Yep, then it needs to be checked. The elements are set in an optical group together into the focussing barrel held in place by the front retaining ring which is where the filter and hoods are screwed into. If the lens has taken a knock to the front its possible that the barrel has come ever so slightly ajar enough to affect. If it were MY lens I'd be into right away. If you're handy with the screwdrivers I could talk you thru it.