Originally posted by GaryH The AFA 1.7 and the DA*300 are probably the best combination for that lens. The Tamron and Kenko with PZ contacts seem to take forever to find focus.
I now have the AFA 1.7x and DA*300 (for one week so far to still working out my technique).
In the combination, the AFA 1.7x does the focusing but it is very quick and accurate. It will only focus across a short range though. You have to manually set the lens somewhere near the range of the subject.
If you have the lens set at say 3metres and the subject is 50metres away the AFA 1.7 wont focus on the subject as it is out of its focus range.
I think that this is actually an advantage rather than a limitation. If you mis-focus on a difficult target the AFA will "zip zip" across its range in 1/2 second. Having the lens go in-out from infinity to 2m and back again is much slower.
If you are having to shoot at 510mm a bird / deer / bear / celebrity or whatever might be hiding in bushes or something so would be a tricky focus anyway. Also your hand might not be so steady if your trying to trying to focus on a Grizly bear close to the minimum. focus dist.
The last thing you want is to have the lens & TC taking forever to rack in/out.