Summary: Bought this to double my lens because a fixed 400mm is to expensive. Picture quality is great. Must use a mono pod or tripod for better quality. Loving it so far. Going to Oregon next week for some coastal shots.
Check Autofocus. Will only work with lenses of 2.8 or faster!
6 March 2013
Summary: This extender will not auto-focus with a lens slower than 2.8. My lens is an EF 70-200 4l IS which is listed on Canon's website and in the literature the extender came with as compatible. There was no asterisk indicating that auto focus would not work. Canon tells me they will correct their literature. Tried the lens but I am returning it.
Summary: Tested this like 5 weeks ago. The Extender 2x III performs better on my 70-200L IS Mk2 than on the Mk1. There is a noticable difference in AF and IQ. I had them side by side. The Extender 2x III gave no improvement on the 70-200L IS Mk1 over the Extender 2x II. I got the Extender 2x III in the end.
Pros: Good build, good image quality, still fast AF