I can't see any advantage Canon had in making the flange 20mm instead of say 17mm (the EF-M is 18mm) and this is the biggest reason I considered moving from EF to Sony instead of to RF.
Not an optical engineer, but a slightly longer flange distance improves the angle of incidence on pixels at the edge of the sensor, potentially resulting in better light collection, and less bending of the light, so possibly better handling of chromatic aberration. I’d love an actual student of optics to correct me. I’m running off physics that’s 20 years of of date and mostly forgotten.
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