canon 1.4 and 2.0x teleconverters question

Are these converters slower to focus than the raw lens?


I have read in many forums that they are 66% slower due to intentional slow down by Canon.


Can someone give me a definitive answer on this?


I have also read that the Kenko 300 pro is not slowed down and therefore faster and a better choice than the Canon brand.


I would love to know the true answer to this...not just unsubstantiated rumor thrown around.





Thanks for any help (esp since I just bought a 1.4 and 2.0 Canon for my 300 2.8)





Peter

Roger Cicala just posted a great article on TCs at http://www.lensrentals.com. In it, there's a link to a modest article about how auto-focus works and how lens aperture affects AF. Smaller effective apertures (i.e. at 420mm, you're at f/4, at 600mm you're at f/5.6, if you combined them with a tube you'd be at 840mm f/8) give the AF sensor less data to use, and therefore AF will work slower. At f/8, most bodies can't auto-focus.


Some TCs lie to the camera and pretend that they aren't in the path. As such, the camera may still AF when it otherwise wouldn't be able to AF, and may Af with faster motor commands than if the TC was recognized. However it may hunt more, and therefore the end AF result is the same or slower than if the TC had properly reported its presence.