The Carl Zeiss Biogon T* 2/35 ZM ($1,087 list) is a relatively wide aperture lens for M-mount rangefinder cameras. The focal length is well-suited for use with rangefinders, as its field of view fills most of a standard 0.72x magnification viewfinder, but still leaves some room around the edges of the frame lines so the photographer can observe what's happening just outside the frame. Rangefinder lenses are also popular for use with mirrorless cameras via adapters thanks to their compact size; the Biogon can be paired with a full-frame model like the Sony Alpha 7 and retain its full field of view, or match with a Micro Four Thirds body like the Olympus PEN E-P5 to act like a short telephoto lens.

The lens measures in at 2.7 by 2 inches (HD), weighs 8.5 ounces, and supports 43mm front filters. It's not quite as small as Leica's 1.4-by-2 inch Summicron-M 35mm f/2 APSH. ($3,195), but it doesn't carry the premium price tag that comes with the Leica lens. Leica includes a hood with the Summicron, but you'll have to pay extra ($84) to get the vented metal hood that Zeiss sells for the Biogon. Like most rangefinder lenses, the minimum focus distance is limited to 0.7-meter.

The Zeiss optic has a metal barrel with a focus ring that's tight enough that it won't move unexpectedly, but still manages to be smooth in operation.  There's a little nub on the ring to make adjusting it easier, and a physical aperture control that can be set from f/2 all the way down to f/22 in third-stop increments. The printed depth of field scale starts at f/4, and while scale focus is certainly possible at narrower apertures, it's not as easy to eyeball the distance between camera and subject and get an in-focus shot with a 35mm lens as it is with an ultra-wide angle optic like the Distagon T* 4/18 ZM.

I used Imatest to check sharpness and distortion when paired with the full-frame Leica M (Typ 240). There's no distortion to speak of and center sharpness is impressive even at f/2. The lens scores 2,240 lines per picture height using a center-weighted testing method, bettering the 1,800 lines we use to define a sharp photo. But at f/2 the edges are soft, recording just 1,107 lines. That's a low enough rating to make them appear visibly fuzzy at full resolution.

Narrowing the aperture to f/2.8 improves things a bit. The center-weighted score jumps all the way to 2,969 lines and the edges start to approach 1,600 lines. At f/4 the lens manages 3,127 lines with edges that are better than 2,000 lines. There's another marginal improvement at f/5.6 (3,313 lines across the frame with edges that hit 2,800 lines) and the sharpest performance comes at f/8 (3,352 lines using the center-weighted method with edges that hit 3,150 lines). As impressive as these numbers are, the smaller, less expensive C Biogon T* 2,8/35 actually offers sharper performance from edge to edge, but its maximum aperture is limited to f/2.8.

If you need the extra light gathering that an f/2 lens provides, the Carl Zeiss Biogon T* 2/35 ZM is a solid budget alternative to the Leica Summicron-M 35mm f/2 ASPH. The Biogon is solidly built, and while the edges aren't that great at f/2, there's a good chance that they'll be out of focus at that aperture anyway, depending on the distance between your subject and camera. But it's not the only alternative you have to Leica glass; Voigtlander makes the Nokton 35mm f/1.2 rangefinder lens that is only a bit more expensive at $1,199, and also has a budget 35mm Nokton f/1.4 ($629) in its catalog. We've not yet had the opportunity to perform a formal test with either Voigtlander lens, but they are popular for rangefinder and mirrorless cameras alike due to their ambitious f-stops and aggressive price points.