For this review, the lens was tested on a Canon EOS 5DS R using Imatest ... starting off at -1.1 stops at f/2.8 but on stopping down to just f/4 reducing to just under half a stop.

A more budget-friendly option, like the rumored Canon RF 400mm f/4.0, would definitely make these lenses more accessible to serious enthusiasts and professionals without breaking the bank.