Metering the Setup
Metering icons aren't the same on every camera but the spot metering icon usually has a dot.
How light or dark a photo appears is determined by your camera's autoexposure system except when you are in manual exposure mode. The autoexposure system uses a light-sensitive photocell to measure the light reflected from the subject and uses the measurement to calculate and set the shutter speed and aperture.
Your camera's exposure system measures light reflecting from the part of the scene shown in the viewfinder or on the monitor. The coverage of the meter (the amount of the scene that it includes in its reading) changes just as your viewfinder image changes, when you change your distance relative to the setup or when you zoom the lens. Suppose you move close or zoom in and see in your viewfinder only a detail in the setup, one that is darker or lighter than other objects nearby: the suggested aperture and shutter speed settings will be different than if you meter the overall setup from farther away.
All parts of a setup are usually not equally important when determining the best exposure to use. The exposure of the foreground subject is usually more important than the exposure of the background. For this reason some cameras offer more than one metering method. The choices typically include the following:
- Matrix metering divides the image area into a grid and compares the measurements against a library of typical compositions to select the best possible exposure.
- Center-weighted meters the entire setup but assigns the most importance to the center quarter of the viewfinder where the most important objects are usually located. This mode is ideal for portraits and larger desktop studio subjects.
- Bottom-weighted meters the entire setup but assigns the most importance to the bottom of the viewfinder. Designed primarily for landscapes to reduce the influence of a bright sky on the exposure, this metering mode has limited usefulness in the digital desktop studio.
- Spot evaluates only a small area in the middle of the viewfinder. This allows you to meter just a specific part of the setup instead of relying on an average reading. This mode is ideal when photographing a subject against a bright or dark background.
Meter weighting can cause a few problems. For instance, a dark object located off center against a very light background may not be exposed properly because it is not located in the area the meter is emphasizing. Or, in some cases, rotating the camera vertically may give undue emphasis to one side of the scene. These occasions are uncommon, but when they occur you can ensure accurate readings and exposure settings by using spot metering or overriding autoexposure.