General Metering Methods for Digital SLR Photography
Metering is the process of sensing how much light is in the scene you want to photograph. The amount of light helps determine what aperture, shutter speed, and ISO sensitivity you or your camera need to set in order to take a good photo.
There are two different ways to measure (or meter) how much light is in a scene: using reflected or incident light.
Measuring reflected light means that you collect light that bounces off stuff and finds its way into your camera or light meter (an external accessory that measures light). Cameras use this metering method exclusively. The upside to measuring reflected light is that you’re able to meter distant objects. You can also meter specific objects in the scene, like a person’s face. The downside to the approach is that not everything reflects the same amount of light, which can fool the metering sensor. This can become a problem when photographing people of different skin colors and tones. Reflected metering can also be called spot metering.
Incident light meters work differently. Rather than sensing reflected light, an incident light meter sits in the scene and measures how much light is falling where it is. Most incident meters have a white-colored dome or disk that bulges out from the meter’s body. Ambient light passes through the dome and is measured by the meter. Cameras can’t do this without special attachments. The upside to this method is that, overall, it is less prone to being spoofed by objects with different reflective properties. The problem with incident light meters is that they don’t take reflectivity into account. If you’re photographing something very bright and reflective, an incident light meter will not take that into account, and therefore may recommend setting the exposure too low.
The long and the short of it is this: Your camera has a reasonably good reflective light meter built right in, but it’s not perfect. You have to be able to switch metering modes (explained next) or manually compensate if the camera misjudges the scene. If you want a more realistic gauge when shooting portraits and other studio-type shots, as well as many landscapes, consider buying a light meter.