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.