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Basic Photo Corrections in Photoshop CS2

Jun 9, 2006

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Manually 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) in the image. 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 displays this 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.

Choose Window > Histogram, or click the Histogram tab in the Navigator palette group to make the Histogram palette visible. Then choose Expanded View from the Histogram palette menu.

Click to view larger image.
Choose Image > Adjustments > Levels to open the Levels dialog box.
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.
The left (black) triangle below the histogram represents the shadows, the middle (gray) triangle represents the midtones, or gamma, and the right (white) triangle represents the highlights. 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

Click to view larger image.
In the Levels dialog box, 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 graph) 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.

Note

You can also scrub to change the Input Levels value: First click in the text box for the value you want to change, and then drag the pointer over the Input Levels label.
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, in the image itself, and in the Histogram palette graph.
Drag the middle triangle a short distance to the left side to lighten the midtones.
Watch the changes in the image window and in the Histogram palette graph to determine how far to drag the middle triangle.

Click to view larger image.
When the image looks good to you (we used Input Levels values of 25, 1.20, and 197), click OK to apply the changes. Then save your work.

About the Auto Contrast command

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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