Today’s Question: How do you copy a layer mask from one layer to another in Photoshop?
Tim’s Quick Answer: There are several ways you can duplicate a layer mask in Photoshop, but one of the simplest is to hold the Alt/Option key while dragging-and-dropping the layer mask to the destination layer.
More Detail: When you apply a targeted adjustment or create a composite image in Photoshop, the “stencil” that causes the layer to only be visible in certain areas is called a layer mask. At times you may want to duplicate a layer mask so it can be used with another layer.
One quick option is to hold the Alt key on Windows or the Option key on Macintosh while dragging-and-dropping the layer mask to the destination layer. If that destination layer already has a layer mask, as would be the case with an adjustment layer, you’ll be asked if you want to replace that existing layer mask.
You could also use a selection as the basis of duplicating a layer mask. To load a selection based on a layer mask hold the Ctrl key on Windows or the Command key on Macintosh while clicking on the thumbnail for the layer mask on the Layers panel. You can then add a layer mask (such as by adding an adjustment layer) with that selection active, and the selection will be used as the basis for the new layer mask.
In some situations you may want the opposite of an existing layer mask. In that case, after duplicating the layer mask, click on the thumbnail for the layer mask you want to invert so that mask is active. Then on the Masks tab of the Properties panel click the Invert button to invert the layer mask to apply to the opposite area of the image.
Note, by the way, that in many cases you can avoid having to duplicate a layer mask at all by using a layer group for the layer mask, with the adjustment layers or image layers you want to mask inside the layer group.
For example, you could create a selection of the area of the image you want to apply a targeted adjustment to. Then add a layer group by clicking on the folder icon at the bottom of the Layers panel. This will add a layer mask to the layer group with that mask being based on the selection you had created. You can then add adjustment layers (or image layers) to the layer group, and they will only be visible in the area defined by the layer mask.