50mm Sonnar-C vs. Summilux Asph

[EdwardKaraa]

And furthermore, leaving aside for the moment that the rangefinder focus mechanism is at its limit in testing to the degree of accuracy of the lensrental post above, on an optical bench I understand that the best focus is found having no regard to the lens's focus hardware, so no focus shift comes into the results, no field curvature etc.

When the Sonnar-C came out, Zeiss's MFT graphs looked quite ok, but what they didn't reveal is that those sharpness results were not available if you were focusing with a rangefinder camera rather than an optical bench.

That is very true, but in my opinion the C-Sonnar is really a mild case compared to the Summilux. I have seen plenty of amazing shots ruined because it's next to impossible to focus it with the RF patch and then recompose placing the main subject in zone B, due to strong field curvature. For this particular reason, I much prefer the C-Sonnar because the shift is predictable and can be easily learned.