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 black point in the image. Nearly every image can benefit from making a Levels adjustment.
To understand Levels, it is 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 is 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 expand the Histogram panel by clicking the submenu (the triangle in the upper right corner of the panel) and choosing All Channels View.
Lets give the command a try:
Step 1. Open the file Levels.jpg (this file is provided only for educational use).
Step 2. Add a Levels adjustment layer by clicking the Levels icon in the Adjustments panel. Be sure to select the Preview check box so changes update onscreen.
Step 3. This photo was shot under shade, but you can reset the black and white points of the image to fix the exposure. In the Adjustments panel, move the black Input Levels slider slowly to the right. The more you move the black slider to the right, the more contrast is introduced into the image.
Step 4. Move the white Input Levels slider to the left. This affects the images white point and allows you to reassign where white should begin in the image.
Step 5. The middle (gray) Input Levels slider changes the intensity of the midtones. This adjustment lets you better expose an image without making dramatic changes to the highlights and shadows. Move the slider to the left to add light; move the slider to the right to subtract light.
Step 6. In the future if you need to edit the adjustment, simply select the adjustment layer in the Layers panel and manipulate the controls in the Adjustments panel.