Sunday, February 08, 2009

Switch Metering Modes

As mentioned earlier, it’s really the camera’s exposure sensor—known as an exposure meter—
that does the majority of the work when figuring out how to shoot your picture. It decides how
much light is needed to adequately expose your picture. So it should come as no surprise to learn
that cameras distinguish themselves by the kind of meter they use. Some meters are better than
others at metering a scene and applying the right exposure.

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 in the front of the picture, so the camera tries to get that part of the scene
exposed properly. You can see a center-weighted light meter in Figure below. A number of digital cameras still rely exclusively on this kind of meter for ordinary picture taking, and if your
camera includes it as an option, it’s a useful backup exposure mode if the multisegment metering
doesn’t work well for an unusual photographic situation.

Multisegment Meters
Better than center-weighted metering for routine photo situations, matrix or multisegment
metering is shown in Figure below. A camera that uses this kind of metering is usually better at
exposing tricky scenes by balancing the lighting needs of several discrete regions within a
picture. Instead of concentrating primarily on the middle, matrix meters gauge the light in many
parts of the scene at once. 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 a spot. The spot meter is never the only kind of meter in a
camera; instead, it’s an option that you can switch to if the center-weighted or matrix meter fails
you. As you can see in Figure below, the spot meter measures light exclusively in one tiny portion of the screen, ignoring the rest of the frame completely. That can come in handy on occasion, but a meter that only measures the light in the central 1 percent of the frame will typically take very poor pictures—either highly under- or overexposed, depending on the situation.Many cameras use multiple focus points in the viewfinder and let you specify one of the
focus points as the spot meter location. The advantage? Say you want to take a picture of a
subject that’s not in the center of the screen. You can lock in focus using one of the alternate
focusing zones and measure the exposure based on that same point as well. Be sure to read your
camera manual to see how this feature works on your camera.So when should you use the spot meter? Any time you are 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 are 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 the 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.
Varying the metering mode—especially the spot meter—is best used in conjunction with the
third technique, exposure lock.

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