Canon EF 400mm f2.8L IS II USM - Manual (Page 11)

ENG-10
6. Power Focus (PF) Mode
Using the playback ring enables smooth focus change.
This is a useful feature for changing focus when shooting movies.
When the playback ring is rotated in a certain direction, focus changes in the same direction as when
the focusing ring is rotated.
You can make a 2-step change in focus shift speed depending on the angle that the playback ring is
rotated to.
With the focus preset switch set to OFF, set the focus mode
switch to PF.
Rotate the playback ring to
change the focus.
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