I received a set of extension tubes for my b'day and today I was seeing what they will do with my 100mm f/2.8 macro lens. I had on the 100mm plus all three (=68mm) extension tubes and was working near 1:1 on the focus. I made a small change and flipped it on AF to get close and was surprised that it went to near infinity to focus. I touched it up a little with MF and here is the shot.
I then manually took it back to almost 1:1 and achieved focus there also. Here is that shot with no changes.
I was amazed.
TF