I own the Biometar (twice, actually, and a couple of Pentaxon Six bodies - it was my second earnest foray into medium format film, that system), but honestly wasn't aware of its character. So, thank you for the information! I've already planned on taking out the Sixes come spring
Small 50mm lenses ("nifty fifties") tend to be character lenses - my Nikon 50mm f/1.8 AI-S (last generation) definitely shows a lot of these "adaptable" (varying) rendering characteristics, depending on distance and aperture. But I find the Nikon lens a bit less "fun" than the 7Artisans (and the Nikkor *is* optically better).