Kiron 28mm f/2 MC (multi-coated)
OM-4 with mirror and aperture prefire
Vignetting = B- @ f/2, A- @ f/2.8, A @ f/4
Distortion = none
Aperture Center Corner
f/2      C+     C
f/2.8    B+     B
f/4      A-     A
f/5.6    A-     A
f/8      A-     A-
f/11     A-     B+
f/16     B+     B+
Notes: Very cool image; moderately high contrast, except moderately low at f/2.