this is also known as Evaluative metering. The camera meters across the frame, and then averages the overall brightness and chooses the exposure accordingly.
This metering mode will be fine in the majority of situations where there aren’t too many bright highlights or dark shadows, or backlighting, and the scene you are photographing is evenly lit.
in this mode the camera still meters the entire frame, but assigns the greatest weight to an area in the centre of the frame. So this would be the mode to choose when your subject is in the centre of the frame, and you don’t want the exposure to be affected by very dark or bright areas around the edges.
Centre-weighted is often referred to as the classic metering mode for portraits, as it exposes for the subject rather than the background (assuming your subject is in the centre of the frame!).
However, this wouldn’t be the mode to choose if you’ve composed your picture with your main subject off-centre.
here the camera meters only a small part of the frame. On my Nikon D700 the area is a circle 4mm in diameter – the exact amount will vary from one camera to another. This means that you can take a very precise reading, getting the exposure correct for your subject, and ignoring brighter or darker areas in the rest of the frame.
See on www.my-photo-school.com