Canon FD 28mm f/2.8 bayonet mount
Canon EF with mirror and diaphragm prefire

Vignetting = B- @ f/2.8, B+ @ f/4, A- @ f/5.6, A thereafter
Distortion = pronounced barrel

Aperture Center Corner
f/2.8    B+     B
f/4      A-     B
f/5.6    A-     B
f/8      A      A-
f/11     A-     B+
f/16     B+     B
f/22     B      B

Note:
high contrast at f/2.8;
very high contrast at f/4 to f/16;
moderately high contrast at f/22.