Some years ago, after seeing the edge performance of a Leica M wide angle lens on an M9 compared to my Canon 5d2 equipment, I became a fan of Leica glass. While the Leica M9 was my camera of choice for a few years, the Sony a7r series' modern electronics and through the lens viewing won me over. The OEM Sony's inability to work well with my Leica M wide angle lenses, however, was a major disappointment. The OEM Sony sensor IR filter/cover-glass was just too thick, causing unacceptable off-axis image softness.
Now, KolariVision has an "Ultra-Thin" replacement glass that is only 0.2 mm thick. I converted my Sony a7r (v.1) to this and am happy to report that my Leica M glass is once again performing very well indeed, with a several caveats. First, there are some color artifacts with at least one of my favorite (28mm) Leica optics, but it can be cured with Cornerfix. Second, the extremely short, relatively symmetrical designs like the ZM 21mm f/4.5 C-Biogon seem to be beyond the limits of what I consider good performance. Third, there is still some edge softening over what I had hoped for, based on published MTF curves, but the UT converted Sony matches or exceeds the M9 edge sharpness most of the time. Frankly, I cannot tell if the cover-glass or the lens is the limiting factor at the edge. At least some of the Zeiss glass for the Sony thick OEM glass (for example the 35mm f/2.8 and the Loxia 21mm, in particular) is better than the Leica-M wide-angle performance at the edge until f/5.6. The Leica M glass seems to be better in the center at all apertures and beyond f/5.6 at the edge also. So, for most of my landscape shooting, the Leica M glass will be my choice.
I have documented the most important findings with the Sony a7r and the KolariVision UT-conversion in this PDF:
http://www.paulroark.com/BW-Info/KolariVision-UT-filter.pdf . I will update the PDF as I get more experience with the system.
At this point the KolariVision Ultra-Think conversion looks like the best mating of the outstanding Leica M wides to modern electronics for those who want relatively light weight and compact full frame, manual focus system, particularly if one already owns the Leica glass.
Paul
www.PaulRoark.com