G'Day
My understanding is that the autofocus is reliant on the aperture of the lens. During focus the lens is wide open and so at the lowest F number for the lens. The 1D series of cameras will autofocus up to f8 while others only autofocus up to f5.6. Therefore a f2.8 and f4 lens should autofocus on a non 1D series camera with a 1.4 converter but only the f2.8 will still autofocus with a 2x converter (The 2x will still autofocus on the 1D series though).
Of course, as David has said you can tape over the pins (but this has been known to burn out the AF motors).
Steve
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