The key to good photography is image quality. To improve the quality of my images I invest thousands of dollars in professional equipment, read how-to-books on photographic technique, and attend photographic workshops with some of the world's top nature photographers. I spend hours analyzing my work and continually challenge myself to improve. However, when it came to Photoshop, it took me years to understand that I was butchering my images with every alteration I made.

Every time an adjustment is made in Photoshop, (or any image editing program) pixels are damaged or lost. In many cases, the changes are so small they are not visible. Other times a stunning image is turned into a horrendous mess. By understanding the tools available in Photoshop, image quality can be preserved.

Image alterations can be made in one of two ways. In the first method, changes are made to the image itself. The computer is told what to do, and pixels are adjusted accordingly. Alternately, changes can be made on a separate layer which alters the appearance of the image without altering the original pixels. These layers are called adjustment layers because they adjust the image in some way. An adjustment layer is similar to a sheet of acetate that is placed on top of the image. In many ways it acts like a filter, though a wider variety of effects are possible. For example, the new layer can increase the saturation of the original image. As long as an adjustment layer is used, the image itself is unchanged and the actual image pixels have not been touched, but the end result is a more saturated image. Adjustment layers provide the computer with a set of instructions about how to interpret and display pixels without damaging the original image.

Let's see how adjustments layers can preserve pixels. To begin with, select an image and open it in Photoshop. Next, open a levels dialog box by selecting Image → Adjustments → Levels. Adjust the white and black points by sliding the outer sliders inward until they line up with the edges of the histogram and click "OK." Now, if you open the levels dialog box again you see a histogram consisting of spikes and gaps. These gaps represent lost data. By narrowing the range between the individual sliders, the computer must stretch the image data so that it covers the entire range from white to black. The result is that the histogram is stretched out and information is lost. It is true that as a single adjustment you probably will not see any negative effects in the image. However, damage is cumulative and becomes more apparent with each additional adjustment.

Now begin again by reopening the same image and apply a levels adjustment layer. To create an adjustment layer select Layer → New Adjustment Layer → Levels or click on the small black and white circle at the bottom of the Layers palette. Once the levels dialog box opens, adjust the white and black points exactly as before and select "OK." At this point the image appears the exact same as when using the previous method. However, when the adjustment layer is reopened (double click on the layer in the layers palette), the original histogram shape appears without any gaps. By using an adjustment layer, changes can be made to an image without a loss of information.

The preservation of image quality is a wonderful feature, but adjustment layers offer additional advantages to the digital photographer. Unlike adjustments applied directly to the image, adjustment layers enable me to readjust my settings at any time. Once an adjustment layer is created, I can double click on it and the dialog box will reopen. From there I can fine tune the effect until I am satisfied. Since the adjustment layer acts as a set of instructions rather then actual image changes, continually adjusting the settings does not damage pixels.

Adjustment layers also let me view before and after. Once an adjustment layer is created, its effect can be turned on and off by clicking the small eye on the left edge of the layers palette. By turning layers on and off I can determine the exact effect each layer has on the final image. Often, I find that I like the overall effect but my setting is too intense. Rather then readjusting the settings, adjustment layers let me decrease the layer's opacity setting in the layer's palette. This decreases the overall intensity of the effect.

Some photographers recommend creating a duplicate layer instead of an adjustment layer. This provides a new layer that is an exact copy of the image. By making adjustments to this layer, I can preserve the integrity of the original image. Duplicate layers do have their benefits, but they also take up a lot of memory. Duplicating an image doubles the size of the image file. For photographers working with high resolution 16-bit image files (only Photoshop CS supports 16-bit adjustment layers), this can quickly move your image files into the gigabyte range! Adjustment layers, on the other hand, take up very little memory.

