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Adjusting the tonal range

The tonal range of an image represents the amount of contrast, or detail, in the image and is determined by the image’s distribution of pixels, ranging from the darkest pixels (black) to the lightest pixels (white). You’ll now correct the photograph’s contrast using the Levels command.

In this task, you’ll use a graph in the Levels dialog box that represents the range of values (dark and light). This graph has controls that adjust the shadows, highlights, and midtones or gamma of the image. You’ll also refer to the Histogram palette, which simply displays the information for you. Unless you’re aiming for a special effect, the ideal histogram extends across the full width of the graph, and the middle portion has fairly uniform peaks and valleys, representing adequate pixel data in the midtones.

  1. Choose Window > Histogram, or click the Histogram tab in the Navigator palette group to make it visible. Then choose Expanded View on the palette menu.
  2. Choose Image > Adjustments > Levels, to open the Levels dialog box.
  3. Make sure that the Preview check box is selected, and then move the dialog box, if necessary, so that you can also see the image window and Histogram palette.

    Three triangles below the histogram represent the shadows (black triangle), highlights (white triangle), and midtones or gamma (gray triangle). If your image had colors across the entire brightness range, the graph would extend across the full width of the histogram. Notice that at this point, the graphs in the Levels dialog box and the Histogram palette are identical.

    A. Shadows B. Midtones or gamma C. Highlights

  4. Drag the left triangle to the right to the point where the histogram indicates that the darkest colors begin.

    As you drag, the first Input Levels value (above the histogram) changes and so does the image itself. In the Histogram palette, the left portion of the graph now stretches to the edge of the frame. This indicates that the darkest shadow values have shifted closer to black.

  5. Drag the right triangle to the left to the point where the histogram indicates that the lightest colors begin. Again, notice the changes in the third Input Levels value and in the image and the changed shape of the Histogram palette graph.
  6. Drag the middle triangle a short distance toward the left side to lighten the midtones.

    Watch the image updates in the image window and the changes in the Histogram palette graph to determine how far to drag the middle triangle.

  7. When the image looks good to you (the sample uses Input Levels values of 18, 1.30, and 232), click OK to apply the changes. Then save your work.

About Auto Contrast

You can also adjust the contrast (highlights and shadows) and the overall mix of colors in an image automatically using the Image > Adjustments > Auto Contrast command. Adjusting the contrast maps the darkest and lightest pixels in the image to black and white.

This remapping causes the highlights to appear lighter and the shadows to appear darker and can improve the appearance of many photographic or continuous-tone images. (The Auto Contrast command does not improve flat-color images.)

The Auto Contrast command clips white and black pixels by 0.5%—that is, it ignores the first 0.5% of either extreme when identifying the lightest and darkest pixels in the image. This clipping of color values ensures that white and black values are representative areas of the image content rather than extreme pixel values.

For this project, you won’t use the Auto Contrast feature, but it’s a feature you should know about so that you can use it in your own projects.

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