Canon EF500mm f/4L IS II USM Instructions Manual page 11

6. Power Focus (PF) Mode
Using the playback ring enables smooth focus change.
This is a useful feature for changing focus when shooting movies.
With the focus preset switch set to OFF, set the focus mode
switch to PF.
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.
Rotate the playback ring to
change the focus.
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