Feature: Master Exposure Metering Modes

How well do you know your digital camera? You're probably pretty familiar with some of the features. No doubt, you've got the stuff like programmed exposure modes, image playback, and flash operation down cold. But how well do you know your camera's metering modes? Do you even know what they do? This week, let's dive into what is probably the most underused feature in digital cameras.

The camera's exposure sensor meter does the majority of the work when figuring out how to shoot a picture: It decides how much light is needed. That data is used to calculate how long the shutter remains open and how large the aperture will be. Not surprisingly, all exposure meters aren't the same--and some are better than others at metering a scene and applying the right exposure. Thankfully, most camera makers put several exposure meters into their cameras, and you can choose the right mode for any given situation.

Center-Weighted Meters

In the old days, most cameras came with a simple center-weighted light meter. This meter measures the light throughout the image, but applies more weight, or importance, to the central part of the scene in the viewfinder. The assumption--often a good one--is that you are most interested in the stuff centered in the picture, so the camera tries to get that part of the scene exposed properly.

Many digital cameras rely on this kind of meter for ordinary picture taking. However, it's the oldest and most inaccurate kind of meter around, so if you have a choice, I suggest that you avoid it in favor of the next kind of meter.

Matrix Meters

Often called matrix or multisegment metering, this kind of meter breaks the scene into multiple discrete parts and measures the exposure in each one independently. The sensor then collects all that exposure information and weights the relative value of each one based upon where it is in the photo.

The bottom line? Matrix metering is dramatically better than center-weighted metering modes at properly exposing a scene, especially one with high contrast or extremely variable lighting.

Even this approach can't provide perfect results in all cases: The contrast may be too great to achieve the ideal exposure in all parts of a photo. But if your camera has a matrix meter mode, you should use it most of the time, since it generally delivers outstanding results under a broad range of conditions.

Spot Meters

The last major kind of light meter is called a spot. You won't use this all the time; it's designed for special situations in which the matrix or center-weighted meter would lead you astray.

The spot meter is designed to measure light exclusively in the center of the image--traditionally, the center 1 percent of the screen. However, most digital cameras have less precise spot meters that read anywhere from 2 to 10 percent of the scene.

This meter can come in handy on occasion, but don't set it and forget it. A meter that only measures the light in the central 1 percent of the frame would typically take very poor pictures, either highly under- or overexposed depending upon the situation.

So when should you use the spot meter? Any time you're trying to photograph a scene in which a small subject must be exposed properly for the picture to work--and its lighting is different enough from the rest of the scene that you're worried it won't come out right otherwise.

Imagine, for instance, that you're trying to photograph someone who is standing in front of a brightly lit window. If you let the camera decide the exposure, the bright light from the window will radically underexpose your subject. So switch on the spot meter and expose the picture based on the subject. Yes, the window light will be overexposed, but that's okay--the important part of the picture is the person.

Switching to anything other than the standard matrix meter mode--especially the spot meter--usually works best when you also switch on your digital camera's exposure lock. This came in handy when I faced one of the trickiest photographic situations I've ever seen: a pair of white wolves playing near some trees, with highly contrasting light streaming into the scene. I tried a shot with the camera in its usual matrix metering mode, but I could immediately see that the image was badly exposed. So I switched to the spot meter, pressed the camera's exposure lock while I was focused directly on one of the wolves, and then recomposed to take the final picture.