As I'm sure some of you will have seen, Canon has bought out some very long, very light and very slow lenses for their new R5 and R6 Mirrorless Full Frame cameras. The 600mm and 800mm lenses have DO optics, their own version of PF....(Phase Fresnel) to make the lenses pretty compact (and inexpensive !) too. https://www.canon.co.uk/cameras/eos-r-lens-announcement/ Who'd have thought a 600mm f11 and a 800mm f11 were on the cards? I didn't know mirrorless cameras could AF with such slow lenses? ...and they have x1.4 x 2 x TCs! I wonder if Nikon can have a reply to these?