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 part of future consumer products. However, I do have it reported as an L lens, so we'll have to wait and see on that one.
As Canon has shown with the RF 600mm f/11 IS STM and RF 800mm f/11 IS STM, they aren't afraid to bring slower long lenses to the masses. Not everyone can afford or justify the purchase of 5 figure big white lenses.