The centre point is a cross type which means it can read both vertically and horizontally when using fast lenses (f 2.8 and faster I think) so is more positive in low light with a fast lens than the outer focus points. My 7D and others have cross sensors on all the focus points but not the 5 series. Having said that my old 40D is still pretty good in low light, the area where it struggles a bit aided by my ineptitude is taking birds in flight. (BIFS) but that is another story.
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