Layer Masks can be one of the tougher concepts of Photoshop to wrap your brain around, but once you understand the basics, anything is possible!  This will take more than one Photoshop tutorial, so we’ll break this up into multiple articles over the next several months.  Read on, and discover one of the most powerful Photoshop editing features.

What’s a Layer Mask and why do I need it?

Layer Masks give you the ability to make adjustments to your image locally, not just globally, by hiding or revealing portions of your adjustment.
 

What adjustments can you localize?  Virtually anything you can do in Photoshop, such as contrast, color, sharpness, brightness, etc.

You work with Layer Masks through Adjustment Layers, such as the Levels or Hue Saturation Adjustment layers.  Think of Adjustment Layers as “clear plastic overlays” sandwiched over your original image, with each layer giving you the global ability to control an adjustment (contrast, color, etc.) to your final image.  In addition to "global" control, you can define what areas to "localize" by shaping your Layer Mask (part of an adjustment layer) using Selections.

Quick Example - How This Works:

In Image “A”, I have a nice image, but the sky is washed out.

In Image “B”, I create a Levels Adjustment Layer, and then make a “Global” Levels adjustment for contrast to punch up the sky - but obviously the foreground is too dark.

In Image “C”, I “Mask Out” the areas that I don’t want affected by the Levels layer via the Layer Mask.  These changes show up as black in the Layer Mask Thumbnail.