The issue: ray angle interacts with the sensor cover glass thickness; a lens is designed assuming a specific thickness. Deviations from that thickness degrades optical performance, particularly off-center. A secondary issue is color shading, along with accentuated field curvature and astigmatism.
One would hope that the L-Mount alliance would have standardized on sensor cover glass thickness and characteristics, so that lenses of either brand could be used with the same results on either-brand camera. But in marketing, form trumps function.
Definitive finding for M-Mount lenses
This shootout between the Leica SL2 and the Panasonic S1R is definitive in proving major image quality differences between the SL2/S1R for M-mount rangefinder lenses, including sharpness and color shading and field curvature.
Includes images up to full camera resolution from f/2 through f/8, plus crops.
Roy P writes:
Not in the least surprised – this is the kind of garbage I got from the S1R using M lenses, which is why I returned the S1R to B&H, following my 3-day trip to Alabama Hills. Some of the multi-shot high-res images were also quite hideous. Stopping down to f/5.6 on the Zeiss 35/1.4 ZM and Leica 50/2 APO were not bad, but still, just not satisfactory.
TW, the guy who used to make the kit for the 6-bit coding no longer makes them. Any other way to fool the SL2 with a non-Leica lens? Maybe just hand-paint a pattern on the lens?!
DIGLLOYD: coder kit for M lenses is no longer made. You'll have to set the lens coding manually.