More interesting information following Artie's discovery on the Kenko converters focusing at f/8 on the canon 1dx.

I just got my Kenko 2.0X PRO 300 Teleconverter DGX a few minutes back.

AF works perfectly well with the 1dx and 600 II. Here is a summary of my observations on AF with this combo:

- AF is very fast. Af fast as with the canon 1.4x vesrion III, as far as i can tell. Much faster than with the canon 2x on the 1dm4. The AF can go from infinity to MFD in a jiffy and vice versa. Snaps in focus with no hesitation. (Of course this is a little subjective, I do not have a way to measure this that is compatible with my laziness).
- The AF appears precise, but this requires more usage to confirm.
- The AF works with all the individual sensors I have tried, all types, including at the border.
- All AF modes work (single sensor, all 61 and all the other modes).
- IS mode 3 works, did not test the other modes.

Unsurprisingly, given the prior observations with the 800 and 1.4.x, the 600 and 2x requires at least a 2 stops exposure compensation. Maybe a little more than 2, not sure 100% about that yet.

Oddly though, the 5D mark III DOES FOCUS with this combo (600 II and Kenko 2x). It is a little slow though and and I did not spend much time on this.

I do not know yet about image quality, will follow up on this in the next couple days.

Hope this helps,

/martin