Originally posted by raider Would not normal teleconverters (installed in between the lens and the body) also present an extra load to the focusing motor?
Physics tell us that anything at rest tends to stay at rest, anything in motion tends to stay in motion.
In order to start something moving we must apply a force to the object equal to the speed that we wish to achieve, in order to stop the object we need to place an equal but opposite force to the object.
The shaft going through a normal tele-converter is very light and easy to move. The lenses that I have seen on eBay would have a very large mass in some cases several thousand times that of a small shaft.
In this case there would be a rotational force suddenly applied to this mass, and then an equal force suddenly applied to stop it. Over rotation would probably be a problem forcing the lens to change directions to compensate adding more of a load to the motor. This would at least shorten the life of the batteries, the worst thing that could happen is it could damage the camera.
The only rear mounted tele-converter that I know of that moves any lenses within the tele-converter, is the Pentax teleconverters that would convert a manual focus lens to autofocus, "these lenses are no longer available from Pentax, although I have seen them on eBay, they go for about $350 each.
I would rather spend my money on a good autofocus lens.
joe1955