Understanding Histograms

The primary tool used to evaluate the tones in a digital image is the histogram. A histogram is a graphical representation of the distribution of the tones in an image. It consists of a graph that ranges from pure black on the left to pure white on the right.

To understand how the graph is created, imagine 256 side-by-side columns, one for each of the individual tonal values. The columns start with pure black (0) on the left and end with pure white (255) on the right. In between, there are 254 other columns representing all the shades of gray between black and white. Together these 256 tonal levels equal the sum total of the tones in an image. The height of each column is governed by the number of pixels in the image having that particular tone. Because these columns are standing right next to each other, they form a graph when viewed as a group. This graph is called a histogram.

Figure 2-1 shows a photo of photographer David Hitchcock. The histogram next to it shows a graph of the tones in this image. In this example, it's easy to see how the histogram is mapping the data. The darker tones in the image are represented by the taller regions on the left side. The lighter regions, which are mostly in the wall behind David, are represented by the tall region on the right. Because the photo doesn't have many middle tones, ...

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