I've got a tamron 17-50 F2.8 VR on my T2i and have noticed for the past while that at 50mm (fully zoomed in) and F2.8 (wide open), I'll lock focus onto my subject with a half shutter press but when I attempt to recompose, the focus slips to where my centre point has been repositioned. I shoot with only a single centre focus point. I catch it right away, of course, because with such a shallow depth of field it's readily apparent the moment it happens. I suspect this isn't normal but don't know if it's the lens or the camera or me. Since the AF motor is in the lens I'm guessing it's a lens issue. Anyone else experience anything like this? Or is there something I'm doing wrong that I'm not aware of?