You may have heard that Fujifilm has opened the door slightly to third-parties regarding their APS-C based X-mount. It seems they've finally given in to both unrelenting pressure from their users and the reality of being able to fully flesh out their lens lineup with more niche lenses in a timely and profitable way. One example: poor sales of the $5995 XF 200mm ƒ/2 with 1.4x Teleconverter, a specialty prime designed for sports and wildlife photographers. It seems very few people "clamoring" for that sort of lens actually put their money where their mouths are and has made Fujifilm gun shy about serving up more high performance, expensive, niche lenses. However, the result is Fujifilm opening up X-mount to approved third parties...
Despite white wall tests of the Canon EF 28mm and 135mm lenses showing acceptable levels of vignetting, my first real world use of them ended in failure.
Widening out to an approximate 22mm in 35mm format terms, the Canon EF 28mm ƒ/1.8 USM does show some hard vignetting that is never fully resolved. It’s very, very slight and occurs at ƒ/4 and smaller at infinity focus. Vignetting is prevalent fully open, mostly disappearing by ƒ/4 at close focus distances.
If the vignetting is a problem for you, the EF 28mm ƒ/2.8 IS might be a better bet but isn’t on Canon’s list of preferred lenses for the 5DSR, so it may or may not resolve well on a 50MP sensor.