Meter in the Middle of the Frame

As I said in the previous tip, the matrix or evaluative meters that are in virtually all cameras these days are very accurate even with scenes that would have many seasoned photographers scratching their heads. There are some situations though when metering only a specific region of the frame will provide more reliable results. One of the options for those situations is center-weighted metering.

As their name implies, these meters concentrate their readings on a small central region in the viewfinder. That are is typically about 8-10cm across, though on some cameras (like my Nikon D90) you can select from three different sized areas--8mm (the default), as well as 6mm and 10mm. The meter doesn't meter exclusively from that area, but typically gives that area about 75-percent of the "weight" or importance and about 25-percent to the remainder of the frame.

The advantage of center-weighted reading is that you can tell the camera to give more importance to one specific subject or area within the frame. In this shot of an old New England headstone, for example, I knew that the slate stone would be a perfect middle tone (middle tones provide the most accurate readings with any meter) and that the snow might otherwise fool the meter into thinking there was more light than there really was and underexpose the stone. The meter read the entire scene, but put most of its emphasis on the middle-toned subject. The meter reading was perfect. The matrix meter might have provided just as good a reading, but why risk it if there is a more accurate method?

Center-weighted metering works best when there is a mid-toned subject of small, but not tiny, size within the frame. By reading off that subject and then using your meter-lock function (usually just holding the shutter-release button down halfway), you can lock that reading and get an accurate exposure for the entire scene.

In the next tip we'll talk about taking meter readings when the subject you want to read is especially small.

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.