Canon's RF lens lineup has been filling up rapidly with both standard choices and more extreme designs, but there are still some notable absences, particularly in the supertelephoto range. That will surely change eventually, and it seems we're on the way to that, as multiple patents have emerged with some innovative and versatile designs.
Canon's supertelephoto primes lenses, commonly known as the "big whites," are popular professional tools for sports, wildlife, and bird photographers. The EF mount came in six common options: 200mm f/2, 300mm f/2.8, 400mm f/2.8, 500mm f/4, 600mm f/4, and 800mm f/5.6. On the RF mirrorless side, we've seen the 400mm f/2.8, 600mm f/4, and 800mm f/5.6, as well as the 1200mm f/8.
However, patents have recently emerged for 300mm f/2.8, 400mm f/2.8, and 600mm f/4 lenses, all with built-in 1.4x teleconverters, which would turn them into 420mm f/4, 560mm f/4, and 840mm f/5.6 lenses, respectively. Lenses with built-in teleconverters have gained some popularity in recent years. They provide a lot of convenience, especially for someone like a wildlife photographer, who may not want to carry multiple large lenses over long distances, but manufacturers can also tweak the individual teleconverters to better optically match each lens, creating better overall image quality than if an external teleconverter was used.
Of course, a patent does not guarantee a lens will make it to the market, but at the very least, I'm sure we'll see a 300mm f/2.8 eventually, and one with a built-in teleconverter would be quite intriguing. I suspect such lenses would be popular.