All light meters, including those built into a digital camera, operate on the same general principles. A light-sensitive photocell regulates the amount of electricity flowing in the metering system. As the intensity of the light reflected from the subject changes, the amount of electricity flowing through the photocell’s circuits changes and is used by the auto exposure system to calculate and set the shutter speed and aperture.
Your camera’s meter measures light reflecting from the part of the scene shown in the viewfinder or on the LCD panel. 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 scene or when you zoom the lens. Suppose you move close or zoom in and see in your viewfinder only a detail in the scene, 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 scene overall from farther away.
Most cameras are equipped with one or more of the following types of metering:
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 for the scene.
Centre-weighted meters the entire scene but assigns the most importance to the centre quarter of the frame where the most important objects usually are located.
Bottom-weighted meters the entire scene but assigns the most importance to the bottom of the frame where the most important objects usually are located.
Spot evaluates only the area within a small area in the middle of the viewfinder. This allows you to meter just a specific part of the scene instead of relying on an average reading. This mode is ideal when photographing a subject against a bright or dark background.