I once had to clean sticky diaphragm blades off a Mamiya 645’s 80mm f/2.8 lens. While putting back the rear lens assembly, a glass element fell out and I didn’t take note of which way the glass was supposed to be facing. I just put it back in the way I thought it should. And surprise, surprise. At wide open, the lens gives edges a blurry look. So I decided to keep it that way. I now have a kind of soft-focus lens for free.