By far the most powerful feature of adjustment layers is the layer mask. A layer mask provides creative control over how an adjustment layer is applied to the underlying image. While an adjustment layer provides a set of instructions about how the underlying image will be appear, a layer mask provides instructions about what parts of the image will be effected. Most adjustment layers are created with a layer mask already attached. The mask appears as a small white box on the layer in the layer's palette. If no layer mask is present, it can be added by selecting the appropriate layer and clicking the Add Layer Mask button at the bottom of the layers palette. As long as the layer mask is white, it will have no effect on either the image or the adjustment layer. This can be changed by selecting the paint brush tool, clicking on the layer mask, and then painting with black.

The rule of thumb for using layer masks is "Black hides, white shows." As I paint with black on a layer mask, the effect of the adjustment layer on that part of the image decreases. The black "hides" that part of the adjustment layer. If I want the adjustment layer to only affect part of the image, I paint the mask black over the area where I don't want the effect and leave the mask white where I do want the effect. If I make a mistake and accidentally paint black over an area I just change the color to white and paint over the mistake for the adjustment layer's effect to come through. This provides a great deal of control over how the adjustment layer will affect the underlying image.

Let's say that I have an image of a bird in flight. After evaluating it, I decide that I want to increase the saturation of the sky but not the saturation of the bird. Layer masks make this easy. Once my image is open, I create a Hue/Saturation adjustment layer and increase the saturation until the sky appears the way I want it. At this point, the saturation of the bird will also have increased. To remove the saturation from the bird, I click on the layer's mask and select the paint brush tool (set to black). Working directly on the image, I paint with black (remember, black hides an adjustment layer's effect) over the area that I don't want additional saturation (in this case the bird). The result is an image where the sky has increased saturation while the flying bird does not.

The ability to use adjustment layers and layer masks depends on the ability to selectively ignore parts of the image. To do this I need to ignore how the layer is affecting the parts of the image that I don't want to change. After all, once the adjustment is complete, I will selectively hide the effect by painting black on the layer mask. By focusing on how an adjustment layer affects only the parts not covered by a layer mask, I am able to use my adjustment layers to their fullest potential.

I have found there are some tricks to producing realistic results when using adjustment layers and layer masks. It is a good practice to rename adjustment layers so that the name indicates what the layer is doing. With complex image, it is not uncommon to have between 20 and 30 adjustment layers. Trying to remember which adjustment layer darkens a bird's feathers is a lot easier if the layer is named "darken feathers." Don't try to accomplish too much on a single layer. If I am making color adjustments to both the subject and the background, it is often better to use two adjustment layers, one for the subject and another for the background. This keeps things simple and organized in case I need to go back and make further adjustments. Whenever I finish an adjustment I turn the adjustment layer on and off to view the effect. It is not uncommon for me to make many small adjustments to an image. Unfortunately, because changes are being compared to the previous setting rather then the original image, it is easy to go overboard without realizing it. By turning the adjustment layer on and off, I have a clear understanding of the effect each adjustment layer has on the image. When using a layer mask, it most effective to use a soft edged brush and to lower the brush's opacity to between 10-25% (these adjustments can be made in the options bar directly over the image). A brush opacity of 15% means that instead of painting black, each time the brush is used a light grey color (15% of black) will be painted on the layer mask. Pressing the mouse button again and painting over the same area adds 15% more black for a darker grey (now 30% black). Using a brush set to 100% produces a complete mask that screams out "fake." By using lower brush opacity and a soft edge, I can gradually build up the masking effect without the results appearing artificial. It is important to remember that since lower brush opacity settings produce gradual changes, the impression is often that the masking is not doing anything. To check the overall effect the mask is having, hold down the shift key and click on the layer mask. This will temporarily turn off the layer mask so you can compare before and after. Finally, be willing to play. I can't begin to count the number of times that sitting down and playing with adjustment layers has revealed new possibilities.

Learning to use adjustment layers is an important step in mastering Photoshop. Through proper use, adjustment layers enable me to optimize my images to more closely match what I saw in the field as well as apply a variety of creative effects while maintaining maximum image quality.

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