Before getting into the different metering modes, it's helpful to understand how a camera light meter works. When you press the shutter button part way, the camera activates an internal light meter, which reads the light values coming into the camera to determine what the proper exposure is. It works this bit of magic by considering the subject as middle gray. How much of the subject is included in this measurement is determined by the mode you select. All dSLRs have some form of evaluative metering and center-weighted metering, and some dSLRs also include a spot-metering mode. Let's take a closer look at each of these and when you'd select one over the other.
Evaluative metering (Canon and Sigma) is called matrix metering in the Nikon world, honeycomb by Sony, ESP by Olympus, and segment by Pentax. By whatever name, evaluative metering works by dividing the image area into sections like those in Figure 4.1.
Figure 4.1. Camera meters work by dividing the viewfinder into segments that are evaluated for lighting values. The number of segments varies by camera model.
The number of segments varies widely from one camera to the next, with some using fewer measurements, like the Sigma which uses 14, up to the Nikon, which uses up to 1,005.
The meter reads the light value from each of these segments and averages them out to obtain the best overall exposure, which is the one that keeps the most information within the range the sensor is capable of recording, from dark to light. This is one area in which dSLR cameras really excel over their compact cousins. Because the dSLR sensor is much larger, it's able to record a wider range of light values (see Chapter 1 for a complete run-down on how the sensor records light).
Center-weightedmetering concentrates the measurement of light on the central area of the image area. This allows you to get a more accurate reading for a critical portion of the scene and works well when there is a fairly even range of light values in the scene. Figure 4.2 shows an example of the area measured by center-weighted metering.
Figure 4.2. In center-weighted metering modes, the camera uses the central part of the viewfinder to make the exposure decision.
A derivative of center-weighted metering is center-weighted evaluative metering, where the entire image area is considered when determining exposure, but more emphasis is placed on the central area of the image.
Spot metering, typically available on mid-level and pro-level cameras, measures only a specific area of the image, often as little as 3 percent of the total scene, to determine exposure. On some cameras, the spot meter is fixed on the center of the viewfinder; on others, such as the Canon, you can set the meter to read from any of the auto-focus points. Figures 4.3 and 4.4 show examples of spot-metering modes.
Figure 4.3. Many cameras also have a spot-metering mode that uses a very small area of the viewfinder to base the exposure on. This mode is best when working in difficult lighting situations.
Figure 4.4. More advanced cameras, such as the Canon 1Ds Mark II, can use any of the auto-focus points as the location for metering. The selected spot is shown in red.
Spot metering is the best choice when you have a specific area of the image that is critical for exposure. An example of this would be photographing a person against a very bright or very dark background, a situation which causes evaluative metering to over- or underexpose the main subject. If your camera doesn't include a spot-metering mode, you would choose center-weighted in such an example.