(multi-coated)

OM-4T with mirror and aperture prefire

Vignetting = B- @ f/2.8, B+ @ f/4, A- @ f/5.6, A @ f/8
Distortion = slight waveforming with distinct barrel
Aperture Center Corner
f/2.8    B-     C
f/4      B+     B
f/5.6    A-     A-
f/8      A-     A
f/11     A-     A
f/16     A-     A-
Notes: Moderately low contrast at f/2.8 and f/16, moderate contrast elsewise.

OM-4 with mirror and aperture prefire, different sample than above
Distortion = slight waveforming with distinct barrel
Aperture Center Corner
f/2.8    A-     C+
f/4      A-     B-
f/5.6    A-     B+
f/8      A-     B+
f/11     B+     B+
f/16     B+     A-