I am getting emails asking whether Leica M lenses* will work well on the Sony A7r and Sony A7. The answer could be different for each camera! One might perform poorly and the other far better: my guess is that the A7R will fare much better than the A7 due to its microlens design which is absent from the A7, as per Sony:
Every gapless on-chip lens (OCL) is designed and positioned to cover every pixel and eliminate the spaces between the micro-lenses to collect significantly more light. Moreover, each on-chip lens is optimally positioned depending on its location to accommodate the sharper angle of light entering the periphery, which is caused by larger sensor dimensions being teamed with the E-mount's short flange-back distance.
The reason involves ray angle to the sensor and the sensor design; see Sony Full-Frame NEX A7r: Rangefinder Lenses, Telecentric Design for detailed discussion. There is existing coverage of ray angle behavior in the review of the Sony NEX. Sensor glass thickness is presumably the same in both models, but Sony makes no statement on that aspect; generally speaking thin sensor cover glass is best for peak sharpness.
Also, wide angle lenses of 28mm and wider are the real issue. Lenses of 35mm and longer generally fare well.
I’ll be testing Leica M lenses on the new Sony A7R and A7 exploring the performance issues, including color shading and sharpness (but note that Leica M and Leica R lens coverages goes into Guide to Leica, regardless of camera platform). I’ll also be testing the various native-mount lenses for the A7R and A7 such as the 35mm f/2.8 and 55mm f/2.8 and 24-70mm zoom; that coverage naturally goes into Guide to Mirrorless along with the A7/A7R review.
PROVISO: I don’t yet have a Sony A7R or A7, and I can’t state with certainty that these adapters work perfectly on those camera bodies. But the lens mount is the same and therefore they should work. But I just can’t say-so as yet.
To use Leica M lenses on Sony A7/A7R/NEX, use a good quality lens adapter like the Novoflex Adapter for Leica M Lens to Sony NEX Camera (Sony A7/A7R mount is the same as NEX). With heavier lenses the Novoflex ASTAT-NEX Tripod Collar for the adapter is useful for balance and reducing strain on the lens mount (e.g., Noctilux). There are other quality adapters, the Voigtlander one I also have and it is of good quality, but lacks a tripod collar option.