With an adapter, the Leica 16-18-21mm f/4 Tri-Elmar-M ASPH makes a compact ergonomically excellent optic for mirrorless cameras like Sony A7R, with a friendly ray angle for very high performance. The Sony A7R and A7 have very few good lens choices, so one is forced to seek out alternatives, particularly in the wide-angle area*.
In Guide to Leica:
My interest here was in seeing just how good the Tri-Elmar is when pressed into use on a 36 megapixel sensor, its wide angle siblings being troubled by ray angle issues at the periphery in particular
This ƒ/4 - ƒ/16 aperture series includes HD and UltraHD images as well as the usual crops.
* Specialty lenses stick to their respective guides.