Behind the Lens

3.5The Tools of Digital Exposure, Part II: The Histogram

The histogram has a long history—dating back to the 1700s, although the term “histogram” wasn’t coined until 1895. Webster’s Dictionary defines histogram as “a bar graph of a frequency distribution in which the widths of the bars are proportional to the classes into which the variable has been divided and the heights of the bars are proportional to the class frequencies.” Basically, early “histograms” were merely bar graphs—and, actually, that’s all a histogram is: a bar graph representing a spectrum of data. In our case, the data represented is the luminance distribution in an image from black to white.

Unlike waveforms, the histogram doesn’t represent the image in a pictorial fashion, ...