(single-coated)

OM-4T with mirror and diaphragm prefire
Vignetting = B @ f/2.8, A thereafter
Distortion = slight pincushion
Aperture  Center    Corner
f/2.8     C+        C+
f/4       B         B-
f/5.6     A-        B
f/8       A-        B
f/11      A         B+
f/16      B+        B
f/22      B         B-
Notes: Moderately low contrast images at f/2.8; moderate contrast at f/4 and f/22; moderately high contrast images at f/5.6 to f/8 and f/16; high contrast images at f/11.