Quote Originally Posted by Feanor


Since people have found that the tape-on-connectors trick to prevent lens reporting to the cameraincreases focus speed, Ican only assume that Canon have slowed down this when using the extender on purpose -perhaps to encourage people to buy the proper lenses.



I don't know the reason for it (I would hope there's more to it than trying to 'force' people o buy more expensive lenses), but yes, it's by design. When used with the 1.4× extender, autofocus speed is reduced to a half. When used with the 2× extender, autofocus speed is reduced to a quarter.