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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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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. –  user2719 Jun 10 '12 at 17:56
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1 Answer

up vote 2 down vote accepted

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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