Canon RF 400mm f/2.8 L IS USM Instructions Manual page 15

7. Power Focus (PF)
By using power focus, you can turn the playback ring to change the focus smoothly.
This is a useful feature for changing focus when shooting movies.
Focus preset switch
Set the focus preset switch to OFF, and the focus mode switch
to PF.
You can make a 2-step change in power focus speed, depending on how much the playback
O O
ring is turned.
Focus mode switch
Playback ring
Turn the playback ring
manually to change the
focus.
ENG-14