The 10D, D30, D60 and 300D have a AF
system requiring a maximum effective lens aperture of f5.6 or better. If adding a convertor reduces the effective aperture to f8 or less, AF
will not work. That's the short answer.
In fact, things are a little more complex than that - the AF
system is essentially an optical device which uses edge contrast to perform its magic - we've all noted how lighting conditions can affect AF
even with fast lenses, and the f5.6 cut off is actually an arbitrary figure, engineered into the system - the lens tells the camera "I'm not f5.6 or better" and the camera says "well sod the AF
In fact, were it not for this limitation, there are conditions where the camera could AF
(if it were allowed) however, performance would be poor and unreliable, hence Canon have put in a cut off point. Canon convertors have pins on the mount which communicate the effective f stop to the camera - if you cover these pins with tape, the camera will attempt to AF
, though in most cases this will just result in a lot of hunting.
Confusion on this issue arises because some cheap 3rd party converters do not provide 100% compatible mounts and cannot tell the camera the effective aperture. Hence you sometimes see statements like "el cheapo convertor lets me use AF
but Canon can't"
Colin K. Work, Pixstel