The point of this lens is to let in a lot of light so that you can go without the flash indoors.
In my first shooting session, I had a really hard time with focus.
Three things learned about focus:
1. The focal point is where the center target in the viewfinder is pointing.
2. Auto focus is your friend (and this lens has an auto focus, AF, setting)
3. AI SERVO is your BEST friend. In AI SERVO, the camera continuously focuses on the target subject as either you or the subject move.
The following pictures were shot without the AF, and AI SERVO options chosen:
in the "P" setting AI SERVO with Continuous Shooting Fixes this issue!
And I think that this is my best photo to date (even though my son is sitting by the safety gate, and it looks like he's in jail!)