Understanding the Histogram

Imagine, for a moment, if we could do a statistical analysis on a digital photo.

We would want some way to count how many dots of each color there are in a given picture. And, once we had that count, we would want to chart it somehow, so we could see at a glance how those dots compare against each other. This chart could show us whether our picture was washed out or too dark before we even printed it.

This chart is called a Histogram. Many digital cameras will generate it for either the picture you've taken or the one you're about to take. And, most photo editing programs will create one, too.

When we view the historgram of a picture, we're looking at the extent of color information contained in a photo. A dark photo will have the bulk of the data on the far left side of the chart, while a "daylight" photo will be somewhere in the middle. The chart could show a single hump, or a series of spikes; it could be very tight, only a fraction of the histogram, or it could spread from edge to edge.

There's no sugh thing as a "perfect histogram" because every photo and every histogram is unique. But learning how to read the lines on a histogram can give us an insight into whether or not we have the image we want.

There's really only one sort of histogram that points out a bad picture, and that's one where the data is up against the edge. The far left edge is pure black, and the far right edge is pure white. If there's a large amount of absolute black or white in the picture, then some detail has probably been lost--because it's very rare to have pure, absolute black or white.

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