As already confirmed several days ago, Canon will announce RF 400mm f/2.8L IS USM lens and RF 600mm f/4L IS USM lens in later this month. Now according to CR, Canon may release two more super-telephoto prime lenses in early 2022. These two lenses will be RF 300mm f/2.8L IS USM lens and RF 500mm f/4L IS USM lens.
Tag: Canon RF 300mm f/2.8L IS USM Lens
Two Super Telephoto Prime “Big White” RF Mount Lenses Coming Along with EOS-R1 in the Second Half of 2021
Canon has already released the EOS R5 and EOS R6 several months ago, next Canon is rumored to announce the flagship mirrorless version of EOS-1D X III – the EOS-R1. Canon EOS-R1 is expected to be announced in the second half of 2021. Along with two “Big White” RF mount super telephoto prime lenses.
RF 300mm f/2.8L IS USM Lens Coming in 2020
Along with Canon RF 500mm f/4L IS USM Lens, Canon may also announce a new super telephoto prime lens RF 300mm f/2.8L IS USM lens sometime in 2020.
RF 500mm f/4L IS Lens will be the First RF Mount Super Telephoto Prime Lens

According to latest rumors on CR, Canon is also developing new RF mount super telephoto prime lenses. And the first RF mount super telephoto prime lens will probably be the RF 500mm f/4L IS USM Lens.
First Super Telephoto RF Lens Could be RF 500mm DO Lens
According to CR, first Canon super telephoto lens for RF mount is in developing, and it will be a diffractive optics (DO) lens for Canon EOS R (B&H/Amazon/Adorama/Focus Camera/WEX/ParkCamera).