Most people who are familiar with photo editing should know about image adjustments. These are used to affect a photo's levels in light & color. The only problem with using typical adjustments is that they can damage an image's pixels permanently.
This brief how-to will explain the benefits of using an adjustment layer instead of a typical adjustment. Non-destructive workflow is crucial to becoming a master of Photoshop. We'll be using many of the same tools and interfaces but they'll be applied onto a separate layer rather than directly onto the image itself.
Basics of Image Adjustments
Regular adjustments can be found in the Photoshop menu Image > Adjustments. These are the traditional options that apply adjustments directly to an image's pixels. Granted this method can be fine when you're editing a duplicate or don't care to save the original image – but non-destructive edits are important for large projects.
Think about how many layers are required for a custom movie poster, magazine cover, or photo composite advertisement design. Adjustment layers make these changes reversible on command.
The best way to understand how this works is through example. I'll be using a photo of some dark suburban streets to change up the colors and tone. I'll be using adjustment layers to simply increase visibility. (Photo by Sage Ross, CC BY-SA 2.0)
You can download a copy of the image from Flickr or grab the sample shot by clicking the thumbnail above. Then open this image in Photoshop and we can get started.
While regular adjustments are found in the Image menu, adjustment layers are found at the very bottom of the layers palette. Locate the small circular icon which looks like a half moon cookie (half black, half white).
This menu's adjustments will be added onto a new layer above the current image. You can adjust layer properties like opacity or blending mode to create different effects.
But the best part about adjustment layers is that they don't affect the image's pixels directly. Instead, they affect the pixels indirectly through a masked layer that applies the adjustments dynamically.
Click the half moon icon menu for fill/adjustments and move up to “Levels…”.
This will open up the traditional dialog box, however, you'll also notice that a new layer has been created. The layer uses a thumbnail icon of the histogram found in the levels box. This layer represents a new adjustment attached to the image itself.
Feel free to cancel the edit box and try playing around with some other effects. You can apply each traditional image adjustment from curves to selective color. Once you've checked out some of these different options let's apply the final step and see how these layers work.
Using Adjustment Layers
Open up the levels dialog menu and take a look at the histogram. You'll notice that much of the data is located over to the left-hand side towards the darker colors. This means our image is very heavy in the darks with not as much value found in the mids or highs.
The way to fix this is to slide the white and grey marker down towards the black marker. Ensure that “Preview” is checked so you can see how the image is being manipulated with each change.
At some point you'll reach a stage that looks good enough. You can toggle the “Preview” checkbox on/off to compare differences from the original photo. Now click OK.
At this stage I'm also going to create a new color balance adjustment by clicking the same halfmoon icon and choosing “Color Balance…”.
I feel that this photo has too many warm colors which take over because of the streetlights. This isn't bad per-se, I just want to see the photo look more natural. So I'm going to slightly boost the cooler colors of Cyan, Green, and Blue.
The Color Balance window defaults to midtones which is fine. Just a small tick in the midtones really does make a world of difference. But I've also gone into the shadows to change some of the redder shades to feel darker and more natural.
Using normal adjustment functions the pixels in this photo would have been permanently changed. You can always go back in the history panel to remove adjustments, but history is only saved in memory while the file is open. If you re-opened the file later you'd be unable to change anything.
The beauty of layer adjustments comes with the editability. If you go back to your layers panel you can double-click either of the adjustment layers to re-open the dialog box. All of your previous edits will be saved and you can adjust from there.
Keep in mind that you should avoid using the same type of layer adjustment twice on the same photo. It's perfectly fine to use many different adjustments together in order to achieve your final result. But using two different Levels on the same photo can result in oddities from already-edited pixels.
Hopefully this guide demonstrates the absolute power of layer adjustments. Serious Photoshop users should work to make this process subconscious and completely natural for all adjustment effects.
To be blunt, I would actually recommend that you never use traditional Photoshop layer effects. Adjustment layers are always a better choice for your workflow. Once they become like second nature you'll question why you ever used traditional layer effects.