Histogram is a very valuable tool for any photographer who wants to have better control over exposure and image quality. It's a graphical chart that shows the brightness levels of an image. The left side of the chart represents the darkest tones, the right side represents the lightest tones, and the middle part represents the tones in between.

Histogram is a graphic representation of the highlights, midtones and shadows of the image. It tells you whether your image has been properly exposed or not.

As opposed to old film cameras, almost all today’s digital cameras have the ability to display a histogram directly on the screen. If your digital camera has a live histogram, turn it on. It shows you how your picture will be exposed if you take it with the present settings.

There is no such a thing as a “bad” histogram

If you are shooting in ordinary conditions with even lighting, you will produce a histogram that has a hump somewhere in the middle, when the subject is properly exposed.

However, there is no such a thing as a “bad” histogram. Histogram only shows you the tonal range in the scene and it's totally up to you to decide what you want to do with it. Sometimes you might want to intentionally underexpose an image to get the desired effect.

Nevertheless, be careful when the histogram spills over one side, either the left or the right side.

A histogram that is heavily bunched up to the right should warn you the photo would probably be overexposed. If a region of an image is way overexposed, that is seen as solid white color on a photo. Even with the best photo editing software you can't recover the lost details. Whenever this happens the highlights are said to be "clipped".

 

Figure 1: Over exposed image and a matching histogram

High key and low key images

Sometimes when you want a dramatic effect on your images you might want to intentionally adjust the exposure in a way that your images appear brighter or darker than the midtones. These images are called high key and low key.

Images where most of the tones occur in the shadows are called "low key". Low key images are considered dramatic and convey allot of tension.

Images where most of the tones are in the highlights are called "high key". High key images are considered happy.

Contrast

Another thing you can read from a histogram is contrast information. If a histogram is broad, it reflects a scene with noteworthy contrast. On the other hand, a narrow histogram shows that a scene contains less contrast and may appear flat or even dull.