Zeiss Biogon 21mm with zero distortion for M mount, the reference ultra-wide using the classic symmetrical design.
The Zeiss 21mm f/2.8 ZM is a wide-angle prime for the Leica M mount from Zeiss's ZM range — the f/2.8 Biogon T* design. ZM = Zeiss M-mount (Leica M bayonet). Biogon formula for low distortion and even illumination. Leica M mount.
f/2.8 maximum aperture. 21mm focal length (full-frame ultra-wide). Biogon T* optical formula. Rangefinder-coupled. ZM (Leica M bayonet). Manual focus. At approximately 320g. Leica M mount.
The Zeiss 21mm f/2.8 ZM Biogon is the faster sibling of the 21mm f/4.5 C ZM (id 2627) — the f/2.8 aperture is larger and the lens physically bigger and heavier. The Biogon formula is optimised for minimal distortion across the ultra-wide field of view. A very capable wide prime for M cameras.
On the used market the Zeiss 21mm f/2.8 ZM is a premium ultra-wide for Leica M. Condition checks: front element, rangefinder coupling. Leica M mount — the f/2.8 Biogon (not the f/4.5 C compact version).