Does it seem far off infinity when you get to the lock point?
In other words, is the furtherest point of focus far from the camera? Given the majority of shooting with this camera is with f16 and above (seeing you have the f8, not the f5.6 lens, I'm assuming that you're not after speed for hand held sooting), you may have a focus distance clode to the hyperfocal distance. If so, forget about infinity and shoot away!!
While you're looking for the data page that Ian suggested, also look for the page with hyperfocal distances for your lens as well. Even if you fix your issue, this information will be vital for any future shooting with this lens/camera combo. I have all of my lenses' HF and DOF data on a simple spreadsheet on my iPhone. I can't stress the importance of using HF over infinity!