Most serious photo-editing programs such as Photoshop and some digital cameras let you use a histogram as a guide when evaluating your images. Since most image corrections can be diagnosed by looking at a histogram, it helps to look at it while still in a position to reshoot the image. Each pixel in an image can record any of 256 levels of brightness from pure black (0) to pure white (255). A histogram is a graph that shows how the 256 possible levels of brightness are distributed in the image.

• The horizontal axis represents the range of brightness from 0 (shadows) on the left to 255 (highlights) on the right. Think of it as a line with 256 spaces on which to stack pixels of the same brightness. Since these are the only values that can be captured by the camera, the horizontal line also represents the camera's maximum potential dynamic range.
• The vertical axis represents the number of pixels that have each of the 256 brightness values. The higher the line coming up from the horizontal axis, the more pixels there are at that level of brightness.

To read the histogram, you look at the distribution of pixels. An image that uses the entire dynamic range of the camera will have a reasonable number of pixels at every level of brightness. An image that has low contrast will have the pixels clumped together and have a narrower dynamic range.

The way a histogram looks depends on the scene you're shooting and how you expose it. There's no such thing as a good or bad histogram. Whether a particular histogram is good or bad depends on what you are trying to accomplish. If fact, you may prefer to trust your visual reaction to the image more than the very numeric image data provided by a histogram. However,even if you never use a histogram, you can learn about digital images by understanding what a histogram can show about an image.

One reason to check a histogram is to see if there are enough pixels in the shadow, midtone, and highlight areas. If there are enough, even if the image is somewhat off, it can be corrected in a program such as Photoshop using the Levels command. This control allows you to adjust the shadow, mid, and highlight areas independently without affecting the other areas of the image. This lets you lighten or darken selected areas of your images without loosing detail.

From the histogram you can determine the image's darkest shadow and brightest highlights; called the black point and white point. In fact, it's the range between these two points that defines the dynamic range (also called the tonal range or contrast) of the image. If the image is low contrast, you can also tell if it's low-key or high-key from the histogram. A low key image has details concentrated at the dark end of the scale. A high-key image has them concentrated at the light end.

When shooting for printed reproduction, the printing process has a lower dynamic range than your camera. Any tones at the extreme ends of the histogram will block up and be pure white or pure black. It's best if you shoot images that have the majority of their pixels in the middle 3/4 of the dynamic range.

In Photoshop the Levels dialog box gives you five triangles you can drag to adjust the distribution of brightness in your image.

The three triangles below the histogram work as follows:

• Dragging the left (black) triangle to the right darkens the shadows.
• Dragging the right (white ) triangle to the left lightens the shadows.
• Dragging the middle triangle to the left or right lightens or darkens the image.

  The two triangles below the Output Levels gray scale bar also adjust the image, having almost the opposite affect of the triangles above.

• Dragging the left (black) triangle to the right lightens the shadows.
• Dragging the right (white ) triangle to the left darkens the highlights.