The Histogram

One of the most useful tools in digital photography is the histogram. The graphs above illustrate how the histogram changes with common “tools” that are provided in every photo-editing package. Essentially the histogram is a distribution of tones of every pixel in the bitmap picture. In the Baseline graph we see very few pixels in the “black” end. As the grayscale on the bottom gets lighter there is a greater and greater occurrence of pixels of that tone until middle gray is reached. At that middle gray, the pixel frequency of the individual tones decreases as the value of the pixels approaches white. This chart forms a stereotypical bell shaped curve. In reality this seldom happens. What is likely to occur, is that the vast majority of “garden variety” pictures will have some number of white pixels and some black pixels. This is such a common occurance that all photo-editing packages provide a tool called Auto-equalize or perhaps Auto-contrast, that corrects a digital photo if true black and white does not exist. What this tool attempts to do is remap the darkest gray to black and the lightest gray to white. In effect this tool stretches the curve in both the lighter and darker direction. We see above that in the Lower Contrast histogram, the curve has been compressed. This is the exact opposite of what I just described. This is more like what is happening in the higher contrast histogram with the curve being stretched. In that particular histogram illustration, the normal baseline has been exaggerated into a higher contrast direction.

This histogram tool is so useful that many digital cameras provide it on the LCD screen. This allows the photographer to “see” the distribution of grays and make judgments on this. A close-up of a face that has no specular highlights in the eyes or teeth might very well not have any pure whites and the photographer must take this into account. With experience it is possible to view a histogram and assess whether or not a reasonable exposure has occurred. We also find the histogram in all powerful photo editors. Often we find interactive variations where the photographer has digital “handles” that allow these curves to be manually tweaked. The tool that IS called “curves” is something different. It is a clever mechanism whereby we can remap tones lighter or darker in a very controlled manor. It uses a different, but equally effective, process from the histogram variants. It is very illustrative to use the curves tool and check the histogram after its application.