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.

Bill I use the 20mm Kenko extension set all the time on my Nikon 70-300VR to get butterflies, so yes I think they should work a treat with the 80-400Vr. You do loose a bit of light, but on the D300, I do not find it a problem. This was taken at f13. Example below

http://www.naturesportal.net/photos/...80_ASYtd-L.jpg

Thanks Nancy. Sounds promising. Seems like I should pay another visit to B&H for the Kenko set. Nice image. I wonder if you focused manually, or if the 70-300 VR allowed you to lock in the AF at f/13?

Bill the AF does work, but the zoom ring helps fine tune the focus.

Hi Bill, I've used the Kenko tubes with my 80-400. They work fine with the exception of the loss of AF if I rotated the camera body the in a way that caused the loss of contact between the lens and body! I don't know if this was because my lens mount was a little worn or what. The combo of lens and more than one tube is clumsy but does work. As for loss of light, there is very little loss because the tubes don't have glass. Even with the full set of tubes mounted, you should loose less that 1/2 stop! Good luck.

Bill, I use the Kenko 25 mm tube with my D300 & D700 with lens from 24-70, 70-300, and 500 f/4. AF works fine with all of them. The 25 mm is about half the cost of the 3 ring set.

Thanks Mack. That's a good suggestion.