1. TC’s always impair autofocus speed/acquisition.
2. TC’s always work best with 2.8 & larger maximum aperture.
3. TC’s always a compromise, but cost effective.
4. More focal length is useless if you can’t focus the shot, or can’t achieve adequate resolution.
5. Where most “pros” (I consider myself of this group) use a TC:
A 1.4 on a 400 2.8; a 1.4/1.7 on a 70-200 2.8; any of the TC’s on a 300 2.8; A 1.4 on the 500/600 4.

So, if you have these lenses, a TC can make a useful companion. I’ve used a TC 1.7 on the 200-400, and didn’t care for the downside. The 1.4 may be adequate, but each shooter different.

Conclusion: Better to use a long lens on a DX body if it’s reach you need, and noise issues not elevated (i.e., you have enough light to shoot action at max. ISO 1250). The 200-400 on DX an excellent overall long-lens compromise to fiddling with TC’s.

Some people are so damned prissy about their gear its a wonder that they ever take any photos at all.

Thank you for this review. I have this lens and 1.7x, absolutely hate the combo. Got to get 1.4x soon.

I have a recent D300 – just before they were discontinued. The 1.10 update primarily issued to fix the “dead battery syndrome”. My 300 came updated, so I can’t speak to any AF improvement. Nikon says that you lose AF with anything beyond 5.6 max aperture. This (inaccuracy) has been the story since the inception of AF. Can you get AF with the 1.7 & 2.0 converters? Yes, but for what I do with the 200-400 – fast-action cars/planes/sports – the AF just hopelessly impaired. That’s why I say: 1.4… barely; 1.7… NO. You can probably MF about as fast/faster when TC’s are involved on an F:4 base aperture zoom. Primes a somewhat better story. See imagepower.de site for some testing results.