We didn’t think the built in extender on the EF 200-400mm f/4L IS 1. 4x was going to be a one off. Now a patent showing an optical formula for a new EF 400mm f/2. 8L IS with a 2 stage built in extender has appeared.
This would give the lens a focal range of 400mm, 560mm Read more. . . . canonrumors.com