The camera can only use data from a single AF sensor to focus. When there is more than one focus point a selection has to be made. This can be done automatically or manually.
If the camera is set for automatic selection of the focusing point, readings are taken from each point as the shutter button is partially pressed. If there are only three or five focusing points, the camera gives priority to the point covering the nearest part of the subject. This point becomes active and its data is used to focus the lens.
However, the more focusing points there are, the less chance the camera has of selecting the one you want. To overcome this problem, Canon introduced a new set of rules (algorithm) with 7-point focusing. Here, the camera does not only give priority to close subjects, it also compares the scene with data taken from a large number of sample photos and analyses the selection probability of each focusing point. The position of each point also affects the weighting given to its data. Focusing points higher up the scene are rated more important than those lower down.
An orientation sensor is built into the camera so that the system knows whether you are shooting with a horizontal or vertical format and can work out the order of the points down the scene.
If several focusing points end up with equal priority, the camera will make active the point covering the closer subject. The active point is visible in the viewfinder, either as a red highlight on the focusing screen or as an icon on the viewfinder read out.
Although automatic selection gives good results most of the time, there are situations where it will fail. If you frame a landscape with a tree branch, for example, the camera might focus on the branch rather than the more distant scene.
Manual selection allows you to choose the active focusing point before you take a picture. Although any one of the points can be selected, the centre point is often the most convenient. You can then operate the camera in the same way as a single-point model, taking the AF reading with the centre point and then using AF lock to hold the focus while you recompose the image. This way, you have full control over the result, rather than leaving important focusing decisions to the camera.
The metering system of your EOS camera is linked to the multipoint focusing. If an off-centre focusing point becomes active, the evaluative metering will also shift to give optimum exposure for the area covered by the point. Canon calls this ‘AIM’ − Advanced Integrated Multipoint control.
If you use the centre focusing point with AE lock, the exposure will be metered for the area covered by the centre point at the moment you activated AE lock. If this is not what you want, focus the lens manually so that you can use AE lock independently of the focus lock.