I also own a Sony a7 and for a change the weekend before last I did some photography of my grandsons using the a7 fitted with a Zeiss Planar 45mm F2 Contax G series lens which I borrowed from my son ( yes we are all into photography). This fits onto the Sony with a rather clunky adapter which has to turn as it operates the manual focussing of the lens.
This particular Zeiss lens is reputed to one of the sharpest lenses ever made and although it is an old design-and this particular lens is probably around 20 years old-it is still an incredible performer. The photos have a distinctly Zeiss look to them which is markedly different to the look from Leica lenses to my eyes.
I am not into pixel peeping and photographing walls for lens tests but I can say that even at very high magnifications the sharpness of photos from this lens is extraordinary even right into the corners and with the lens wide open. The sharpest lens I have ever used? I can't say but it certainly is up there.
You can find,with luck,find one of these lenses for sale for US$250-$500 depending on condition.
All the photos below were taken with the lens wide open at F2.