Any 'good' lens = any lens with good performance across the frame - will do just fine with 36Mp sensors in full frame, and for example the FE55 shows virtually the same performance on the 54Mp equivalent NEX7 as it does on the a7r.
Sony Lens: Primes - Sony FE 55mm f/1.8 ZA Carl Zeiss Sonnar T* SEL55F18Z (Tested) - SLRgear.com! (click blur charts to observe at f5.6-f8 where you can see the crop area in the a7r image = NEX7 chart)
The 'across-the-full frame' need is the reason (I believe) we are seeing at the same time - give or take a year - not one or two, but four fabulous 50-55mm lenses with cross-frame excellence, from Sigma, Sony, Zeiss and Leica. I talk too much about this stuff so will simply provide a few quotes:
DxO:
'Once the Nikon D800 was tested in the labs, it became clear that *with a whole range of lenses*, the Nikon D800 was out-performing every other camera tested..' (in pre a7r days anyway)
Zeiss - (and you can plug in 36Mp and other Mp counts and get the same trend - as it is optical science):
"We used the 12 MP camera with the Macro-Planar 2/100 ZF at aperture stops 8
versus 22, a combination which certainly yields optimal performance and on the
other hand a setting with diffraction-limited reduced resolution. These two cases will then be compared to the large apertures, the optimal aperture stop 8 and aperture stop 22 on the 85 lens on the 24MP camera.
This comparison turns out clearly in favor of the 24 MP camera with all aperture
stops...combining any optical performance with a camera with a higher resolution limit and a low-pass filter designed to match improves the transfer function.
It is particularly interesting to compare the two curves for aperture stop 22 where the optical resolution of both cameras is limited solely by diffraction and is approx. 75 lp/mm, i.e. clearly less than the resolution limit of the 24 MP sensor. *The difference in resolution between the cameras is maintained.*
1. Doubling the number of pixels improves the transfer function even if the sensor resolution is better than the resolution of the lens.
2. The curve for the poor lens on the 24 MP sensor is almost as good as the
curve of the good lens with the 12 MP sensor.
Thus concerns that today's good lenses may in general not be able to cope with a
24 MP sensor appear somewhat exaggerated. Of course the full potential of the huge data files can only be used with a very good lens. But we can expect some improvement of image quality not only for the optimal aperture stops but also outside
of the range of best performance..."
Concerning the differences between 12 and 24 MP you will have noticed that they are sometimes surprisingly small. But this is also a question of the motif, not all of them reveal the differences of the high spatial frequency transfer. But they exist: extremely fine line and dot patterns are the nightmare of all digital cameras."
How to read MTF curves? Part II, by H. H. Nasse.
