Canon FD 28mm f/2 bayonet mount
Canon EF with mirror and diaphragm prefire

Vignetting = C+ @ f/2, A- @ f/2.8 and f/4; A thereafter
Distortion = waveform with moderate barrel tendency

Aperture Center Corner
f/2      B      C+
f/2.8    B+     B-
f/4      A-     B+
f/5.6    B+     A-
f/8      B+     B+
f/11     B+     B
f/16     B      B
f/22     B-     B

Note:
moderate contrast images at f/16 to f/22;
high contrast images at f/2.8 to f/4;
very high contrast at f/2 (perhaps from underexposure) and f/5.6 to f/11.