Canon teleconverter

[kennchris]

I am looking at buying a teleconverter mainly to use on my 100-400 lens for birding, would like some advice from anyone that has used the MK111 converters in both the 1.4 and the 2.0,
Thanks

[Gollum]

Your camera’s AF system won’t work if you use a TC on a f5.6 lens.

[fabian628]

have you got a 1d body becuase a TC on the 100-400 will produce f/8.

I used a 1.4x TC on the 300 and 400 2.8 lenses with 1d bodies and the AF is noticably slower (it is not too bad but slower). These lenses already focus very quickly, I am not sure using a 1.4 and 2x on 100-400 will render very good AF speed. From much of what I have read on other forums use of teleconverters on the 100-400 is not so successful, either IQ degredation is almost similar to croping or the AF speed is not so good.
Alas it is hard to get long fast lenses without shelling big dollars

[Bennymiata]

I use a Kenko 2X with my 100-400L, and in good light, it still auto focusses OK, although slower than it would without the converter.

IQ is also suprisingly good with the Keno, and for far less than the cost of one Canon converter, I could buy both the 1.4X and 2X Kenkos and have enough left over for a good meal!
The Kenko's also work on lenses that can't be fitted with the Canon converters.
Having said that, they can't be used on Canon EF-S lenses, but do work on lenses made for crop sensors from other brands.