Originally posted by jmbower Thank you all for the suggestions. I have managed to get a couple shots I like, but mostly they were larger wading birds. I realized I had an issue when, not too far from me, couldn't have been more than 25 feet or so, a prothonotary warbler flew up, perched on a tree, with a dragonfly in its mouth...a fantastic shot, but I couldn't get close enough to really get the detail I wanted even at 300mm.
One follow up questions, I've noticed some teleconverters say something about AF only working at f4 or higher...is this common? Is it just an assumption that it's manual focus for the most part at long tele lengths?
So when you add a TC to your lens you are reducing the amount of light that comes in. With a 1.4x TC you are reducing 1 stop and with a 2x TC you are reducing 2 stops. Most (not all) cameras are made to give reliable AF up to f5.6, what that means is that the max aperture of the lens has to be f5.6 for good AF. If you add a 1.4x TC to a f5.6 lens then you essentially have an f8 lens.
Now there is wiggle room here. Depending on the lens and camera you can still get AF to work with a f5.6 lens and 1.4x TC. What you will find is that it will be slower to AF and it will hunt (sometimes a lot) if it starts to get dark out. The acceptable limits of the AF hunting is really up to the photographer. I've used a 1.4x TC with a Tokina AT-X 400mm f/5.6 prime lens (on a K10d and K20d) and it has been OK.
Hope that helps
John