ABCs of Photography – Histogram
“In the world of statistics, a histogram is a kind of bar graph in which the bars vary in both height and width to show the distribution of data. In the Levels dialog box, it’s a bit simpler. The central histogram contains exactly 256 vertical bars. Each bar represents one brightness value, from black (on the far left) to white (on the far right). The height of each bar indicates how many pixels correspond to that particular brightness value…”
Deke McClelland ©2004 Adobe Photoshop CS One-On-One, Sebastopol, CA: Deke Press/O’Reilly Media To see more from Deke, visit deke.com
This is by far the best description of a histogram that combines the mathematical usage with the photographic usage of a histogram.
This grayscale image only has three tones, black, middle gray and white. They correspond to three bars in the histogram, 0 for black on the left, 128 for middle gray in the center and 255 for white on the right.
This continuous tone image was created in Photoshop using the Gradient tool and reveals a much different histogram with a bar for each of the 256 tones in the image.