LeVel-headed you!

The ImageOAdjustmentsOLevels command (^+L/Ctrl+L) gives you control over shadows, highlights, and your image's overall tonality individually. Using a slider with three controls, you adjust the picture both to suit your eye, and with an eye on a histogram for reference. You even have numeric fields in which you can type exact values, should you find the need.

To perform the basic Levels correction, spreading the image's tonality over the full range of values available, you simply drag the slider controls under the histogram in the Levels dialog box inward until they're under the point where the histogram begins to rise in a mountain shape. Ignore those little flat tails that extend outward — they represent individual stray pixels — and drag the little pointers under columns that are at least a few pixels tall. The histogram in the Levels dialog box (as shown in Figure 5-9) is for reference as you make changes. Note, however, that while you work in Levels, the Histogram palette updates, showing you the "before" (in gray) and the "after" (in black).

Figure 5-9: Compare the Levels histogram and the Histogram palette.

Defining white and black

The Options button in the Levels (and Curves) dialog box opens a door through which you might never need to walk. However, should your path lead you to that door, I want you to know what lies behind. Neither pit nor tiger awaits you, only the possibility of controlling your highlights and shadows — or making a total mess of your image, of course. (Adobe does a good job of hiding those it-could-cost-me-a-fortune-if-I-screw-it-up features.) The Options button gives you control over the behavior of the Auto button (in both Levels and Curves), but I suggest that you leave the defaults alone. Even more importantly, the Options button lets you define what colors Photoshop should use for the lightest pixels and for the darkest pixels. That's right, white and black are not always the same.

If you print to an inkjet, you want the full tonal range of the printer, so you should leave white and black set to the extremes, as far apart as possible. However, if you prepare images for commercial offset presses, you might need to adjust the values used for highlights and shadows. If your white is too light, clouds and white blouses and snow have no detail — they are completely washed out. If your black is too dark, every shadow is impenetrable, and there's no texture to the surface on which the shadow falls.

If your print shop (or another person handling your images prior to press) asks you to redefine the highlights and shadows, open Levels (or Curves), click the Options button, click the highlight (white) color swatch to open the Color Picker, and then input the suggested values. Click OK and then repeat for the shadow swatch. Make sure to click the buttons that retain the values as your defaults as you OK your way back out of all those dialog boxes.

Dragging the middle slider to the right moves the bulk of the histogram toward the left, indicating that the overall appearance of the image is slightly darker.

Also note the lower Output Levels slider in Figure 5-9. You generally use that only when preparing an image for a commercial printing press that requires you to compress the image's tonal range. Otherwise, ignore that slider and its two fields except for special effects. And make a mental note of that pop-up menu at the top of the Levels dialog box — you can apply Levels to each color channel of your image individually, changing this tonal adjustment tool to a color correction feature. (Fixing the color in your image is covered in Chapter 6.)

When you're working in Levels (or just about any dialog box), remember that holding down the Option/Alt key changes the Cancel button to Reset. When you click Reset, all values in the dialog box are restored to the defaults, letting you start over without having to cancel and reselect the command.

Earlier in this chapter, I mention that you can use the Histogram palette to avoid introducing problems into the image. Note in Figure 5-9 that the Histogram palette shows slight gaps appearing among the darker columns in front. Technically called posterization, these gaps represent tonal values that are being squished together into a single value. The pixels at one brightness level are being shifted to the next higher or the next lower value, leaving that empty column in the Histogram. Is this a problem? No, as long as you don't see wide gaps, representing a number of consecutive tonal values not in use. (Extensive posterization ruins the subtle transitions between colors in your image.) And that's why you want to keep an eye on the Histogram palette — to make sure you're not creating wide gaps in the histogram and noticeable posterization in your image.

Here is an easy way to minimize that posterization, one that lets you make your Levels adjustment but keep a pretty histogram. Immediately after using Levels, use the EditOFade Levels command and change the blending mode from Normal to Luminosity. As you see in Figure 5-10, the posterization goes away with a minimal change in the effect of the Levels adjustment. Remember that the Fade command is available only immediately after applying an adjustment (or filter or tool) — you can't even use the Save command between. (Read more about the Fade command in Chapter 15.)

Figure 5-10: Change the Levels adjustment blending mode to Luminosity with EditoFade.
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