The only time lens quality really jumped out at me was a few years ago, when I first acquired my Bessa-R. I shot a retirement ceremony indoors with it, switching between the 35/2.5 Skopar and a Jupiter 8, and my Nikon FG, with a cheap zoom on the latter. When I got the 4x6 prints back I could easily tell which camera shot which, just from the higher level contrast on the Bessa. Couldn't tell when I switched lenses on the Bessa, but I could tell when I switched cameras.

BTW, would anyone like a cheap, offbrand 35-70 zoom in AI mount?