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.
As for the lens itself, the focus is snappy on the TechArt Pro adapter, though becoming very unsure in AF-C. While it works with face detect, it doesn’t work with eye AF. The focus ring is direct and not drive by wire, with a 120 degree throw lock to lock, making it easier to manually focus.
ƒ/1.8 at minimum focus
ƒ/1.8 at infinity focus
ƒ/4 at minimum focus
ƒ/4 at infinity focus
ƒ/8 at minimum focus
ƒ/8 at infinity focus
ƒ/32 at minimum focus
ƒ/32 at infinity focus
I bought this lens for the ability to shoot ultrawide landscapes and astrophotography, and for that, the minimal vignetting should be tolerable, despite the issues at infinity focus. If I need to, I can crop to 5:4 or 3:2 to eliminate most of the vignetting. It also works perfectly fine in the 35mm crop mode. This lens is small, light and cheap, something the Sigma 28mm ƒ/1.4 Art is not, and is ideal for backpacking.