Gauging Tones with the Histogram on a Digital SLR Camera

A histogram - a chart that appears on your digital camera's LCD - displays the number of tones being captured at any brightness level. The number of pixels at each brightness level is shown on the histogram as a vertical bar, and there are 256 of these bars. The far-left position represents the darkest tones in your image, and the far-right slot shows the tones in the very lightest parts of your image.

The histogram helps you determine if a photograph is or will be under- or overexposed. The trained eye can also see whether an image is likely to have excessive contrast or look particularly flat, based on the distribution of tones in a histogram.

Typically, a histogram looks something like a mountain, as shown in Figure 1. Most of the tones are clustered in the middle of the image because the average image has most of its detail in those middle tones. The bars are shorter at the dark or light ends of the scale because most images have less detail in the shadows and highlights. However, images that have a great deal of detail in the dark or light portions can have histograms that look very different, reflecting that particular distribution of tones.

Figure 1: With a well-exposed image, the histogram looks like a mountain.

Tips
It isn't really possible to manipulate the shape of the histogram in your camera. For that, you need to use an image editor. What dSLR (digital single lens reflex) camera owners can do with a histogram display is use it to judge whether the current exposure is correct for the image.


Checking for correct exposure on a dSLR is fairly easy to do:

1. Take your picture.

You have to take the picture first because, unfortunately, dSLRs can't display a histogram "live" (in real time). The sensor doesn't see the image until the exposure is actually made. (Point-and-shoot digital cameras might show a live histogram.)

2. Examine the histogram with your picture review function.

If an image is overexposed, the graph is shifted towards the right side of the histogram, with some of the pixels representing lighter tones clipped off entirely, as you can see in Figure 2.

An underexposed image has the opposite look: The tones are crowded at the left side, and some of the shadow detail is clipped off.

3. If you see either condition, compensate by changing the f-stop, shutter speed, or EV (exposure value) to correct the exposure error.

If you're shooting in the Camera Raw format, you might be able to adjust exposure and contrast when importing the image into your image editor. However, most of the time you'll want to get the exposure correct in the camera.

Figure 2: An overexposed image clusters all the information at the right side of the graph.

tech.yahoo