Mode, like Overlay or Multiply, to the new layer and adjust the opacity if necessary.
Don't forget to name your layers and keep things organized.

Give this a try and make your images really POP!

To learn more creative techniques in Photoshop CS4, enroll in one of our digital darkroom classes or outdoor workshops at Aperture Academy, or come in for some private tutoring. Hope to see you soon!

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