Histograms

A histogram is a simple graph that displays where all of the brightness levels contained in the scene are found, from the darkest to the brightest. These values are arrayed across the bottom of the graph from left (darkest) to right (brightest). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level. Possibly the most useful tool available in digital photography is the histogram. It could also well be the least understood. In this article we will look at what a camera histogram tells the photographer and how best to utilize that information.

Virtually every digital camera, from the simplest point-and-shoot to the most sophisticated digital SLR has the ability to display a histogram directly, or more usually superimposed upon the image just taken.

This histogram shows an almost perfect distribution of tones covering about a 4 stop dynamic range — from deep shadows on the left to just short of bright highlights on the right. This fits comfortably within the approximately 5 stop dynamic range capability of most digital imaging chips.

A light meter reading tells you what exposure will render a standard 18% gray reference card as a mid tone. This reading may have been made because the camera read a variety of areas of the scene and averaged them out, or because you read the highlights, the shadows and some other areas and decided that a particular setting would yield the best compromise exposure for that scene.
This setting, like every other that you or your automated camera makes, is a compromise. In most real world situations there is no such thing as an ideal or “perfect” exposure. There is simply one that places the tonal values found in the scene most appropriately within the capability range of the camera’s imaging chip. And "most appropriately" means that the mid-tones found in the image fall roughly half way between the darkest and the brightest values. Hold that thought while we digress for a moment and look at the concept of dynamic range.

if a part of the image receives too much light it becomes burned out, and if too little light it is rendered as black. A recognizable image is only recorded if the light hitting the chip falls within a range of about 5 F stops. (Remember — each F stop is a doubling or halving of the amount of light hitting the film). With digital things are much the same and even the dynamic range is about the same as for slide film; about 5 stops. Also keep in mind that the total range of brightness values encountered in the real world is only about 10 stops — from the dimmest light that you can read in to the brightest beach or snow scene in which you might find yourself).

Histograms