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Monday, November 16, 2009

Understanding the Three Types of Light Meters

Many digital cameras (including virtually all DSLR cameras) offer a choice of three light-metering options, typically including: matrix (also called evaluative metering), center-weighted and spot metering. Understanding how these metering types differ from one another and knowing when to use each is an important part of getting a good exposure. In today's tip I'll discuss matrix metering which is, by far, the most frequently used metering method (and with good reason).

Matrix meters work by dividing the frame up into a complex pattern grid that covers the entire frame. The camera then uses a series of algorithms to compare the various brightness regions of the frame and arrive at a good exposure. The way that these algorithms are programmed is nothing less than astonishing in their sophistication and complexity. The cameras have been programmed with the results of hundreds of thousands of potential exposure situations and when you point your camera at a scene, the camera's micro-computer compares the scene you're shooting to all of those exposures to find the one that most closely matches your subject. Sounds like science fiction, I know, but it's all true--and it happens in the blink of an eye. Many matrix meters also take into account not only the colors of your subject but, using information from the lens and focusing system, determine which parts of the scene are closest to the camera and are likely the main subject. Wheww!

What the camera is looking for as it analyzes a scene are familiar landmarks of shape, color and tonality (brightness). In a really simple scene, for example, if it sees a tall vertical subject of average tone in front of a bright background in the upper half of the frame and a slightly darker foreground below the subject, it makes an educated guess (a very educated guess, since it was probably a team of supergeeks from MIT that programmed it) that what you're shooting is a person standing on a green lawn in front of a bright blue sky (and wearing a red shirt). The camera no doubt also knows that your subject just got back from a week in Miami and has a nice tan. And in most cases, it would be correct. It then rummages around in its memory banks to find what it "thinks" is the best exposure for that subject.

The amazing thing about matrix metering though, is that it can handle far more complex scenes than that. In my shot of Notre Dame above, for example, the meter has determined that what I'm shooting is a bright yellow subject (it probably even knows it's a piece of architecture based on the shape pattern of the subject) against a blue sky. But even that is a mild challenge for a matrix meter. I commonly trust matrix metering when shooting subjects like a bright blue dragonfly sitting on a dark green leaf in front of a black pond with sunlight reflecting off of the water. And the exposures are (as the Brits say), spot on.

Matrix meters are extraordinarily accurate in a huge range of lighting situations, both complex and simple, and that's why they are the default metering systems of most cameras. In the next few tips I'll talk about the other types of metering methods and why you might choose them over the genius of matrix metering.

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