The exposure meter in your EOS measures the brightness of the light passing through the lens.
This then sets, or helps you select, the shutter speed, lens aperture and ISO value for correct exposure. Most current EOS cameras feature four metering modes; check your Instruction Manual that came with you camera. The differences between these are described below.
Evaluative metering: A good, general-purpose metering mode. It divides the image area into a number of zones. Metering sensors measure the brightness of the light in each zone to suit the type of scene being shot.
Partial metering: Readings are taken from the central area of the viewfinder (6 - 10% depending on camera model). Useful when the background is much brighter than the subject, e.g. backlighting.
Spot metering: Used when metering for a specific area of your subject or scene (1.5 - 4% of the viewfinder depending on camera model). For similar situations to Partial metering but when the subject is a smaller part of your image.
Centre-weighted average metering: The camera takes more account of the central area of the viewfinder by weighting exposure to the centre and then averaging for the entire scene.
See the difference in Live View
Digital photography shows you the image you have taken within a second or two of the exposure. However, Live View shooting can show you what the image will look like before you take the picture.
If you press the 'Info' button during Live View to bring up the histogram, you can check the exposure. The aim is to keep the lines of the histogram within the graph boundaries. If the lines extend off to the left you are losing shadow detail. If the lines extend off to the right you are losing highlight detail.
Norfolk Hawker, © Miles Bostock 2012, Canon EOS 550D