Hi all,
Canon metering attempts to consider every photon in the scene, combine all of the values obtained, find the average, and meter that average for a midtone (13% in the case of Canon). From this midtone point, there are defined points where saturation (over exposure) occurs, and the point where noise becomes greater then the signal. Dynamic range.
So if we place the focus point over an area of the scene that is predominantly darker then the average, the Camera will attempt to meter this area closer to a midtone. And the same occurs when we place the focus point over an area of the scene that is predominantly brighter then the average. This is useful if we decide we want to place priority on the shadows or the highlights. In other words, let's place (some exposure) priority in these areas, so they don't get crushed to black, or over exposed.
Thanks