No existing 300mm tele prime lens goes beyond the f/2.8 aperture. But Canon is playing with the idea to break that barrier and patented a 300mm f/2.3 lens. The overall length of that lens would be close to 18cm (not bad) thanks to the Reflex. The trade off is surely a image quality drop compared to classic designed lenses. Still, I hope they will really make this lens!
via Asobinet