There are two ways to apply the adjust commands. They can be applied directly to the current layer (or to a selection on the current layer) or they can be applied via an adjustment layer. We prefer the latter method because it offers the most flexibility.
Unlike normal layers, an adjustment layer affects all the visible layers below it-not just the current layer. But the beauty of an adjustment layer is that it doesn't actually change pixels until it's merged with the layer below it (Ctrl-E/Cmd-E), so you can use it to try out various effects. We think of adjustment layers as a handy way to preview color and tonal adjustments.
Here we explain how to create and use adjustment layers.
To create an adjustment layer:
Choose the layer above which you want the adjustment layer to appear.
Choose an adjust command from the "Create new fill or adjustment layer" pop-up menu at the bottom of the Layers palette (see Figure 2).
Choose a command from the Layer menu > New Adjustment Layer submenu, then click OK.
Make the desired adjustments, then click OK.
To modify an adjustment layer:
On the Layers palette, double-click the adjustment layer thumbnail (the thumbnail on the left, see Figure 3).
The slow way: Click the adjustment layer name, then choose Layer menu > Layer Content Options.
Make the desired changes in the adjust dialog box, then click OK.