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If both a camera body and lens have motorized autofocus, would one be used over the other or would both work in tandem to speed the focus process along?

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The lens.

There are two cases to consider, assuming a built-in focus motor in the lens:

  • The lens does not have a mechanical link for the in-camera focus motor: In this case, the camera motor cannot focus the lens at all. This is the case for the vast majority of lenses.

  • The lens has a mechanical link for the in-camera focus motor: In theory any one motor could do the job but camera makers assume the in-lens motor to be better, so they use the lens' motor. It has been requested to have a camera setting added to change this in case of a failure of the in-lens motor but no camera provides this yet AFAIK.

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If the lens has it's own focus motor it likely isn't capable of being driven by the camera motor so it'll be the lens motor getting used.

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The lens. In every case like that I've seen the spot on the mount where the body motor would engage the gearing in the lens, there's nothing there for it to connect to, just empty space.

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    \$\begingroup\$ Pentax makes lenses that use screw-drive (body motor) on cameras that don't support lens motors (K100D and earlier lack the eletrical contacts). But bodies which support both (ie all current ones) will not use screw drive if the lens has SDM — even if that happens to be broken! \$\endgroup\$
    – mattdm
    Mar 23, 2011 at 20:47
  • \$\begingroup\$ @mattdm ah, I haven't used a Pentax in years and years... good to know. \$\endgroup\$
    – cabbey
    Mar 24, 2011 at 21:36
  • \$\begingroup\$ — more like, frustrating than good, at this point. Hopefully they'll actually listen to that petition linked below Itai's answer. \$\endgroup\$
    – mattdm
    Mar 24, 2011 at 21:52

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