Photoshop® CS4 Bible

Chapter 9. Histograms, Levels, and Curves

IN THIS CHAPTER

About histograms

The Levels command

The Curves command

Most images are made up of lighter, darker, and in-between color values, which can be called tones, brightness levels, or intensities. The collection of all an image's tones is called its tonal range. A nighttime photo may have mostly dark tones in its tonal range (a low-key image), while an illustration of bright white clouds may have mostly light tones (a high-key image).

A histogram is a bar graph that shows the amount of each brightness or color intensity level in an image and illustrates the image's overall tonal range. This graph can be seen in numerous places in Photoshop, including the Histogram palette and the Levels and Curves dialog boxes. Viewing histograms, as a supplement to viewing the image itself, can be helpful in determining how to adjust an image for its intended purpose.

The Levels and Curves commands can be used to manipulate the tones in an image to make it lighter or darker, or to increase or decrease its tonal range. Grayscale images can be adjusted and color images' individual color or composite color channels can be adjusted with Levels and Curves.

The light and dark qualities of an image can be changed, as well as the color quality. By making individual color channels lighter, darker, or higher or lower in contrast, the colors in an image can be altered with Levels and Curves.