The Soviet-era Helios 44/2 lens (actually a 58mm lens, and a direct copy of the Zeiss Biotar 58mm lens) has a cult following for the way it renders out of focus areas at wide apertures. I recently got one that had been adapted to work on the Nikon F mount and still focus to infinity, and I have started to experiment with it. The person who did the work on the lens has an interesting YouTube video that explains the process.
Nikon F4, Helios 44/2 58mm lens
Rollei RPX25 film