This is weird. My D5100 is supposed to be able to auto focus with all AF-S lenses, including the AF-S 28-70mm f/2.8D. For some reason, I borrowed my friends 28-70, I couldn't autofocus with it. All focal length were okay except the ones near 70mm. It focused well with his D1x but on my D5100, it hunts and hunts and hunts and never stops. I wonder if this is an issue with my camera. I thought it was the problem with the side focus points but I tried the middle one but it doesn't work either. I have a full frame lens and it worked well. Is it the D that is causing the problem? Seems unlikely. Any help?