My 300mm autofocuses well with the 1.7x teleconverter.
In low light, it hunts a little, but at least it hunts quickly . I've found the most effective thing to do is to give it a head start by roughing in focus with the focus ring and letting the AF take over from there. To be expected from a f/6.7 aperture.
In good light, it snaps right into focus.
In either case, it autofocuses faster than the 80-400VR I've used before. There is no other affordable way to get to 500mm.
I can't tell you about performance with the 1.4x, I've never used it... but my goal was maximum reach, so the 1.7x was a compromise between the 1.4x and the 2x.
If you go by Nikon's recommendations, they do not advise using the 1.7 with an f/4 lens. The 1.4 is what they recommend.
That said, the combo does work quite well. As stated, it isn't the best choice in low light, but at f/6.3 I wouldn't expect it to be. I've found some early morning photo shoots to be nearly pointless with this combo, as it simply couldn't focus well enough to do the job. I do like it, but it just needs enough light to work. Keep in mind that 500mm is a bit to handle, as you don't have VR to help you out. I guess that's why they made tripods!
It just comes down to if you want an f/5.6 400mm lens, or an f/6.3 500mm lens. Both setups have their place, and an individual's choice is based on their own needs. I needed (wanted) the length, so I chose the 1.7.
Hey 'blue1'... I just got and tested the 300 f4 AF-S w/ Kenko 1.4x DG on a D200. AF is
hardly affected and here's a 100% crop straight out of camera. The Nikon TC's were too
rich for my blood as I'd only use 'em occasionally but I never cared for anything more than
a 1.4x on f4 lenses. Of course YMMV.
. I've found the most effective thing to do is to give it a head start by roughing in focus with the focus ring and letting the AF take over from there. To be expected from a f/6.7 aperture.
