One of the most powerful tools photographers have for proper exposure is understanding how to read the light falling on a scene and make it work within the abilities of the camera's imaging sensor. In the past, photographer's have used the Zone System of Exposure Control (pioneered by legendary photographer Ansel Adams) with film photography to gain incredibly precise control of exposure. With the advent of digital cameras, shooters have tried to adapt the principles of the Zone System to use with this technology. Please keep in mind, this is a system that rewards thoughtfulness and careful photography. It is not a fast way of making images, in fact it's just the opposite.
Evaluating the tonal range of a scene
The heart of the Zone System is the idea of breaking the scene into zones based on tonal gradations and then exposing the image to bring values into the proper zones. (Note: Adams based the Zone System on working with individual sheets of large format film and often deliberately mis-exposing it then correcting during development and printing to a specific type of paper chosen to work with particular image. The Digital Zone System discussed in this article is a simplified approach geared towards digital photography and does not attempt to be a complete digital version of Adams' system.)
There are 10 zones used (1-9) with zone 5 representing a mid-tone gray, zone one representing pure black and zone pure white. Each zone represents a change of one f-stop and either a doubling or halving of the light. Rather than just relying on the camera's light metering system to figure out the best exposure, the photographer considers the tonal range of the scene and bases exposure on what part of the scene he or she wants to place in a particular zone. This often puts the photographer's exposure at odds with the exposure recommended by the camera, but that's okay. The whole idea of the Zone System is to reward the photographer for being smarter than the light meter.
Zone System proponents often work with spot meters. These are hand held light meters that are used to meter off a tiny part of the scene and are much more precise than built in camera light meters. It's not necessary to have a spot meter to use the Zone System with a DSLR though (although it can help). Instead just zoom in to the area you want to meter off and consider the resulting reading. Often you'll have to meter off a mid-tone, a highlight and a shadow area to figure out the tonal range of the image you're planning.
Using the system
The first challenge to using the Digital Zone System is to develop an understanding of what a mid-tone gray looks like in a color world. Here are some real world examples of things that approximate a middle gray: gray stone, dark skin (average Caucasian skin meters to Zone 6 or one f-stop lighter than middle gray).
Once you've decided what part of the scene is a middle gray, you can compare other values and establish the tonal range of the image. If you find it is within the capabilities of the camera (usually about 6 f-stops difference between brightest highlight and darkest shadow) you're good to go.
The system also lets you control the tonal relationships. Rather than relying on the overall exposure of the scene to determine your settings, you identify the middle tone of the scene and expose that to middle gray if appropriate. (This works for an average scene.) When dealing with something more unusual, say a field after a snowstorm, the Digital Zone System really pays off. Here you'll consider the tonal range of the scene and decide which zone you want to put different elements in.
Another consideration of the Zone System is manipulating exposure as the photographer makes the image. Here, the photographer decides what's most important in the scene and then determines which zone that element should be placed in. The photographer then visualizes what zones the other elements of the scene fall into (extrapolating the shift in tonality accordingly).
In this case, the photographer may meter a scene and find the most important element falls into Zone V. As part of his or her artistic vision, the shooter may decide that element needs to be rendered brighter (the eye is normally drawn to brighter objects) and will decide to place the element into Zone VI. He or she then needs to determine how this will affect the other elements of the photography. This step effectively over exposes the image based on a normal light meter reading. In Adams system, the photographer would then either over or under develop the negatives to boost or contract the tonal range and print on the particular paper he or she had determined would best render the image. With digital photography, the photographer make similar adjustments during RAW conversion (recommended) or through levels or curves adjustment layers in Photoshop. The idea of visualizing the final image before setting the camera's exposure controls is a fundamental aspect of the Zone System. It requires the photographer to look closely at the scene, recognize basic tonal values (not necessarily every value) and then imagine how the image would look if the exposure shifted the tonal range of the photo.
It takes some practice to get the hang of using the Zone System, but if you can become comfortable with it, it can really help the quality of your exposures.