Canon FD 50mm f/1.8 S.C. breech mount
Canon F-1 with mirror and diaphragm prefire, 2nd sample

Distortion: slight barrel
Vignetting: C @ f/1.8, B+ @ f/2.8, A- @ f/4, A thereafter

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

Note:
moderate contrast at f/1.8;
moderately high contrast at f/2.8;
high contrast at f/4 through f/16.