Originally posted by Canada_Rockies I use manual focus. The only shots at 680mm I have taken were with my K10, and it wouldn't even think about AF at f/9.5. I haven't been out with the K3 and combination, but I suspect it won't care for it either. I learned to MF back in the 1960's on an original Pentax. It doesn't bother me much. I chimped on the K10 for the first shot or two to ensure I was looking at the right thing, but when pushed I just shoot and save.
I have sort of operated on the assumption that f/5.6 glass was needed to get AF to work, though sometimes you can get a little more.
This implies that if you are using an f/5.6 lens to start with getting af to work with a TC is doubtful, though on some subject matters it will. A 1.4 TC eats about 1 f stop of speed
IT's the reason I spent some serious dollars to buy an FA 300 f/2.8 lens. --- I can put the HD TC on it and the resullts are happy as you can see of the photos of thet Sealions a couple pages back.
of course the lens that I have that has 'put lots of TC's on me' is manual focus to start wtih--- than is the Pentax-A 400mm f/2.8. It's useful ness is not so much taking photos in really low light where you need the f/2.8, It's usefull
ness is to jam a 2x-L TC on it and it puts you out there at an effective 800mm at less than f/8 or if you get desperate you can stack the TC's and lurch out to the 1100-1200mm range. places you can't go
http://www.vannattabros.com/12-05/a400-217.jpg
This photo was taken with a 2x-L converter stacked with the 1.7x adapter of pentaxes' which I think given an effective 1360mm focal length.