There's some appreciative discussion over in the Sony 7r threads about the late model (1990 - 2006) Elmarit-R 19 mm lens, and examples shot with it look pretty nice. It's a thoroughly retrofocus lens, doesn't cause problems in the corners on the Sony (or with a Nikon D800E). But for the M there are other options. The 16 and 18 mm range of the 2006 WATE appear from the technical specifications (MTF and distortion curves which max at 3% about 2/3 of the way to the edge of the frame) to be very similar to the older but faster R design. The newer (2008) SEM 18/3.8 would seem to render a little differently from the other two designs, sharper over a wider circle and with a different set of distortion characteristics. More like the very sharp (2011) SEM 21/3.4 from the same design group. Curiously, the prices of all three of the 16 to 19 mm superwide options seem to be about the same these days.
Has anyone got evidence or strong opinions on how differently each of these render? Not outdoors in Kodachrome country but indoors in complex well-lighted scenes, like the one Guy M exhibited in the Sony thread.