To me, A7 and A7R do have their good and bad points for adapting with older lenses:
A7 - With the 24mp sensor it is less demanding on the MTF performance of the lenses, especially for older wide angle lenses with MTF to be dropping very rapidly towards the border areas. The effect of "Red Gate" could be minimized as well because the pixel size is relatively larger than A7R. However, as mentioned by Sony, A7 do not have the special arrangement of shifted microlenses on the sensor as in the case of A7R, and that would certainly make vignetting to happen more likely. In addition, because of the existence of the AA filter in A7, the thicker filter in front of the sensor would certainly degarde the performance of the symmetrical design wide angle lenses with very steep incident angle of light ray path on the focal plane. (Please refer to the article by Dr.Nesse which explained why Leica had made the filter on M8 to be so thin:
http://blogs.zeiss.com/photo/en/wp-cont ... stagon.pdf on page 12)
A7R - As mentioned by Sony, the 36mp sensor on A7R had been designed specifically for lenses with shorter back focus distance. These include shift microlenses arrangement to reduce the effect of vignetting, thinner sensor filter by removing the AA filter layer etc. However, we should be aware that the pixel density of 36mp is really no kidding, for which the demand on the MTF performance is ultra-high! This had been proven in the situation of Nikon D800E already. For the symmetrical wide angle designs like the Biogon or Super Angulon, the distortion had been corrected to extremely low level, but with the trade off on the vignetting performance as well as border resolution. This had been proven already in days of film (e.g. the test of G Biogon 21/2.8 vs CY "Apo" Distagon 21/2.8 ) but with the characteristic of digital sensor the effect is simply magnified now!
So for a good result with 3rd party lenses on the A7 / A7R bodies, it is always better to check:
1. The clearance of the rear lens element to the film plane - as a rule of thumb the longer the better for the result. The shorter one the easier to create smearing effect especially in case of A7 with thicker sensor filter
2. The MTF of the lens - pick one with a high MTF value, and especially for one which could maintain a good result up to the border region. Otherwise it could not meet the resolution requirement of the A7R 36mp sensor
Failing to meet either one of the requirements may end up in disappointing result. And please also remember the very precise focusing requirement in case of the 36mp sensor, and you need a very steady handheld technique to avoid even a very slight hand-shaking
Therefore, if one insist to use the legacy M lenses with digital sensor, the ulimate choice should be still the Leica M8! Or if one is crazy enough, you may send your A7 / A7R to have all sensor filters to be removed. (But then you would need to add UV/IR cut filter in front of the lenses for colour pictures, just as the case of Leica M8)