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.
- 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 aside, as needed, so that you can also see
the image window as you work.
In the middle area of the dialog box, the three triangles directly beneath 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, from black triangle to white triangle. Instead, the graph is clumped somewhat toward the center, indicating there are no very dark or very light colors.
You can adjust the black and white points of the image to extend its tonal range and then adjust the midtones.
- 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.
- 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.
- Drag the middle triangle a short distance toward the left side to lighten the midtones. Watch the image updates in the image window to determine how far to drag the middle triangle.
- 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.
- Choose Image > Histogram to view the new histogram. The tonal range now extends throughout the entire range of the histogram. Click OK to close the histogram, and 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.