Vivitar 24mm f/2.8
OM-1 with mirror lockup
Vignetting = C- @ f/2.8, B @ f/4, A- @ f/5.6
Distortion = pronounced barrel with slight waveforming
Aperture Center Corner
f/2.8    C-     D
f/4      C+     C-
f/5.6    B      B-
f/8      B+     B+
f/11     A-     A-
f/16     B+     B
Notes: High contrast, but at the expense of resolution.