I have compared my 400mm f/5.6 to my 70-200mm II with 2X TC III. For static shots in the central part of the frame, I see more variation from frame to frame (AF before each test shot) than between the two lens combinations. On the other hand, if I am shooting BIF, the prime wins hands down. The zoom can get lost in the blue sky and focus down to MFD. At that point I have no hope of reacquiring focus since there is no longer a hint of a visible image through the viewfinder. I purchased my 400/5.6 a few years ago after giving up on the zoom/TC combo for BIF, particularly raptors. The 400 prime is an old design, but it is also very sharp. Additionally I have a 100-400mm II that is wonderful for slower moving wildlife. I only wish it had a second focus limit option of 10 meters, as it can also lose lock and focus down to 3 meters. At that point any hope of re-locking on a raptor is gone. Since a used 400/5.6 could be as old as 1993, I chose to buy a new one for under $1200, and I couldn't be happier with my purchase.