I have an XSI, and usually use the 50mm f/1.8 mkii lens for portraits. One thing that's been bothering me lately is that I always have to use manual focus when doing portraits, which really turns out to be hit and miss. Autofocus just never gives me sharp photos. Is it because of the lens that I have to use manual focus? I know that using an aperture like 1.8 is going to give me a shallow DOF and therefore make it harder to focus on a precise area, but I have to manual focus even when the aperture is set to f/5.6 to get a sharp photo. Is this normal?

http://www.flickr.com/photos/48939759@N07/4907065533/


Above is a link to a picture I particularly like; it was just a fluke that I focused well.