The Voigtlander 15mm f/4.5 Super Heliar II is quite the unique little lens. There you have the first part: its the only possible way to shoot ultra-wide on an M series camera with a physically small lens (2 inches and 6 ounces as compared to the fairly huge Zeiss 15mm f/2.8). The only real downside is that its f/4.5, but it is quite sharp wide open so that’s not an issue except in low lighting.
The new version is rangefinder coupled and will activate the 28/90 framelines, but we do rent the Voigtlander 15mm Viewfinder separately in needed. Most people tend to set it at f/5.6 and use zone focus to just snap away — everything from about 5 feet to infinity should be in focus at that setting.