In photography, a zone system is a system which is used to control the varying tones produced in a finished image. Many explanations of the zone system are convoluted and highly confusing, which turns amateur or beginning photographers off because they think it's too hard. In fact, the zone system is very simple, and mastering it can radically improve the quality of someone's photographs. Ansel Adams, the developer of the original zone system, is famous for the richly textured tonality of his images, illustrating how valuable mastery of the zone system can be.
The idea behind the zone system is that by controlling exposure of a film negative, a photographer can reduce the amount of correction which will be needed when making prints. The photographer's goal is to capture the desired tones when taking the image, rather than trying to bring them out later. As Adams put it, the zone system exposes for the shadows, allowing photographers to develop the highlights.
According to the zone system, tones in an image can be divided into 10 categories, from pure black to pure white. The zones run from 0, pure black, to IX, pure white. Using the system requires photographers to use a light meter, and to be familiar with operating the manual settings on their cameras.
Light meters work by detecting the various levels of light in a scene, and averaging them to come up with an exposure recommendation. It is also possible to use spot meters to get information about a specific object. The light meter is not a terribly smart instrument, although it tries awfully hard, and it usually assumes that the default tone in an image is around a Zone V, right in the middle of the zone scale. Sometimes this works out just fine, but when someone is photographing something with a lot of light or dark tones, an exposure designed for a Zone V is going to look dark or washed out.
Using the zone system, a photographer picks an object in an image and decides where in the zone that object should fall when the picture is printed. For example, someone photographing snowy mountains around a lake might decide that the mountains should be around a Zone VII in the finished image. Next, he or she would meter the mountains to get recommendations from the light meter for a Zone V, and then adjust these recommendations upwards by two F-stops, giving the mountains more exposure. Conversely, the photographer could meter the darker lake and reduce the F-stops to get the lake to show up as a Zone II in the finished image.
Using the zone system ensures that dark and light tones will show up in finished images as desired. Light tones can always be brought out later during processing, but if a camera fails to capture dark tones, it is impossible to add them in. The process can be used for both film and digital photography, and while people may find it limiting and cumbersome at first, once they get used to it, the system can start to feel like second nature.