Extension tubes with 80-400 zoom?
Does anyone have experience with using extension tubes on a mid-range tele-zoom, particularly with AF function? I have had a number of occasions where I have been carrying the AF Nikkor 80-400 VR 4.5 - 5.6 and have been able to get closer than the 7.5 feet minimum focusing distance only to find that I can't lock in on the subject. The Nikon PK-12 and PK-13 tubes won't support the AF function on that lens; but apparently the Kenko "DG" set will do so as long as I maintain an effective aperture of 5.6 or less. Does it make any sense to use that combination? I am planning a trip to the Galapagos next spring and thought it might be a possible option.
Also, what effect do the extension tubes have on exposure?
Thanks in advance.