I know this is a somewhat common problem with this lens w/ the built-in focus motor. But what about without the focus motor? I'm using the version w/out it and feel like I have been getting some front-focusing problems @ 50mm, but haven't had a chance to test it yet. Maybe it's just soft, as I've only noticed it when photographing objects at a distance @ f/4 or lower. Is it possible to have this problem without the motor, or is it limited to those with the built-in motor?