Let’s take an imaginary example: You’ve been shooting the school’s most important football game, hoping to get a shot that could lead off the sports section of your yearbook. You want something dramatic, not just the usual running and passing pictures. Long shadows from the bleachers cover the field in dark patterns as the team huddles for the final play of the game. The score is tied. The players shift slightly, and suddenly their yellow helmets come ablaze with color from a small patch of sunlight. Everything else is in deep shadow. You manage to squeeze off a shot just before the team moves into its scrimmage position. What a shot—the bright yellow helmets set off by the contrast of dark shadows.

You send the film to be processed and cross your fingers. A few days later the photographs come back, and you nervously look for that one great shot. There it is! But something’s wrong. The yellow helmets are almost colorless. That great photo just isn’t.

What happened? To understand what really went wrong, you need to know how a camera's light meter works. The process is the same for film or digital cameras. A light meter is merely an instrument that is sensitive to light; it reacts to the amount of light that strikes the photo cells inside the camera. Most meters "read" all the light reflected from a subject and the background. This range of values, from dark to light, is "interpreted" by your meter as being an average, middle-gray tone. This means that most subjects containing a normal range of light and dark areas will be considered (by the meter) as middle gray. In these cases, you just point the camera and shoot for a good exposure every time! Well, almost … read on.

What happens when your main subject is partially in bright sunlight and mostly in dark shadows, as was the case with the football helmets mentioned earlier? A light meter would be more influenced by the greater dark areas and would indicate the exposure necessary to make the shadows middle gray. Of course, this would overexpose the brightly lit subject; hence the colorless yellow helmets.

There are several methods of solving these kinds of exposure problems. One method requires you to move in close and take a reading from just the main subject, press the exposure lock button, then move back to photograph the entire picture area. However, this would be impractical in a situation like a football game. An alternative method would be to take a reading from an object similar to the main subject that is close at hand and in the same kind of lighting.

But what if the entire subject is very dark or very light? If you use the exposure given to you by your meter in these kinds of circumstances, you will get grayed whites or grayed blacks respectively.

The answer to this dilemma is something called the zone system of exposure. This is a fairly complex system, but you can use a modified simple version of it and still get accurate results. You should use a camera that has the option of manual exposure settings to get the best results using this system. However, can can "trick" your automatic camera into reading substitute "zone" areas.

This zone system allows you to compensate deliberately for light or dark subjects and the in-between ones so that you have predictable control of the final outcome. Using this system you can be sure of bright, deep colors, white snow, and black shadows.

Let’s see how it works. First we divide a scale of values from black to white into five steps. This is a zone value scale. 

Zone 1 represents the blackness of deep shadows, and black, dark objects. 
Zone 2 represents normal shadows, dark foliage and dark grass
Zone 3 represents middle gray -- usually a mixture of normal objects
Zone 4 represents light skin tones, and light bright colors
Zone 5 represents white and very light highlights (not glaring white). 

The key to understanding this system is to realize that zone 3 represents the point of deviation. In other words, zone 3 is the zone your camera is programmed to read. It is the average from dark to light of a normal scene.

Using a zone value scale, we can determine which color matches with which zone. Then we can adjust our exposure in the camera to match that color. For example, the yellow helmets would probably be about zone 4. This is one zone away from middle gray. Each zone represents one f/stop of exposure, so all we have to do is open up the camera lens (or shutter) 1 stop, and we’re ready to shoot for perfect exposure.

We could approach it from the opposite side and read the shadow area (about zone 2) and  we close down 1 stop. Interestingly enough, either adjustment would give you the same final exposure setting.

All you are doing with this system is using any value from black to white to determine proper exposure. You’re not stuck with middle gray. And since middle gray is not always available, you have a system that will give you correct exposure information at those times, too.

Of course, there are times when you can use a substitute object that is middle gray to take a reading. Then you can just shoot with the reading obtained without adjustment. A lawn is usually close at hand and is very near to middle gray. You can also use clear, blue, northern sky in the same manner.

However, it is still usually best to take your reading from the main subject itself and adjust to fit it. This will give you the color saturation (deepness of color) that you want in your subject.

Some professionals photographers underexpose 1/2 stop in addition to the adjustments made as described in this article. They feel the extra 1/2 stop gives them a richer color rendition. I have even underexposed 1 to 1 1/2 stops to achieve deep tonality and contrast in special subjects. You should experiment to find what works best for you and your equipment.