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The Levels command corrects tonal ranges and color-balance issues. With this command you can fix poor exposure. Additionally, you can perform color correction by manually identifying a white point and a black point in the image. Nearly every image can benefit from making a Levels adjustment.

To understand Levels, it's essential to be able to read a histogram. This graph works as a visual guide for adjusting the image. The Levels adjustment has its own histogram that's visible when working in the Adjustments panel. You may also want to call up the Histogram panel (Window > Histogram) and leave it open while color-correcting. You can also choose to expand the Histogram panel by clicking the submenu and choosing All Channels View. Let's give the command a try.

  1. Close any open files, and then open the file Ch10_Levels.tif from the Chapter 10 folder on the CD (see Figure 3).
  2. Add a Levels adjustment layer by clicking the Levels icon in the Adjustments panel (see Figure 4). Levels is also available from the Adjustments menu (Image > Adjustments), but the adjustment layer is more flexible for future modifications. Be sure to select the Preview checkbox so changes update onscreen.
  3. This photo was shot under low light, but you can reset the black and white points of the image to fix the exposure. In the Adjustments panel, move the white Input Levels slider to the left, where the histogram starts to rise (see Figure 5). This change affects the image's white point and allows you to reassign where white should begin in the image.
  4. Move the black Input Levels slider to the right, where the first amount of black starts to rise (see Figure 6). The more you move the black slider to the right, the more contrast is introduced into the image (see Figure 7).
  5. The true power lies in the middle (gray) Input Levels slider (see Figure 8). By moving this slider, you can modify the gamma setting. Effectively, you can use the middle Input Levels slider to change the intensity of the midtones. This adjustment can be made without making dramatic changes to the highlights and shadows, and lets you better expose an image (see Figure 9). Move the slider to the left to add light; move the slider to the right to subtract light.
  6. If you need to edit the adjustment later, simply select the adjustment layer in the Layers panel and manipulate the controls in the Adjustments panel.

Auto Levels

While working with the Levels adjustment layer, you may have noticed the Auto button. This command button triggers an analysis of the histogram data by Photoshop that is then used to modify the individual controls of the Levels adjustment. In many cases, this results in an image that's properly adjusted for color balance and exposure issues. In others, it will get you closer to a corrected image.

  1. Close any open files, and then open the file Ch10_Auto_Levels.tif from the Chapter 10 folder on the CD (see Figure 10).
  2. Add a Levels adjustment layer by clicking the Levels icon in the Adjustments panel.
  3. Click the Auto button (see Figure 11) to perform an automated adjustment for the image. The image's levels and color are adjusted (see Figure 12).
  4. To refine how the automatic adjustment works, hold down the Option (Windows: Alt) key and click the Auto button again. A new dialog box opens, as shown in Figure 13. Choose Find Dark & Light Colors and Snap Neutral Midtones to create a very natural balance of colors for the image (see Figure 14).
  5. Click OK to close the dialog box.

Color Cast

In the first Levels example, you made a Levels adjustment to all the channels evenly. In the Auto Levels example, you let Photoshop adjust the levels and remove color cast using an automated algorithm. The Levels command can be isolated to a specific channel by clicking the drop-down list in the center of the Levels dialog box (see Figure 15). This option allows you to tackle color-cast issues, such as spill from a background, a bad white balance, or a photo shot under mixed or colored lighting.

Follow these steps to try adjusting the color cast:

  1. Close any open files, and then open the file Ch10_Levels_Color_Balance.tif from the Chapter 10 folder. Notice that the image has a greenish tint, as shown in Figure 16.
    Figure 16

    Figure 16 Photo by James Ball.

  2. Add a Levels adjustment layer by using the Adjustments panel. You will use the Levels command to fix color and exposure issues.
  3. Select the Set White Point (white eyedropper) in the Levels dialog box (see Figure 17). Then click an area in the image that should be pure white. For this image, click a bright area in the white pillar, as shown in Figure 18. If you click too dark an area, the whites in the image will overexpose. (You can click the Reset button—it looks like a circular arrow—at the bottom of the Adjustments panel if needed to reset the Levels command.) After you click, you'll see that some of the color spill has been removed.
  4. Select the Set Black Point (black eyedropper) in the Levels dialog box, as shown in Figure 19. Then click an area in the image that should be pure black. Choose an area such as a jacket or a dark shadow. This action will adjust the color balance and the exposure.
  5. The image's color balance should be better now. Adjust the middle Input Levels slider to brighten the image (see Figure 20).
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