The only test of a wide angle lens on RF I am aware of is Optical Limits' test of RF 24-105 which has a maximum aperture of only F/4. Measured light fall-off (vignetting) at f/4 was 2.6EV at 24mm and 2.1EV at 105mm, which would be considered pretty awful for a F4 DSLR lens. For comparison, light fall-off for EF 24-105L ii was measured at 1.9EV at 24mm and 1.45EV at 105mm. I would be very interested to see vignetting measurements on R bodies for faster wide angle lenses. I guess RF 16-35mm f/2.8L and RF 24-70mm f/2.8L IS will be available shortly, but there are no fast wide angle RF lenses on the Canon roadmap. DSLR lenses mounted with an EF to RF adapter would not have the angle of incidence issue, so Zeiss/Leica M mount lenses with the RF adapter are probably the only fast/wide options until Sigma decides to enter the native RF mount market.
As I recall it fast M-mount lenses have serious vignetting issues as well as corner sharpness issues until stopped down quite a bit
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