The Carl Zeiss Biogon T* 2,8/21 ZM ($1,428 list) is one of two 21mm lenses available from Zeiss for Leica and other M-mount rangefinder cameras. It's quite compact compared to similar SLR lenses that cover a full-frame field of view, and manages to show very little distortion. Via a simple adapter it can also be paired with mirrorless cameras, ranging from Micro Four Thirds models from Olympus to pro-level full-frame Sony Alpha 7R($629.99 at Amazon). Its field of view will narrow a bit if you use it on a camera without a full-frame sensor: its field of view is roughly 42mm on Micro Four Thirds and 32mm on APS-C.

The lens measures in at 3 by 2.1 inches (HD), weighs about 9.9 ounces, and supports 46mm front filters. It's available in a silver or black finish, but there's no hood included. You can add the Zeiss hood for $84; that hood can also be used for the Distagon T* 2,8/25 ZM lens. The aperture can be narrowed to f/22 in third-stop increments. It can focus to 0.5 meter, which is a bit closer than the closest equivalent Leica lens, the Super-Elmar-M 21mm f/3.4 ASPH. ($2,995), which is limited to 0.7-meter focus—but if you use a rangefinder camera, you'll only have coupled focus to 0.7 meter. Voigtlander makes one film rangefinder with 21mm framelines in the finder, but most shooters using cameras that don't support Live View will need to invest in an external optical finder in order to get accurate framing—Zeiss sells one for $375. Like all M-mount glass, only manual focus is supported.

Compared to SLR lenses that cover a similar field of view, the Zeiss Biogon is impressively compact. But it's not the smallest 21mm you can get for a rangefinder camera. Voigtlander sells the 21mm Color-Skopar f/4 ($419), which is small enough to qualify as a pancake lens, and Zeiss also has the compact C Biogon T* 4,5/21 ZM( at Amazon) in its lineup. If size isn't your primary concern, there's also the 21mm Voigtlander Ultron f/1.8 ($1,149), which captures more than twice as light as the Biogon T* 2,8/21 at its maximum aperture.

I used Imatest to check the sharpness and distortion characteristics of the lens when paired with the full-frame Leica M (Typ 240)($2,969.00 at Amazon). At f/2.8 it is impressively sharp using our center-weighted testing method, scoring 2,367 lines per picture height. But this high score (much better than the 1,800 lines we require for a photo to be called sharp) doesn't hold up at the edges of the frame. The right and left sides of our test chart are noticeably fuzzy, and Imatest shows that the lens manages to just 462 lines along the edges at f/2.8.

Sharpness improves as you narrow the aperture. At f/4 the center-weight score is 2,750 lines, but edges are still fuzzy (538 lines). Moving to f/5.6 improves the average score to 3,070 lines, and the edges improve to 797 lines. Close examination shows some fuzziness, but edges are more defined than they were at f/4. At f/8 the lens delivers its best overall performance: 3,066 lines using the center-weighted score, and the edges improve to 1,253 lines. There's some more improvement along the edges at f/11—they hit 1,675 lines, which is visibly crisp to my eye—but the center and overall score drops to 2,800 lines due to diffraction. There's just a little bit of distortion, about 0.7 percent of the pincushion variety, but that's negligible in field conditions.

If you like a wide field of view and are a rangefinder shooter, the Carl Zeiss Biogon T* 2,8/21 ZM is a solid option, especially if a Leica lens is out of your budget. There are other options available for this focal length, including a smaller lens from Zeiss and a pair of lenses from Voigtlander, but the Biogon T* does a good job balancing image quality, cost, and size. Its edge sharpness at wider apertures leaves a bit to be desired, but it does deliver at narrower apertures. If you want a lens that does a better job from edge-to-edge in this price range, the Zeiss C Biogon T* 4,5/21 ZM is worth a look, but keep in mind that it shows color shift in the corners that requires some work with software (or black-and-white conversion) to remove.

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