Canon's RF-mount 400mm f/2.8 — the mirrorless flagship sports telephoto.
The Canon RF 400mm f/2.8L IS USM is a super-telephoto prime released in 2021 for the RF mount, one of the first big-white primes for Canon's mirrorless EOS R system. It shares its optical formula with the EF 400mm f/2.8L IS III USM of 2018, repackaged with the RF mount and updated control ring.
25 elements in 17 groups including fluorite and Super UD glass, ring-type USM autofocus, optical IS rated at 5.5 stops, 2.5m minimum focus, 52mm drop-in filters, weight 2,890g. Compatible with the RF 1.4x and 2x extenders, and carries a lens function button set and customisable control ring.
Its weight matches the EF III version, roughly a kilogram lighter than earlier EF 400mm f/2.8 generations, which makes hand-held sports and wildlife work realistic. On R-series bodies it supports eye and animal detection AF at f/2.8 where slower zooms cannot.
Used examples are scarce and mostly from professional sports shooters; check the front element and hood for impact marks, the drop-in filter slot, IS and USM noise, and tripod foot wear. The EF 400mm f/2.8L IS III with an adapter is the cheaper route; the RF 600mm f/4L suits longer reach.