Understanding Metering

Modern camera systems have come far in recent years. Most SLR camera systems on the market now, be it digital or film-based, automate many of the processes that were strictly manual just a few years ago, such as automatic metering. In the not so distant past, metering was an essential process involving a variety of gray cards and meters…even some calculations were in order to ensure the correct exposure was selected. This is not to say these steps are no longer important. To the contrary, understanding metering is just as important today as it was in years past. The difference between now and then are the systems that are available to the photographer. As with all things that are designed to automate a process, there is a risk of experiencing some skill loss caused by automated dependency. This article will cover metering and how metering has an effect on the quality of your photographs.
There are surely some people reading this who may become quite defensive over the suggestion that photography has become an automated process. It is not my intent at all to suggest a photographer should allow the camera to make all the decisions. Rather, I submit that the photographer must not only understand the principles of traditional metering, but they must also be familiar with how the automated process works in order to better understand the process.
There are three basic forms of automated metering on SLR camera systems; Matrix Metering, Center-Weighted Metering, and Spot Metering.
Matrix Metering
Matrix Metering, also known as Evaluative Metering, is the workhorse of metering. Matrix Metering uses a generalized approach to handle exposure. In this mode, the camera evaluates the totality of light within the field of view and tries to find the best balance for the exposure. Generally, if a photographer were taking photographs of a landscape setting using Matrix Metering, the camera would perceive the highlights at the top of the shot as sky and would apply the darker areas to set the exposure levels. This mode is best used with general view shots, such as landscapes and other full-frame shots. The dangers associated with Matrix Metering is that specific subjects tend to become underexposed. For instance, if a photographer were to take a shot of a person standing outside, the camera would not set priority to the subject, which would almost surely result in an underexposed subject.
Center-Weighted Metering
Center-Weighted metering uses approximately 13% of the center field of view to set exposure. This mode is best when taking shots where a more refined exposure area is required, but not so much that Spot Metering is necessary. This exposure method works best when taking group shots, or perhaps when a photographer wants to have a specific subject exposed with less regard to the entire field of view.
Spot Metering
Spot Metering is by far the most misused metering function. This metering method focuses on a very limited portion of the field of view; approximately 3%. When a photographer uses Spot Metering, especially when used in conjunction with a zoom lens, he or she sets the exposure to the very center of the field of view. This method of metering works best with portrait photography where the background is of little concern. Since this exposure setting is so specific, many photographers use gray cards to set the exposure to ensure the metering is set to a known standard.
Understanding metering is essential for the photographer. Although most camera do an excellent job setting the exposure levels, it is up to the photographer to predict the outcome and select the proper exposure setting for the scene.