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.