I've been taking some shots with my prime lens. Its a 50mm f1.4 I was shooting in f4.5. I focussed in auto on a persons face about 3 metres away. when I looked at the shots on the computer later, the people behind the subject I was photographing was in focus instead. I am absolutely sure I focussed on the main subject so why was that out of focus, and teh background probably 4-5 metres were in focus more? is this something all camera lenses do, using auto focus?