A few of the most recent Canon telephoto lenses a feature called "Power Focus" or PF. What is the purpose of the setting and does it benefit photography at all?

I have found the following lenses currently have this feature:

  • Canon EF 300mm f/2.8L IS USM II
  • Canon EF 400mm f/2.8L IS USM II
  • Canon EF 200-400mm f/4L IS 1.4x USM

The manual for the Canon EF 300mm f/2.8L IS USM II notes on page 10:

Using the playback ring enables smooth focus change.

This is a useful feature for changing focus when shooting movies.

Power Focus is described on Canon's Infobank page as "a mode that allows you to drive the autofocus motor electronically instead of having to use the focus ring." As you indicate in your question, it's meant for changing focus from one point to another very smoothly, for use while shooting video.

As one doesn't normally change focus in the middle of taking a photograph, it's hard to see how Power Focus would be useful for still photography. The same page goes on to say: "It is aimed at users shooting HD EOS Movies with their DSLR cameras..."

  • What does "drive the autofocus motor electronically" mean? – dpollitt May 14 '13 at 19:08
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    It means you push a button that activates the AF motor to change focus instead of turning the focus collar with your hand. – Michael Clark May 14 '13 at 19:11
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    It's interesting to see this coming out on lenses. In the video world, it used to be the upscale feature to add a focus ring to allow for manual adjusting of focus. Now they are adding the ability to use a power focus to move instead of manually using the ring... – AJ Henderson May 14 '13 at 19:39
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    @dpollitt What Michael Clark said, except that in this case you twist the knob rather than push a button. Judging by the description of the two speeds, it sounds like the knob is probably spring-loaded, like a jog wheel: twist a little for slow, or a little more for fast. – Caleb May 14 '13 at 20:25

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