HistogmPrimer

By Ed Chop

A histogram can be very intimidating and confusing. What is it? It’s a tool you can use to evaluate and adjust your luminance and colors in your digital photo.

The luminance histogram is probably the one you are most familiar with. It will show you a graphical view of the brightness and contrast of your image. The values on the left represent black and dark tones. The values on the right represent white and light tones. The higher the values on the graph, the higher the pixel count for that tone. Thus a low key photo will have a higher peak (or peaks) on the left side. A high key image would have higher peaks on the right. At the opposite ends, the pixel counts would be low and would have very low peaks or, more likely, a gradual curve.

A more balanced image would have its peaks in the center of the histogram. The falloffs would be on each side.

If your camera has a histogram available for viewing your image, it can be used to determine if you have a properly exposed picture. Your LCD screen cannot be counted on to give you a true representation of your image. The screen is too small and the contrast changes dramatically as you shift from one side to another.

There is no right or wrong histogram for all images. Look at the graphs for all your images and you will soon see the correlation between good and bad images.
The histogram can also be used to adjust colors in your image. The RGB histogram is useful for these adjustments. Basically, you can adjust any of the red, blue or green channels to change the colors or hues of your scene in the same manner as the luminance histogram.

So, remember, in general, for better images avoid clipping, or peaks, on the sides of the histogram. You will lose details in the shadows or highlights. Unless you have a high key or low key photo, you want to avoid high values on one end or the other.

For a more complete discussion of histograms, visit sphoto.com.