Photoshop Adjustment Layers 101
I've mentioned adjustment layers a number of times when writing about Adobe Photoshop here on Earthbound Light, but hadn't yet dedicated an article to the fundamentals of their use. That is, until now. If you've not yet discovered the benefits of using adjustment layers, you owe it to yourself to read on.
New users of Photoshop tend to make any needed adjustments to their images using the appropriately named Image >> Adjustments menu. When they make a change this way to an image such as increasing the saturation or brightening it up, the pixels in their image get changed accordingly. Make another adjustment, and they get changed again. With each change though, information will be lost as the values for red, green and blue for each pixel get rounded to the nearest integer. After all, it's a digital world in the digital darkroom and there's no such thing as fractions. Even if you make an image bluer and then change your mind and made it more yellow again you can't get back exactly where you were because of these unavoidable rounding errors.
As they learn the ins and outs of Photoshop, users soon become acquainted with the History palette as a way to successively remove changes they have made to an image. This seems to offer a solution to the buildup of errors from repeated edits, but there are significant downsides to this approach. Returning to a prior History state will cause you to lose even unrelated changes you've made in the mean time. If you don't like how something on one side of an image looks and decide to revert back to a prior History state and you will also lose changes made to objects on the other side of the image. That's a sure recipe for frustration as you get forced into redoing things that weren't even part of the problem you are trying to fix. And once you close an image, all of your history is gone for good. When you re-open that image, you will start all over with an empty History palette. If you feel a need to revise some of the edits you made the last time you were working on that image, you can't undo it.
The real solution to this dilemma is the use of a feature known as adjustment layers. These are a special kind of layer that contain adjustments rather than image pixels. Turn off all the other layers and an adjustment layer don't look like anything. Instead, it affects whatever image layers lie underneath it. An adjustment layer can contain most any modification that can be made using traditional Image >> Adjustments edits. But unlike the traditional method, these edits can be revised without loss or degradation at any point in the future, even after closing and re-opening an image.
To create an adjustment layer, you have two choices. Both get you the same thing, so which you choose to use is up to you. To create one via the menu, go to Layer >> New Adjustment Layer and select the type of adjustment you wish to make. Or, to save a mouse click, you can create a new adjustment layer by clicking on the small half-black/half-white circle icon at the bottom of the Layers palette and select the desired type from the pop-up list. Either way, the normal dialog for Levels, Hue/Saturation or whatever type you selected will still open up so you can make your changes. When you click on "OK" the dialog will close and the appearance of your image will look just the same as it would have had you used a traditional adjustment. The magic comes from the fact that the change you just made hasn't actually changed the underlying image pixels at all. It only looks like it has. In reality, your changes are saved simply as a list of settings in your new adjustment layer. You can see your new adjustment layer in the Layers palette directly above the layer that was active at the time you created it.
Click on ththis icon at the bottom of the
Layers palette to create a new adjustment layer
The resulting new adjustment layer
To see the magic of adjustment layers, double-click on the black and white circle icon of what you just created in the Layers palette. The original editing dialog will re-open just as you left it with all your settings still intact. If you created a Levels adjustment layer, the black point, white point and all other sliders and settings will still be set the same when you re-open the layer. If you decide you need to raise the white point a tad, you can do so safely and easily, and what you will then end up with will be identical to what you would have gotten had you chosen these same settings in the first place. And you can do this even after closing and re-opening your image.
Unlike with traditional edits, repeatedly tweaking an adjustment layer is completely non-destructive. Errors due to rounding do not build up since changes are never actually made to your underlying image pixel data. Instead, they are made on the fly as image data gets sent to your monitor. Thus, it looks like the image has changed, but it hasn't really. The same thing happens when you print an image with adjustment layers. The data from all visible image layers gets merged on the fly with your adjustment layer choices on the way to the printer. No matter how many times you revise the settings for an adjustment layer, the only time the edits are actually applied in any sort of permanent fashion is when you flatten the image into something like a jpeg to post on the web. But as long as you keep the image in a format such as Photoshop's native PSD file type that supports adjustment layers, your changes remain completely editable. And if you create multiple adjustment layers in an image, each remains independently editable, in any order, any time you want.
I've only just scratched the surface this week of what you can do with adjustment layers. More on adjustment layers next week.