Advise please on which tele-extender

condyk wrote:
.... You lose AF of course with either, so you need a good eye for MF

Canon says "With the EF 2x II, AF is possible with any EOS body if the lens has an f/2.8 or faster maximum aperture."

?????

MALI

I have a hard time picturing many occasions when the 800mm of a 2x extender would be needed...

Leo

Tarpontim
As many others have advised go for the 1.4 extender with your lens, you will not be disapointed, don't forget to keep your shutter speeds up, increase your ISO if needed.

BusterBoy
you are spot on about the quality with 300mm f2.8 IS and using either extender 1.4x or the 2x.
What you may not realise is the canon 2x extender was originally designed and built specifically for just the 300mm f2.8 (non IS version) so not suprising the quality is so good, as shown in your superb picture above.
the other long fast lenses were built to make use of this extender.
I also use the 300mm f2.8 IS with the 1.4 and its supperb, I do not think any noticeable drop in performance.
Any other points about extenders / converters , check out the sticky where I have supplied some useful info previously.

Happy new year to all
Cheers
Martin (Huckaback Photo)

Just to re-iterate,..

the Original question was:

(emphasis added)

For the 100-400mm the better choice by far would be a 1.4X

(emphasis added)

But the lens in question is not f/2.8 or faster,. it is a much much slowr @ f/5.6... so no AF with either of the T-cons.

Check the FAQ if you have further questions regarding what lenses will AF on which bodies with which t-cons,. it's all in there.