Monday, October 19, 2009
Choose Your Camera’s Correct Metering Mode—10/19/09
TTL metering—do you really know how to use it?
Though TTL metering is powerful and accurate, it does depend on some photographer intervention to ensure its providing the right type of exposure information. How? By selecting the right type of metering mode for a given lighting situation.
There are three primary metering modes—and even more in the many cameras (particularly high-end models) that sport multiple variations on these basics. Knowing when to deploy each mode can make a big difference in the indicated meter reading—whether the exposure is then manually adjusted by the photographer or set automatically by the camera.
The first and simplest metering mode is the spot. This mode simply measures a small circle in the center of the frame. How small the circle dictates how precise the reading. It’s ideal for exposing a subject in the center of the frame in a tricky lighting environment, particularly when you’re unconcerned about the exposure in other areas of the frame. For example, a dark subject against a white background (like a backlit portrait with the subject’s face in shadow) or a light subject against a dark background (say, a subject in sunlight with the background in shade) will benefit greatly from spot metering.
When an overall reading is more important than a single spot, most photographers prefer to use what’s known variously as matrix, multi-segment, averaging or full-frame metering. These metering modes all utilize different techniques for determining the best exposure for the overall scene—rather than just a spot in the center. Basic averaging can be easier to fool by extremes of contrast, whereas more advanced multi-zone versions of full-frame metering can take into account variations of brightness in the corners and at specific points throughout the frame. This is a good and versatile metering approach for many uses, although it still has its shortcomings—as it’s possible to fool the meter by brightness and contrast changes in unimportant areas of the frame.
Improving full-frame metering accuracy to keep contrasty scenes from fooling the camera is why many manufacturers incorporate evaluative comparisons. A D-SLR with this feature will make an exposure reading before comparing the metered scene quite literally to an in-camera database of other scenes with similar readings. The results are factored in to determine the most appropriate exposure.
Another modification to full-frame metering is the multi-segment approach, which is a bit like several spots (or zones) throughout the frame being predetermined as important, and therefore factoring in to the indicated meter reading. These zones are sometimes user-selectable and tied to active focus points—making the camera’s meter able to tell what the photographer deems important in the frame since. After all, that’s where he’s focusing.
The third broad category of metering modes is a bit of a combination between the first two. It’s usually manifested as partial or center-weighted metering, and it values a predetermined area in the center of the frame. With partial metering, it’s as if the single spot has been enlarged, with the exposure exclusively based on the center of the frame. The key to understanding center-weighted metering, then, is to think of the center spot having feathered edges and its exposure value heavily weighted (say, 80%) with the remaining 20% of the exposure information determined by the readings taken nearer the edges. These modes often are valuable for splitting the difference between the extreme accuracy of spot metering—which can be harmful to exposures where the overall scene is contrasty yet important—and the overall considerations of full-frame averaging (which doesn’t give priority to the center of the scene).