Layer masks
Mark Galer, Philip Andrews · 2004
A layer mask is automatically attached to every adjustment layer. The effects of an adjustment layer can be limited to a localized area of the image by simply clicking on the adjustment layer’s associated mask thumbnail in the layers palette and then painting out the adjustment selectively using any of the painting tools whilst working in the main image window. The opacity and tone of the foreground color in the toolbox will control whether the adjustment is reduced or eliminated in the localized area of the painting action. Painting with a darker tone will conceal more than when painting with a lighter tone or a dark tone with a reduced opacity.