Carl Zeiss Biogon T* 21mm f/4.5 ZM

Carl Zeiss Biogon T* 21mm f/4.5 ZM

Among super wide rangefinder lenses, the Carl Zeiss C Biogon T* 21mm f/4.5 ZM stands out for packing a very wide angle of view into a barrel barely longer than a body cap. The "C" denotes a compact, classic design, and the lens revives the symmetrical Biogon concept that Zeiss has used for wide angles since the 1950s, brought up to date with modern glass and T* multicoating [1][2]. It was announced at Photokina on 28 September 2004 and began shipping the following year as part of the ZM line built for Leica M mount cameras. Reviewers have repeatedly singled it out for unusually low distortion and high sharpness for a lens of its size and price.

Optically the lens uses eight elements in six groups, with a moderate maximum aperture of f/4.5 that keeps the front group small and allows a 46mm filter thread, the same size shared by several Leica M lenses. The diaphragm has ten straight blades and clicks in third stops, which produces distinct ten-point sunstars on bright point sources. Build is typical of the Cosina-made ZM series: metal barrel, engraved and paint-filled markings, a smooth focus action with a small focus nub rather than a tab, and a focusing range down to 0.5 m, although most M rangefinders only couple to about 0.7 m so the closest range is set by scale. The lens is rangefinder coupled and meters through the lens on TTL bodies; as with all 21mm M lenses, an accessory viewfinder is needed for framing because M bodies lack 21mm framelines.

The lens was offered in black and silver finishes, carrying part numbers 1419-575 for the black version and 1419-574 for the silver, and it was produced by Cosina in Japan. It shares the bayonet square hood used by the wider-aperture ZM 21mm f/2.8 and ZM 25mm f/2.8, though that hood is short and mainly cosmetic. It has long been discontinued and is comparatively uncommon on the used market.

Optical qualities

Rendering This is a slow, symmetrical wide angle, so depth of field is generous and the character is defined more by geometry and contrast than by background blur. Documented strengths are very low distortion, strong cross-frame sharpness once stopped down, and good flare resistance with high contrast even in backlight.

Sharpness Performance is good across most of the frame from maximum aperture on a Leica M, with the corners improving and peak cross-frame results reached around f/8. On film and on Leica M digital bodies the center is already strong wide open.

Distortion and vignetting Distortion is among the lowest of any 21mm M-mount lens, which makes the lens well suited to architecture and other angular subjects. As is normal for a small symmetrical wide angle, vignetting is heavy, with light falloff measured at over three stops wide open and only modest improvement on stopping down.

Flare resistance The T* coating gives generally good flare control; strong light sources can produce a faint ghost or crescent, but contrast holds up well in difficult light.

Digital use The classic Biogon design places the rear element close to the sensor, which causes problems on digital M bodies that use non-BSI sensors. On the M8, M9, M240 family and M10, this produces an asymmetrical color cast toward the frame edges that must be corrected in software, while film and the newer sensor in the M11 fare better. For this reason the lens is often recommended for film or black-and-white work, or as a deliberate choice for shooters completing a ZM kit.

History

Development and Launch The C Biogon 21mm was conceived as a compact, distortion-corrected super wide for the ZM system that Zeiss introduced for the Zeiss Ikon rangefinder and other M mount bodies. It draws on the long-running Biogon family of nearly symmetrical wide angles and was presented as a modern realization of that classic concept using current optical glass and T* coatings. Public announcement came at Photokina in late September 2004, with deliveries starting in 2005.

Special editions No major factory special editions are widely documented for this lens. It was sold in standard black and silver finishes during its production run.

Collector Notes The two factory finishes carry distinct part numbers, 1419-575 for black and 1419-574 for silver, which helps confirm originality. The ZM lenses use a small blue index dot for mounting rather than a red one, a useful identification detail. Buyers intending to shoot digital M bodies older than the M11 should be aware of the edge color cast inherent to the design and budget for software correction; those shooting film are unaffected. Worth verifying before purchase are the matching front cap and the optional shared square hood, plus 46mm filters, since the filter size is common across several M system lenses. Published weight and length figures vary slightly between sources, but they cluster closely around the figures recorded here.