Many modern digital and film cameras uses one or more metering patterns to calculate the most appropriate exposure for any given scene. The metering system is what that is in the camera that determines the shutter speed and aperture to be used for any given scene. More often than not, the brightest or white region is largely used to calculate the exposure setting.
The followings are different kinds of metering systems:
1. Spot and partial metering.
In high contrast conditions and where you need only to sample one small portion of the scene, this is probably the best mode to use.
2. Multi-segment or matrix metering.
The metering area is divided into many small segments. The camera evaluate the brightness within each segment before deciding on the exposure setting.
3. Center-weighted metering.
As the name implies, more weightage is given to the center portion of the frame.
Most camera automatically set the metering system. However, some more expensive models allows you to select the type of metering to use. You can improve the success rate by selecting the right metering mode for the given conditon.
Typically, the trick for achieving good exposure is to determine what is the Subject of the scene and adjust the exposure setting correctly for it.