In their simplest form, adjustment layers work just like the similarly named commands on the Adjustments submenu (under the Image menu) that we discussed at length in the previous chapter. However, there are two important differences:
Figure 7-3 shows a simple set of adjustment layer edits using one Levels and one Curves layer. We start by adding a Levels layer, which we use to adjust the color balancethe image is a little blue-greenand to make a small midtone lightening. We add a Curves layer to increase the shadow and midtone contrast, then we go back to the Levels layer and tweak the black point. It was easier to get the shadow contrast with the Curve when the shadow detail was left open, then go back and tweak the black point in Levels, than to set the black point and then try to tweak the contrast.
Figure 7-3. Simple Adjustment Layers (part 1)
Figure 7-3. Simple Adjustment Layers (part 2)
The ability to go back and refine previous edits is one of the strong points of adjustment layers. But what really floats our boat (or "snaps our bellybutton" or whatever your local colloquialism is) is the ability to turn global corrections into local ones using layer masks. But we've seen brain surgeons and physicists blanch at the terms "mask" and "alpha channel," so a little explanation is in order.