The Zeiss 21mm/2.8 Biogon ZM is longer and narrower (46mm filter thread) than the Leica 21mm lenses, so it might be a better option physically with narrow RF base cameras like the Leica CL, Minolta CLE and the Voigtländer Bessas. Optically it's better than the 21mm Elmarit-M, and perhaps better even than the 21mm Elmarit-M ASPH. I love mine.