17 December 2007
How To Read and Understand a Histogram
Posted by Mikhail Esteves under: Photography .
What is a histogram and what does it tell us about a photograph?
A histogram is nothing more than a bar graph. It shows how the luminance values in a in a digital or digitized photograph are distributed. The linear scale in a histogram runs from black at one end to white at the opposite end. With the exception of a scanned negative, the scale runs from left (black) to right (white). About 99 percent of the histograms we use in photography today have 256 increments, corresponding to 8-bit data depth. The histogram maps the distribution of the luminance values either as a composite of the red, green, and blue channels or in each channel, as you may have seen in the histogram display on some cameras and as option in Photoshop.
The horizontal scale of the histogram measures exposure latitude, and the vertical scale measures quantity: it tells us how many pixels in the image have a specific luminosity value. While the horizontal scale is measured in absolute values (0 to 255) the vertical scale is effected by several factors: the color space, bit depth, and if you are shooting jpegs, the compression level.