Autofocus: Power Focus
Introduced with the announcement of the Canon EF300mm f/2.8L IS USM II and the EF400mm f/2.8L IS II USM telephoto lenses in autumn 2010, Power Focus is a mode that allows you to drive the autofocus motor electronically instead of having to use the focus ring. It is aimed at users shooting HD EOS Movies with their DSLR cameras as it makes it possible to produce a very smooth and controlled focus pull from one focus distance to another.
Power Focus mode is selected via a sliding switch on the left hand side of the lens barrel, close to the lens mount, that offers a choice of Autofocus (AF), Power Focus (PF, the central setting on the sliding switch) or Manual Focus (MF). Turning the knurled focus preset ring, found in front of the lens' main focus ring, then activates Power Focus. Two speeds are available, and these are selected by the amount of rotation that's applied to the focus preset ring.
The EF300mm f/2.8L IS II USM lens was one of the first Canon lenses to feature Power Focus mode that allows the lens to adjust the focus smoothly by driving the AF motor. Note the PF setting on the right hand side of the image.
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