The Leica 18mm f/3.8 Super-Elmar-M ASPH (at about $3195) might just be a must-have optic for the Leica M shooter, and possibly for the Sony A7R shooter. It makes a terrific ultra wide angle complement to a 24mm lens.
This f/3.8 - f/11 aperture series in Guide to Leica shows the performance of the 18/3.8 SEM with Ultra-HD images and generously sized crops.
Leica 18mm f/3.8 Super-Elmar-M ASPH Aperture Series (M240, Beaver Pond in Lundy Canyon)
On a Retina display, the image quality sings.
Lundy Canyon Beaver Pond, November 7, 2013
Leica M Typ 240 + Leica 18mm f/3.8 Super-Elmar-M ASPH