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Canon's new Rebel T4i (a.k.a. EOS 650D) has a new "hybrid AF" mode, which allows continuous AF during video, but only when used with one of the new STM lenses.

I've seen chatter on the web forums where it's suggested that this is a greedy move from Canon in order to force people into buying new lenses. I understand that STM allows a better AF experience during video, but is there a technical reason an STM motor is a hard prerequisite?

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    \$\begingroup\$ I don't know if you'd get a definitive answer, but I'd suggest it's a trade-off in customer complaints. Some people will whine about the lens restriction if they make it a hard requirement (call it the "typical greedy corporation" whine). Others will complain about the AF noise if they allow C-AF with all lenses (the "Canon is technically incompetent" whine). It's probably marginally better from a PR perspective to be seen as greedy by some than technically incompetent, even if the noise is a RTFM problem. \$\endgroup\$
    – user2719
    Commented Jun 10, 2012 at 17:56

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No, there is no hard prerequisite. You can use any lens you want. Just that STM "allows a better experience" as you say, because:

  1. It is really silent. (no noise during video recording because of vibrations etc.)
  2. Better design for Video (contrast-based) AF - smooth focus etc.

Also see the follwoing hands-on experience:

We found the focus to be much snappier than on the T3i, in particular it was very smooth with the new 18-135 STM lens attached to it. During video, focus was nearly silent and smoothly moved to grab the subject we wanted. With a non-STM lens the focus isn’t as smooth, but it’s still accurate, though the same trouble with low-contrast subjects exists.

Source here.

So for tl;dr readers: it works also with non-STM lenses but if you can, use STM ones.

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I cannot say anything official about hybrid AF working or not with non-STM lenses, but I have one clear experience with a third-party "Tamron SP 70-300 F/4-5.6 Di VC USD" lens, and this is awful:

In phase detect AF mode the lens needs 1-2 seconds to focus, in Live-View mode on the EOS 650D it takes 2-4 seconds, which is plainly slow. Especially above 200mm or with moving objects it has severe problems focusing at all in LiveView mode. I often observe that the lens seeks its focus in the wrong direction first, focusing to the close end, then back to the far end when trying to re-focus on a distant landscape. It is my understanding that if it would use a hybrid AF then the camera should know at least sort of where the focus should be, instead of searching back and forth through the full range.

I therefore conclude in practice that either hybrid AF is not working with 3rd party lenses or that it is working too poor to even notice.

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