Absolute Beginner's Guide to Digital Photography

Introduction to Histograms

A histogram is a bar graph that shows the number of pixels at each brightness level

in an image, with the darkest pixels to the left and the lightest pixels to the right

(see Figure 14.2). The quantity (amount) of pixels in the image that uses a specific

brightness value is shown by the height of the vertical bars. The higher the bar, the

more pixels there are at that brightness level.

CHAPTER 14 THE IMPORTANCE OF BRIGHTNESS/CONTRAST 207

FIGURE 14.2
A histogram
of an image.

In addition to giving you information about an entire image, a histogram can show

information about the pixels in a selected part of an image. To view a histogram for

an image, choose Image, Histogram or Image, Adjustments, Levels. The Levels dia-

log box also includes a histogram.

You can only modify the histogram in the Levels dialog box by moving the black,

50% gray, and white triangular sliders with the mouse, or by typing in numbers in

the Input Levels and Output Levels boxes. The three sliders beneath the histogram

are used to lighten and darken the image, increase contrast, and to lighten and

darken its midtones. The two sliders on the Output Levels bar are used to decrease

contrast.

You can change a single color’s brightness and contrast by selecting the color from

the Channel pull-down menu at the top of the dialog box. This lets you make

changes to the color balance. You will rarely change levels for a specific color chan-

nel, however. This technique is mainly used when there is some type of color cast—

for example, when the image is predominantly one color because of lighting.

Correcting Brightness and Contrast for Dark Images

The histogram in Figure 14.3 graphically shows that the image is too dark by dis-

playing a large number of pixels bunched at the left side of the graph. The his-

togram also reveals another problem: no pixels on the right end of the graph. This