Canon FD 85mm f/1.8 bayonet mount
Canon EF with mirror and diaphragm prefire

Vignetting = B- @ f/1.8, A- @ f/2.8, A thereafter
Distortion = pronounced pincushion

Aperture Center Corner
f/1.8    B-     B
f/2.8    B+     B
f/4      A-     B+
f/5.6    A      A-
f/8      A-     B+
f/11     B+     B
f/16     B-     B-

Note:
moderate contrast images at f/1.8 and f/22;
moderately high contrast images at f/2.8 through f/5.6
(the optimum aperture) and f/11 to f/16;
high contrast images at f/8.