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Possible Duplicate:
What are the aperture limits for autofocus on Canon DSLR cameras?

It is mentioned on page 79 of the EOS 60D manual that the center point AF is twice as accurate at apertures up to f/2.8 and all other 8 cross type points works with lenses having maximum aperture of up to f/5.6.

Does that mean AF will not work at all with this zoom sigma lens : Sigma 18-200mm f3.5-6.3 II DC OS HSM, as maximum aperture at telephoto end is f6.3 ?

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marked as duplicate by mattdm, whuber, Itai, dpollitt, MikeW Mar 16 '12 at 7:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

    
See my answer to the linked question: "The limitations ... generally do not apply to third-party zoom lenses that have maximum apertures of f/6.3 at the long end of the zoom range, as they actually report a maximum aperture of f/5.6 to the camera...." –  DragonLord Nov 10 '12 at 0:58

1 Answer 1

AF will engage, as the lens lies about its maximum aperture in the long end to keep the body happy. Since the aperture is narrow, obtaining focus lock may fail (especially at close distances or in low light levels) and the lens continues hunting. Use the center point in tele end to increase your chances, as its position enables it the best view.

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