Adjustment Layers
To create an Adjustment Layer, choose Layer > New Adjustment Layer, and then select any of the available adjustments (Levels, Curves, Hue/Saturation, etc.), or click on the Adjustment Layer pop-up menu at the bottom of the Layers palette (Figure 11.3). Once you've completed your adjustment, it will appear as a separate layer in the Layers palette (Figure 11.4). You can think of it as if you are standing at the top of the Layers palette looking down and that the Adjustment Layers are like filters that you attach to the lens of a camera, or like a pair of sunglasses. Anything you see through that Adjustment Layer (filter) will be affected by the adjustment, while layers that appear above the Adjustment Layer will not be affected (Figure 11.5 to 11.7).
Figure 11.3. The Adjustment Layer pop-up menu at the bottom of the Layers palette.
Figure 11.4. The top layer shown above is an Adjustment Layer.
Figure 11.5. The original image is made from a total of four layers.
Figure 11.6. Adding an Adjustment Layer.
Figure 11.7. The layers under the Adjustment Layers are the only ones affected by the adjustment.
The adjustment is in its own layer; that means it is isolated from the underlying image, which allows you to retain the unmodified original. At any time you can simply turn off the eyeball icon on the Adjustment Layer and the image will return to its unmodified state. You can lessen the effect of the Adjustment Layer by lowering its Opacity setting. Since the adjustment has not been permanently applied to the image, you can also double-click the Adjustment Layer icon to modify the adjustment settings. There are also many features that can be added to the Adjustment Layer to limit which layers are affected by the adjustment, limit which areas of the document are affected, or change how the adjustment interacts with the underlying layers (that's what the rest of this chapter is all about).
What makes Adjustment Layers so much more useful than direct adjustments is that the image can be saved (in formats that support layers) and then reopened in the future for more editing of the adjustments. Adjustments made through Adjustment Layers only become permanent when you merge them into the underlying image, or save the image in a file format that doesn't support layers.
 | If setting the opacity to 100% doesn't produce a strong enough effect, try duplicating the Adjustment Layer, which will often double the effect of the adjustment (depending on the type of adjustment being applied). A generic Adjustment Layer icon (which looks like a half black and half white circle) will appear when the thumbnails used in the Layers palette are too small to allow room for the full sized versions. This can happen when the document is much wider than it is tall. |