Digital SLRs utilize “through-the-lens” light metering; what you see through the viewfinder, you meter. TTL is reflective, measuring the amount of light bouncing off a subject. It’s accurate, and it means that subtle changes in composition—which can create dramatic changes in a reading—create instant adjustments to the exposure. This is great if you’ve set your camera on the ideal metering mode. If not, it’s a disaster.
Most SLRs offer variations on a few primary metering modes—spot, center-weighted and multi-zone metering. How they work dictates how they should be used.
Spot metering is simple. The light meter measures a small circle in the center of the frame. Usually, the spot measured is from 1% to 5% of the overall frame, so a subject silhouetted by strong backlight can be metered accurately—only the shadowed subject at which you’ve pointed the spot is factored in to the exposure.
Partial metering still places emphasis on the center of a scene, but the circle is larger. Center-weighted metering, however, blends this partial zone into an overall reading of the entire frame. The center is given emphasis, but the edges are considered, too. The third metering mode could broadly be categorized as averaging, accounting for the entire frame. In its purest form, it reads the overall scene and averages the reading, ideal for flat and even lighting. In truth, though, most SLRs rely on a smarter variation, reading multiple zones in the scene and weighting them according to particular presets. These matrix or multi-zone modes are more precise than a blanket average, so they better handle contrasty scenes.
Matrix metering evaluates information from various points in a scene (often hundreds or thousands of segments) to determine the correct exposure. These modes factor in various bits of information (such as autofocus point, focus distance and even stored scene information in the camera’s internal memory, often called “evaluative” metering) to determine what area should have principal effect on the exposure. Implemented well, multi-segment metering should deliver highly accurate exposures in a variety of lighting scenarios.
Each manufacturer utilizes its own proprietary technologies to gain what it hopes will be a slight advantage in metering. Nikon’s 3D Color Matrix, for example, accounts for brightness, contrast and focus distance in making readings, as well as color of the subject and RGB values in every section of the frame. Values are then compared to an in-camera database of 30,000 images to determine the most appropriate exposure.
Olympus’ Digital ESP metering also reads light in several areas of the frame before cross-referencing a database of images. Canon’s multispot metering not only links spot metering to the active viewfinder AF point, it can connect up to eight total areas within the frame to determine the best exposure setting. It’s clear that manufacturers are working overtime to create smarter metering—in hopes of delivering more accurate exposures in every shooting mode.
No matter which metering mode you typically choose, taking the time to practice working with each will help you get the best results when it counts.