When you create an adjustment layer, you will notice that there is a small grey square with a white circle in its center. This square is located in the spot next to the eye icon. When you see this square, you know that there is a mask associated with the layer. Masks will be addressed in more detail in section 4.

In the layer item itself, you will see two squares, one with a small graphic representing the type of adjustment, and another square that is initially blank. Between the squares you will see a small section of chain.

The blank square and chain tell you that there is a mask linked to this adjustment layer. As a matter of fact, every time you create an adjustment layer, there will be a mask associated with it. These masks are extremely powerful tools that enhance the utility of adjustment layers even further.

Basically, this mask allows you to paint on or paint off the effect produced by the adjustment layer. If your foreground colour is white (the default) you can select the brush tool, and paint on the adjustment effect. If the foreground colour is black you can paint off the adjustment effect. That is to say, as you paint the effect will be removed. Black is the background colour by default, but you can use the small curved arrow at the upper right corner of the colour squares in the tool box to switch the background and foreground colours.

This gives you extremely fine control (as fine as your smallest brush size) to manipulate the extent of the adjustment effect. You can paint on the effect in one area of the image, and you can paint off the effect in others.  When you look at the small white square in you adjustment layer that represents the mask, you can see the black strokes where you painted off the effect. Black areas of the small square will not have the adjustment effect, and the remaining white areas will.

For even finer control over your adjustments, you can paint with a grey foreground colour. Grey can be thought of as a blend of black and white. The strength of the effect that you paint on will be in proportion to the lightness or darkness of the grey. Lighter greys will produce more effect, darker greys will produce less.

As a rough example, consider the image shown above. The photo filter adjustment layer was applied making the image appear through a Warm filter. The upper part of the image, above the horizon, was painted with the foreground colour set to black. This removed the sepia effect from the upper part of the image. You can see the black area in the square in the layers palette, representing the area that the adjustment layer will not apply to.

An alternative way to create adjustment layers is to choose Layers->New adjustment layer from the menu bar. This will reveal the same menu options as the New Adjustment layer icon in the Layers palette.

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