A video histogram is a graph showing the distribution of dark areas, middle areas, and bright areas in a video image as it is being recorded or played back. By analyzing the video histogram, the cameraman doing the shooting can determine if the image is properly exposed and, if not, adjust the video camera accordingly. Videographers who really know how to use a video histogram not only achieve better exposure but also shoot cleaner video images than those who don't.
Histograms have been around a long time as ways to visually analyze data. They are bar graphs which represent the number of elements with a given value. For example, in a group of ten people, three of whom do not own a home, four own one home, two own two homes and one owns three homes could be expressed on a histogram with the leftmost bar going to three, the next bar to the right going to four, the next bar over going to two, and the rightmost bar going to one. This would graphically demonstrate the point that more people in the group own one home than any other number and that the majority of the people either rent or only own a residence, as opposed to owning vacation or investment homes. A video histogram works on the same principle.
Rather than graphing how many homes a person owns, video histograms show the number of pixels which have a given brightness level. The leftmost side of the histogram represents dark tones, usually referred to as shadows, while the middle represents midtones, and the rightmost side shows the distribution of highlights, which are bright tones. So, a very dark image would have a histogram with a peak on on the leftmost side, and a very light one would have a peak on the rightmost side. The histogram of a picture of a piece of black paper next to a piece of white paper would have two peaks — one on the far left and one on the far right.
By looking at the video histogram, a videographer can ensure that the image is not overexposed, which would show up on the histogram as a large number of pixels going all of the way to the right, or underexposed, which would show the opposite. In either of these instances, the cameraman could adjust the video camera's exposure or add additional light. Videographers also use the histogram to shoot as bright a video as possible without overexposing it. Not only does bright video appeal to many viewers, but it also provides a sharper and clearer image with less video noise.