I've been going crazy with focus on my 5100. After trying all the suggestions on the various forums and not being able to a picture of my dog with the 5100 that came anywhere near close to the quality of the picutures I could get of him with my point and shoot Canon A610, I was seriously obsessing about using the 610 to make a video of me using the 5100 and all the lenses as rifle targets.
Then I took a look at the settings on the Canon and saw that the ISO was set to 50. The Nikon, however was 3200. So I set the ISO to 100 and shot another pic. Again the Nikon ran to 3200. So I set the max ISO to 100 and shot the pic again. This time the pic was great. As goot as the Canon and at least twice the size. Went in again and found the Auto ISO and set to OFF and set the max ISO back to the maximum and shot the pic again. Again, great pic shot at ISO 100. Took a lot of photos over the weekend and the focus is fine.