The Carl Zeiss C Biogon T* 4,5/21 ZM ($1,201 list) is a compact wide-angle lens that was designed for use with rangefinder cameras like the Leica M (Typ 240), but can also be used with mirrorless bodies via an adapter. Its field of view is a wide 21mm when paired with a full-frame camera, but narrows if used with bodies with smaller sensors like the Micro Four Thirds Olympus OM-D E-M1 or the APS-C Sony Alpha NEX-6.

Zeiss states that the "C" in the model name stands for both compact and classic. There's no argument about size—the lens measures just 2.2 by 2.1 inches (HD), weighs a mere 7.4 ounces, and supports 46mm filters—and with a design that dates from 1952, classic is appropriate as well. Like all M-mount lenses, it's a manual focus design; it can focus as close as 0.5 meter, but most rangefinder cameras are limited to a 0.7-meter focus distance, so you'll have to scale focus or engage Live View (if your camera supports it) when working at the closest distances. There's no lens hood included with the C Biogon, so you'll have to set aside an extra $84 if you want to add the one that's recommended by Zeiss. Aperture is adjusted via a manual ring that can go as narrow as f/22 in third-stop increments. The lens feels solid thanks to its all metal build and tight, smooth focus ring.

I used Imatest to check the sharpness of the lens when paired with the full-frame Leica M (Typ 240). Its f/4.5 maximum aperture is a bit narrow to begin with, but the lens notches 3,110 lines per picture height using a center-weighted scoring method at that setting, nearly twice the 1,800 lines we use to call an image sharp. There is a little bit of fuzziness at the edges of the frame—they record just 1,158 lines at f/4.5—but they sharpen up nicely at f/5.6. There the lens scores 3,417 lines, with edges that manage 1,535 lines. Diffraction sets in at f/8, dropping the center-weighted score to 3,179 lines, but the very edges of the frame are tack sharp at that aperture, recording 1,844 lines on average.

Distortion is nonexistent. The lens shows absolutely no pincushion or barrel distortion. What it does show, at least when paired with the M (Typ 240), is color shift at the edges of the frame. The lens design puts the rear element close to the shutter, taking advantage of the rangefinder design that omits a mirror box. The lens design dates back to the days of 35mm film, and when paired with film there are no issues. But digital cameras require light to hit the sensor at a fairly straight angle, and even though Leica puts microlenses in its sensors to help straighten light towards the edges of the frame, they can't fully correct for the design of C Biogon. This causes the edges of the frame to show a bit of color shift, which tends to be purplish in color. Black and white shooters, especially those who own the M Monochrom won't be bothered by this, but if you shoot in color then the Cornerfix software application can help remove the unwanted color from images. Not every camera will show the shift; I was only able to test it with the M (Typ 240), but experience with similar lens designs tells me that you likely won't see noticeable color shift using the mirrorless Sony Alpha 7 and NEX-6. However, the high-resolution Alpha 7R and NEX-7 will almost certainly exhibit color shift.

The Carl Zeiss C Biogon T* 4,5/21 ZM is a tack-sharp, compact wide-angle lens. It's also very well built, with a metal barrel and a focus ring that is tight, but smooth in operation. It doesn't capture as much light as the f/2.8 Biogon T* 2,8/21 ZM or the Voigtlander Ultron 21mm f/1.8, but it's tough to argue with its size. If you shoot with an M (Typ 240) or another camera that exhibits color shift it's probably not the best lens for you. If you're still shooting with 35mm there's very little downside to the lens apart from its f/4.5 maximum aperture, and if you work with the Leica M Monochrom give this wide-angle optic serious consideration; color shift won't be a problem with a camera that only shoots in black-and-white.