RandonOrbits, that applies to the Canon 1.4x, because of its design. The Kenko design is different, and allows it to be used over the entire 70-300L range. I don't have either tele-extender, so this isn't from personal experience, but that seems to be the consensus. And as for the extender eating another stop of light and impacting sharpness, I think that's the expected price you pay for gaining the reach.