But just thinking about the 1200mm f/8 with a 2X, assuming it can still AF. And the fact the R3 noise performance is so good. It is mind numbing that a lens with that kind of focal length is as usable as it is!

Yes the aperture is narrow and you would not be shooting is super dim light. But still, that is incredible to have that kind of usability, still lower weight than the EF 800mm f/5.6, shorter and still have 2400mm!!!!!

If shot on a R5, it can be used in 1.6 crop mode to reach an even more staggering effective focal length.

Besides all the obvious downsides of such a long focal length. Just having it as an option is amazing.