Rocky,
The more "open" (aperture) your lens is, the more problems will occur with (lens) focus shift. In other words, a subject shot at F8 will have less turmoil than one shot at F2.8. Many lenses, even high-end ones exhibit this problem. On some cameras the issue can be resolved by focusing with a more open aperture and then re-adjusting to a more closed one, similar to "depth-of-field".
I am no photography expert but I try to eliminate problems like this by:
* Backing up
* Moving my image farther away from the camera
* Closing down a few F stops...
....&....
* Shooting at a higher ASA.
Good luck, Capt. Chris
PS: Where's "Litch" when we need him??