Photoshop’s Adjustment Layers
by David Arnold & Gail Rutman
Anytime you adjust an image in Photoshop (or any other image editing software) you lose valuable pixels, leading to image degradation. Multiple adjustments to an image (Levels to set the black point and white point, Shadow/Highlight to pull out shadow detail, Curves to tweak contrast or to darken or lighten all or specific areas of the image, etc.) multiply the loss.
How can you avoid this loss? Use adjustment layers.
Adjustment layers are like transparent sheets of glass or acetate stacked on top of each other, each of which contains an adjustment and its settings. You can see through to the layers below, and you can add, remove, or modify layers at will, as well as shift their position in the stack.
The layer palette gives you a cross-section view of the layer stack. If you see one aspect of an image you want to modify further (contrast, for example), you can go back to that particular layer and just make the desired change, without having to dump the whole effort and start again from scratch.
What’s more, you can save your image in layers. So if you learn a new technique, or have a client who requests a different look, or Adobe has added a new feature to Photoshop that you want to take advantage of, you can go back later and simply remove or modify the appropriate layer.
The adjustments aren’t permanent—nor are any pixels actually lost—until you flatten the image into a single layer for delivery. Always do this to a copy of the layered image, and archive the unflattened version. Though working in layers adds an extra step to each adjustment (and increases file size), in the long run you’ll save time and improve the quality of your images.
To create an adjustment layer click Layers>Adjustment Layers (or the Adjustment Layers icon in the Layers palette) and the specific tool you want to use. Almost every Photoshop book spends at least a few pages on adjustment layers, with illustrations to help you understand the x-ray view of a layered image displayed by the layers palette. Just check the index for "Adjustment Layers."
Here are some resources to get you started: Ben Willmore, Adobe Photoshop CS2 Studio Techniques (Adobe Press, 2005, Chapters 3 and 11); Dave Cross, Photoshop CS2 Layer Techniques (DVD: http://www.photoshopvideos.com, $34.99, 2 hours); Michael Ninnes, Photoshop CS2 Essential Training (2 CDs: http://www.lynda.com, $149.95, 11 hours total, 94 minutes on layers); and online tutorials at http://www.phong.com/tutorials/adjust (brief introduction to adjustment layers) and http://www.arraich.com/ps6_tips_llayers1.htm (based on Photoshop 6, but still an excellent introduction).
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David Arnold and Gail Rutman are Oregon-based photographers who have been writing about photography and computers since 1980. You can contact them at www.dgfotos.com.
Sidebar: Expand Tonal Range With Layers
Another valuable use of layers is to create an image with a wider contrast range than your camera can capture unaided. Using a tripod, make one exposure for the highlights, another for the shadows. (To maintain constant depth-of-field vary shutter speed, not aperture.) Then using layers and a Layer Mask (explained in any good Photoshop book) you can combine the best of both versions in a single final image. With raw you can even take a single image, process it twice (once for the highlights and once for the shadows) and combine the two file—though this approach won’t handle as wide a dynamic range as two separate exposures. Tutorials at http://www.luminous-landscape.com/tutorials/digital-blending.shtml, http://www.outdoorphotographer.com/content/2004/janfeb/tonal_range.pdf, and Rob Shepard, Camera Raw for Digital Photographers (Wiley, 2005 pages 281-285).
—David Arnold and Gail Rutman
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