EOS AF and max ap

[Ken Durling]

I need a refresher.  Could someone ploease explain, or point me to an 
explanation, of why it's the *maximum* aperture and not the working 
aperture of a lens that decides AF speed and accuracy with different 
bodies, and how those bodies differ?  For instance, the 1 series is 
said to be able to "focus to f/8" (making an f/4 lens on a 2xTC 
feasible) while other bodies need f/5.6 or so?  - this is what I want 
to  understand better.

Thanks

Ken

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