Rumor: A Canon RF 500mm F4.0 IS USM is in development

A new rumor from Canon Rumors states that there is a 500mm F4.0L IS USM in development for the Canon RF mount.

They suggest that the lens may come out around the same time as a RF series 1 camera, which seems to be sometime in the distant future.

The fact that it's under development or on an internal timeline is probably no real surprise, lenses take years to develop, test, and release.  Canon most likely has 15 to 25 lenses currently under development for the RF mount.  All of them in some specific stage of development.

Canon Rumors states;

In the past, we’ve also been told that an RF 300mm f/2.8L IS USM was also under development, though we have yet to see any RF related patents for super telephoto lens design.

Canon tends to announce super telephoto lenses in pairs, so an RF 300mm f/2.8L IS and RF 500mm f/4L IS being announced alongside each other wouldn’t be a surprise.

Most super telephoto designs have a backfocus distance that exceeds even the Canon EF mount which is 44mm.  These could very well be used on the RF mount and we'd never know it.  However, we did discover patent applications for the Canon RF mount just today that includes the 400mm 2.8, 500mm F4.0 and the 600mm 4.0.