I was curious to see how the Mamiya-Sekor CS 135mm 1:2.8 compares with the Sigma Art 60mm 1:2.8 at 2.8 when everything is scaled down to the resolution of the MS lens. The MS lens has a resolution roughly 2/3 that of the Sigma. Applying a 1/3 of a pixel of Gaussian blur to the Sigma picture takes it more or less at the the resolution of the MS.
The original of the MS picture is a jpeg captured with the minimum amount of sharpening the XE1 allows but I applied later a fair amount of sharpening and I increased the contrast slightly.
The level of the Sigma picture was adjusted to match more or less the MS picture.
The depth of field of the Sigma on the 4/3 format camara is about three times the DOF on the MS lens mounted on an APS-C camera. The Sigma picture is in focus from the tip of the nose to behing th eyes. By contrats, the MS picture is sharp only on a tiny strip including the eye.
My conclusion that the factors that account for the differences in sharpness of the original pictures are the 2/3x resolution difference combined with a 30% difference in contrast coming from the lighting and finally by the depth-of-field differences in both setups.