The Shadow/Highlight command, which debuted in Photoshop CS, is probably the single most useful addition to Photoshop's Adjustment submenu since version 1.0. Unlike all the other commands on that menu, all of which treat each pixel the same way, Shadow/Highlight is an adaptive command that evaluates and adjust each pixel differently depending on the values of its neighboring pixels. (That's why it isn't available as an Adjustment layerShadow/Highlight has to calculate the value for each pixel, and it would have to recalculate not only whenever you changed the Shadow/Highlight settings but also when you changed any layers underneath, leading to agonizing slowdowns!)
Shadow/Highlight is a contrast-reducing tool: It lets you recover detail from nearly-blown highlights and nearly plugged shadows in a way that Curves or Levels just can't do, because where Levels and Curves shift all pixels at the same level by equal amounts, Shadow/Highlight evaluates local contrast and preserves it by making adaptive, context-sensitive adjustments.
In its default form, the Shadow/Highlight dialog box is very simplefrankly, too simple to be of real use. Turn on the More Options checkbox to reveal the full capabilities of Shadow/Highlight (see Figure 6-71).
Figure 6-71. Shadow/Highlight dialog box
In its expanded mode, Shadow/Highlight offers Amount, Tonal Width, and Radius sliders for Shadows and for Highlights; a Color Correction slider (on grayscale images, it's replaced by a Brightness slider); a Midtone Contrast slider; and clipping percentage fields for black and white. The interaction between the controls is fairly complex, so let's look at them in turn.
The Amount sliders for Shadow and Highlight control the strength of the correctionthey're the "volume knobs."
The Tonal Width sliders dictate how far up the tonal range from black the Shadow correction applies, and how far down the tonal range from white the Highlight correction applies. They let you constrain the correction to the tonal range that needs it.
The Radius slider dictates the neighborhood that the command evaluates for each pixel, thereby affecting local contrast. A good rule of thumb is to set the Radius to the size of the features you're trying to emphasize. If you set too large or too small a radius, you tend to lose detail rather than emphasizing it. The easiest way to master the Radius slider is to experiment with it.
The Color Correction slider is a saturation control, but it operates only on the corrected colors, not on colors that are unchanged by the correction. Increasing its value saturates the corrected colors; reducing it desaturates them. We generally find that the default value of +20 is usually about right, but don't be afraid to experiment.
On grayscale images, the Brightness slider replaces the Color Correction slider. It lightens or darkens the imageunlike the Color Correction slider, it affects the entire image, not just the pixels that are affected by the Shadow and Highlight adjustments.
The Midtone Contrast slider adjust contrastnegative values decrease contrast; positive ones increase it. While its effect is most pronounced on the midtones, it affects the entire image, so use it carefullyif you push it too far, you might just end up undoing most of the effect of the Shadow and Highlight adjustments.
Black Clip/White Clip
The clipping percentages dictate how many of the corrected pixels are mapped to black (0) and white (255), so these controls let you adjust the contrast of the pixels close to black and white. Note that unlike the clipping percentage fields in Levels, these use three decimal places. Use them to fine-tune the contrast near the endpointswhen they're set too low, the image can turn muddy.
Figure 6-72 shows a step-by step Shadow/Highlight correction of an image with blown highlights and plugged shadows. The result would be impossible to obtain with Curves or Levels!
Figure 6-72. Shadow/Highlight adjustment
Shadow/Highlight is an amazingly flexible tool. Figure 6-73 shows some additional examples of its power.
Figure 6-73. Shadow/Highlight adjustments