Understanding how to use layer masks can allow digital photographers to easily make selective adjustments to their images. For example you may want to make your overall image brighter but not certain highlight areas. Or perhaps you want to apply sharpening to the image but not to the background. Conversely you may want to apply noise reduction to the background but not the subject. Layer masking is an easy way to achieve these goals.
WORK IN LAYERS
The first step required to use layer masks is to make sure that the adjustment you wish to selectively apply is done so on a separate layer. To do this select LAYER > DUPLICATE LAYER. You are then free to go ahead and make the adjustment you would like to make to the entire image (it will be applied only to the top layer).
For the purposes of this article lets assume that we want to apply sharpening to the image - but only to the birds facial area and not to the background or branch.
Above: Create a duplicate layer in order to use layer masks.
ADD A MASK
To add a layer mask to the layer that you made the adjustment select LAYER > LAYER MASK > REVEAL ALL
In the layers panel click on the icon to "Add Layer Mask"
Once you have added the layer mask you will see a white rectangle to the right of the layer that you are working on in the layers panel - this is the "mask".
Above: Apply a layer mask to the top layer.
The next step to layer masking is to select the brush tool and paint in black over the areas in the image that you DO NOT want to apply the image adjustment to (in our case sharpening).
If you are applying the adjustment to only a small area of the image you can also choose to paint black over the entire image and then add the sharpening to only the areas you wish by switching to white and painting in the sharpening.
This is the great thing about layer masking. It is completely reversible. Painting in black reveals the layer beneath. Painting in white covers it up (revealing the top layer).
You can have amazing control over the area you apply an adjustment to by changing your brush size, softness and opacity.
You can always see the area that the mask is being applied to by clicking the forward slash ( \ ) button on your keyboard. The area covered up in red is what you have painted black (i.e. displays the un-altered layer beneath). The area that has no red will be the area that the adjustment is applied to (in our case the area that will receive sharpening).
Above: Paint in black over the areas you DO NOT wish the adjustment to be applied to (seen in red).
Once you are finished painting in the adjustment you can flatten the image and proceed with the next step in your digital workflow.