The RF 1200mm F8L IS USM's maximum aperture is one stop smaller than that of the EF 1200mm f/5.6L USM, but this has considerable benefits for handling. "The f/8 maximum aperture contributes to the substantial reduction in size and weight compared to the conventional EF model," Tomohiro Ino confirms. "As the RF 1200mm F8L IS USM supports Dual Pixel CMOS AF, it is capable of fast and accurate autofocus despite the darker maximum f-stop. High image quality, autofocus and IS are also still achievable when the Extender RF 1.4x or RF 2x is attached to the RF 800mm F5.6L IS USM or RF 1200mm F8L IS USM."
Indeed, the only discernible effects with the lens extenders attached are the drop in maximum aperture and a reduction in the AF coverage. "When the RF 800mm F5.6L IS USM is used with the Canon EOS R3, EOS R5 or any other camera that supports Dual Pixel CMOS II AF, you get full focus tracking across 100% of the frame, even with Extender RF 1.4x attached," explains Mike. "With the Extender RF 2x, you still get 80% AF coverage horizontally and vertically.
"The RF 1200mm F8L IS USM still gives you 80% AF coverage horizontally and vertically with the Extender RF 1.4x, and it drops to 60% x 40% coverage with the Extender RF 2x. So, even with a 2400mm f/16 lens, you're getting a focusing area that's roughly the size that DSLR users are accustomed to. There's no impact on AF performance, apart from the size of the area."