Ray angle of rangefinder lenses interacting with the relatively thick sensor cover glass of the Sony mirrorless cameras introduces severe astigmatism in outer zones, along with lateral chromatic aberration.
Astigmatism can be understood as “two planes of focus generating a double image”. These are the sagittal and tangential foci, as used in MTF charts.
A specially modified Sony A7R II (KolariVision) and a stock unmodified A7R II were used to explore the performance of the Leica 28mm f/1.4 Summilux-M ASPH. This is not a lens review, and is targeted strictly at showing how the modified camera performs.
This detailed analysis of the Leica 28mm f/1.4 Summilux-M ASPH is likely to be a good proxy for the performance differences with most Leica M and Zeiss ZM wide angle lenses. Similar behavior was seen with the Zeiss ZM 35mm f/1.4 Distagon in field shooting.