Keith over at Northlight uncovered a USPTO patent showing various optical designs for catadioptric lenses, better known as mirror lenses. The advantages to mirror lenses are reduced size, weight, and cost at long focal lengths.
Some of the historical disadvantages to mirror lenses tend to be low contrast, fixed aperture, and doughnut bokeh. That’s not to say that Canon hasn’t improved or eliminated those issues in their designs.
Optical designs that appear in this patent:
Canon RF 400mm f/3.6 IS
Canon RF 800mm f/5 IS
Canon RF 1200mm f/8 IS
Canon RF 1200mm f/10.5 IS
Canon RF 2000mm f/15 IS
Interestingly, a Canon RF 1200mm f/8 appears on my Canon RF lens roadmap. This patent may actually be...