T3i-how to choose AF 9 points?
In auto focus point selection, the camera normally chooses the closest point coveret by a focus point.
Pre t4i rebels only have cross type focus point in the center.(+) and only vertical in others (-)
To get more consistant focus, you should focus point manually, covering the point you want in focus.
Watch this Series of 3 video's explaning canon autofocus.
First one is