I started testing my 5D MK III for autofocus beyond f/5.6 yesterday. I decided to start by using my 1.4X and 2X Mark II TC's stacked together rather than taping pins. The day was overcast, and dim, but not exceptionally dark either.
I had my 135mm f/2 L handy, so I put both TC's on it (F/8) and it had no problem autofocusing about 40 feet away, but it was quite slow. Then when I tried it on some flowers 6 feet away, it chattered rapidly, focusing back and forth as though in a loop. Back to a 20 ft distance it focused slowly again. I tried it numerous times with the same results.
Then I tried my 70-200mm f/4 IS (F/11 equiv), it did not even try to focus (my 1D MK III would snap in with this combo when focus was close).
Next, my old non is 70-200mm f/2.8, (f/8 equivalent). It autofocused reasonably fast and reliably every time with the 2 TC's stacked.
Finally, my 100-400mm L at 400mm (f/16 equiv). No go not even a attempt to focus as expected.
Next, I'll tape the pins and try the 70-200mm f/4 + 2X TC (f/8) and the 100-400mm L + 1.4X (f/8). I'm thinking it might work.
i'll post the results.
I had my 135mm f/2 L handy, so I put both TC's on it (F/8) and it had no problem autofocusing about 40 feet away, but it was quite slow. Then when I tried it on some flowers 6 feet away, it chattered rapidly, focusing back and forth as though in a loop. Back to a 20 ft distance it focused slowly again. I tried it numerous times with the same results.
Then I tried my 70-200mm f/4 IS (F/11 equiv), it did not even try to focus (my 1D MK III would snap in with this combo when focus was close).
Next, my old non is 70-200mm f/2.8, (f/8 equivalent). It autofocused reasonably fast and reliably every time with the 2 TC's stacked.
Finally, my 100-400mm L at 400mm (f/16 equiv). No go not even a attempt to focus as expected.
Next, I'll tape the pins and try the 70-200mm f/4 + 2X TC (f/8) and the 100-400mm L + 1.4X (f/8). I'm thinking it might work.
i'll post the results.