Why Use Adjustment Layers?

Whenever you apply a Curves or Levels tweak (or even a Hue/Saturation adjustment) to an image, you're degrading it a little by throwing away some image data. Once that data is gone, you can't get it back. In the digital darkroom, this degradation is no longer an issue because you make edits to layers above the image rather than to the image itself.

If you're a photographer, think of it this way: Your unedited image is like a negative that you can print through many different filter pack combinations on many different contrast grades of paper. You can make huge changes from print to print, but the negative itself doesn't change. If the unedited image is analogous to a negative, adjustment layers are like enlarger filter packs on steroids. You can change the color balance as you would with a filter pack, but you can also change the contrast and do local, selective editing akin to dodging and burning. However, unlike their analog counterparts, you can always undo digital dodging and burning.

There are several other reasons why we love working in Photoshop's digital darkroom.

• Changing your mind. Adjustment layers give you the freedom to change your mind. If you make successive edits with Curves or Levels on a flat file, your image will quickly degrade. With adjustment layers, you can go back and change your edits at any time without further degrading the image. This gives you endless freedom to experiment, and because you can fine-tune your edits with no penalty, you're more likely to get the results you want.

• Instant before-and-afters. You can always tell exactly what you're doing when you use adjustment layers. Because all your edits are on layers, you can easily see "before and after" views by turning off the visibility for the layer you're working on, and then turning it back on again (by clicking on the eyeball in the left column of the Layers palette).

• Variable-strength edits. The Opacity slider in the Layers palette acts as a volume control for your edits (this is similar to using the Fade feature; see Chapter 2, Essential Photoshop Tips and Tricks).

• Applying the same edits to multiple images. You can use the same adjustment layer on a number of different images, and even script the layer with actions to batch-apply the effect to a folder full of images.

• Brushable edits. You can make selective, local edits to a particular area of an adjustment layer. This means you not only have essentially unlimited undo, but also selective and partial undo.

• Doing the impossible. You can use adjustment layers in conjunction with blend modes to do things that are usually extremely difficult, if not impossiblesuch as building density in highlights or opening up shadows without posterizing the image.

You can use adjustment layers as effectively on CMYK or Lab images as on RGB (though we still typically work with RGB images when we can). If you prefer to work by the numbers, the Info palette shows before-and-after values while you're working an adjustment layer's controls, and you can place color samplers to track key values just as you can on a flat file. If you'd rather work visually, you can use Proof Colors to see how your edits will work on the printed result. In fact, Photoshop even lets you preview the individual CMYK plates while working in RGB.

Adjustment layers and high-bit images

One of the biggest changes in Photoshop CS was the ability to use layers, including adjustment layers, on high-bit files. We love this capability and use it extensively, but we'd be remiss if we didn't point out that doing so can create extremely large filesthink gigabytes rather than megabytes.

In Chapter 11, Building a Digital Workflow, we discuss strategies for dealing with ballooning file sizes in much more detail. For now, a relatively simple rule of thumb is to make your biggest corrections on the high-bit data. After that, we suggest staying in high-bit mode until it hurts, or until you need to do something that you still can't do in high-bit mode, such as running Extract, or some of the more esoteric filters.

The techniques in this chapter work in both high-bit and 8-bit modes. You need to make your own call on when to downsample to 8 bits per channel based on the quality needs of the job at hand, how much time you're willing to spend on an image, and the capabilities of your hardware. You may elect to downsample to 8 bits right in the camera by shooting JPEG, or you may decide to preserve your high-bit data all the way to the output process. However, unless you're shooting JPEG (in which case, you have no high-bit data to start from), we strongly recommend that you make your big initial edits in high-bit mode, and save the high-bit file. That way, if you run into the wall in 8-bit mode, you'll still have a fallback position.