A histogram forms a graphical representation, a simple graph, of the brightness levels in an image from darkest (on the left side of the graph) to the brightest (on the right side of the graph). The vertical axis (the height of points on the graph) shows how much of the image is found at any particular brightness level.

A Very Useful Tool
Start using the histogram review feature of your digital camera. Set your camera to display a combined thumbnail and histogram for 5-10 seconds after every frame. Get in the habit of glancing at it. Many photographers consider the histogram display after an exposure the greatest invention since the built-in light meter.

Many photographers are curious that I frequently look at the LCD after taking an exposure when I am using a DSLR, (which I usually am these days). It is not the image on the LCD that is of most interest, but rather the histogram that commands by attention.
 

'Wave Cloud Way I'

Histogram for 'Wave Cloud Way' Note that the histogram fits well within the extremes of the graphical space indicating that the image contains the full range of photographic information available in the scene.

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 I digress for a moment and look at the concept of dynamic range.

Using the Histogram
The histogram really comes into its own if there are no flashing highlights. If the image on the LCD display is clearly too dark, retake the picture with an increased exposure, repeating until you find the maximum you can give and keep your highlights sweet.

The histogram is a bar chart of sensor cell densities in the image, generally corresponding to the minimum exposure at the left and the maximum at the right. Histogram shapes depend on the tonal distribution within each image, but ideally they should come down to zero at or near each end of the scale.

If there is a large empty area at the right of the scale, then you should increase exposure, as this will decrease noise in the image and normally improve tones. However, keep an eye on the actual subject values on the screen, and don't let the highlights get too light. So long as the highlights are right and the LCD image looks fine, there is no need to worry about empty areas at the shadow end.

If your histogram has non-zero values at the high (right) end, then you need to reduce exposure. Non-zero values at the low end mean you are underexposing. With non-zero values at both ends you have a subject with too much contrast for the sensor to cope with, and you will either have to add fill-in lighting to reduce the lighting contrast or reduce exposure and accept a loss of detail in the shadows. For much normal photography, an exposure correction of -2/3 stop is a good starting point, though I find values from about +1 to -2 are occasionally indicated. When working under pressure it isn't possible to check every shot, but a single setting generally works unless the lighting changes.

Digital Camera Dynamic Range
Put simply, dynamic range can be though of as how many f-stops in an image will contain detail. In this example there are five zones labeled as: Very Dark / Dark / Medium / Light / Very Light. Of course, each zone actually graduates in multiple gray tones of more than 50 discrete brightness levels which yields the mathematical gray scale of 256 shade of gray. As a practical matter, consider about 4 - 5 points at the very bottom (black) and another 4 - 5 points at the very top of the scale (white) to be so close to the extremes that they cannot really be part of the image-forming segment of the graph.

One f-stop increments (5 zones) on a 255 step gray scale
recordable by a digital camera.