I made all the photos with Canon EOS 1 Ds Mark III handheld (much harder work than with EOS 6D in silent mode)
At f/4 this lens is good, at f/2.8 far corners are soft.
When working at full aperture f/1.8 at close distnaces everything is fine. Sharp in the center and soft in the corners but softness is mixed with shallow depth of field.
At greater distances softness of the edges and coma at the edges are quite visible.
ef 20/2.8 at f/5.6 or even f/8