Vivitar 17mm f/3.5 (multi-coated)
OM-1 with mirror lockup
Vignetting = C- @ f/3.5, B @ f/5.6, B+ @ f/8, A- @ f/11
Distortion = pronounced barrel with slight waveforming
Aperture Center Corner
f/3.5    C      C
f/5.6    B+     B-
f/8      A-     B
f/11     B+     B
f/16     B-     B
Notes: Moderately low contrast wide open and at small apertures, very good contrast at approx. f/8