Originally posted by heliphoto Lee, I have no idea if this is correct, but my guess is that the DOF at 400mm f/2.8 may be so shallow that the focus detection system doesn't believe it's in focus even when it's really really close.
Edit with another question: Do you have a proper filter in the rear filter holder? I have heard that these rear filters are part of the optical design of this sort of lens, and sharpness suffers without one in place (however on my Tamron 400mm, I couldn't detect a difference with or without the filter).
I just looked at the lens diagram at Dimitrov's site and it shows the rear filter in place. So, you are probably correct, that it is part of the design. A rear filter would somewhat flatten the focal plain and also increase backfocal distance.
Nevertheless, I also think - as wildman qwrote - that a "flickering" of the focus confirmation is not unexpected, given the extremely shallow depth of field. On the other hand to get something into focus at near infinite distances is not too easy too, as it is very dependend on the weather (clraity of the air, local turbulences etc.). With my old 500/4.5 I can get rarely sharp shots of objects farther away than app. 100 meters, simply because of seeing influences.
Ben