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 & 680mm.

Patent Publication No. 2016-191761 (Google Translated)

Published 2016.11.10

Filing date 2015.3.31

Focal length 392.57

F-number 2.90

Angle of view 3.15

Image height 21.64

Overall length of the lens 371.15

BF 44.00

The first state magnification 1.40

The second state magnification 1.70

We had heard previously that an extender was being tested in a new 800mm lens, but this may make more sense.