Curves

Curves aren't easy, but they are powerful. The Levels adjustment just gives you three control points (highlights, midtones, and shadows). The Curves dialog gives you up to 16 control points, opening significantly more possibilities. One option is to add a single control point and pull it up to lighten the image or down to darken. The adjustment is applied evenly throughout the entire image. Multiple points can be employed for contrast adjustments based on tonal range.

Let's take a look at Curves in action. I've chosen a grayscale file for the exercise because it is easier to work only on contrast issues when first using Curves. When you feel comfortable with grayscale images, switch to color ones, being sure to keep an eye on color shift.

Step 1. Open the file Ch09_Curves1.tif.

Control Points Made Easy

^ When the Curves Editor is open, you can automat-ically add control points by ^^+clicking (^B+clicking) within the image. The control points will appear in the editor. These can be moved up to light en or down to darken.

Step 1. Open the file Ch09_Curves1.tif.

Step 2. Launch the Curves dialog box by choosing Image> Adjust>Curves or by pressing (X+© (L+©).

Step 3. Under Photoshop CS3 (or newer) click the Curve

Display Options triangle and make sure that the Curves is set to Show Amount of Light (0-255). If you are using an older version of Photoshop, click on the small right-hand triangle on the bottom of the x-axis. This will place white at the top and right in the Curves dialog box, thus using the more familiar 0-255 scale.

Currently your curve has two points on it: one representing the black point, the other, the white point. When you add additional curves and move them, you are reassigning values.

Step 4. Click in the middle of the Curve line to a control point at the midpoint, then move that point up (towards the lighter area on the y-axis); the image will lighten. Notice that the input and output values update as you drag.

What differentiates Curves is that you have precise control over what points get mapped, whereas in Levels, you do not. Additionally, a Curves adjustment uses a curved line (rather than Levels' linear) to make adjustments that are eased through the image. It is possible to create adjustments that are easier to blend in with existing data, as opposed to Levels' frequent problem of hard clipping.

So how do you know what points to pick? You click on the image itself (actually, jj-a+click on the Mac and click for PCs). In the Curves dialog box, you can then move these new points up to lighten or down to darken.

You will notice as you drag on a point that the others react as well. Be careful, because as you add contrast in one area, it is removed from another. Radical adjustments will leave you with an undesirable posterization effect.

Because Curves are so complex, most people use a Curves adjustment layer instead of applying the adjustment directly to the source layer. An adjustment layer affects everything beneath it and has the added ability to support masks and blending modes. The results are just as effective as a traditional adjustment, but significantly more flexible. We'll look at adjustment layers in greater detail later in this chapter.

Special FX

• Curves (with arbitrary settings)

• Channel Mixer (load presets from Photoshop's Goodies folder)

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