The Carl Zeiss Distagon T* 4/18 ZM ($1,428 list) is one of a pair of 18mm lenses currently in production in the M-mount, the native lens mount for the full-frame digital Leica M (Typ 240) and other rangefinder cameras. Because of the compact nature of rangefinder lenses, they are also popular choices for other mirrorless cameras, like the full-frame Sony Alpha 7 and Micro Four Thirds Olympus OM-D E-M1. This 18mm Distagon isn't the widest lens available for these systems, and the only other 18mm lens for the mount is one that costs twice as much: The Leica Super-Elmar-M 18mm f/3.8 ASPH. ($3,250).

The field of view that the Distagon covers on a full-frame camera is vast. It's not a fisheye (in fact, the lens shows almost no distortion that's worth talking about). You can use it to capture wide landscapes, or to take photos in interiors that show a good portion of the room. It can focus to 0.5-meter, but you won't want to use it for close-up portraiture, as it will draw your subjects in an unflattering perspective. If you use the lens on an APS-C camera its field of view isn't quite as wide, it's like using a more traditional 28mm wide-angle, and on Micro Four Thirds it's further narrowed to a moderate wide-angle (roughly 36mm). And there's the Leica M8, which has a APS-H image sensor; on that camera the lens is more like a 24mm on full-frame.

If you use a Leica rangefinder you won't be able to focus to the minimum distance with coupling (digital M bodies only focus to 0.7-meter); owners of the M (Typ 240) can switch to Live View when working at those close distances. Live View, or the M (Typ 240)'s add-on EVF, is your best option for this lens when working at nearer distances, as it's really the only way you'll get accurate framing. The add-on optical finder ($419) that you'd normally use to get an idea of framing with this lens won't offer any type of parallax correction, but will be perfectly fine if you're shooting wide-angle landscapes and scenes. The lens hood isn't included either; it'll cost you $84 to add the metal hood that Zeiss has designed for use with this lens.

The lens is downright small compared with SLR lenses that cover a similar field of view on a full-frame camera, but is larger than others designed with rangefinder use in mind. Like more modern mirrorless cameras, there's no mirror box in a rangefinder, so wide-angle optics like this one can be made smaller. The Distagon measures 2.8 by 2.6 inches (HD), weighs 12.3 ounces, and supports 58mm front filters. Compare this to the Carl Zeiss Distagon T* 3,5/18 SLR lens: It measures 3.3 by 3.4 inches, weighs 1.1 pounds, and requires the use of much larger (and pricier) 82mm filters. Like all M-mount glass, it's a manual focus design, and also sports a manual aperture ring that can be set from f/4 through f/22 in third-stop increments. The depth of field scale, located at its base, has markings for f/4 through f/11 in full-stop increments. As you would expect with a lens this wide, zone focusing is quite practical. The lens barrel itself is metal, and the focus ring is tight, yet very smooth when turned.

I used Imatest to check the sharpness and distortion characteristics of the Distagon 18mm when paired with the full-frame Leica M (Typ 240). Its center-weighted sharpness exceeded the 1,800 lines we require to define an image as sharp even at f/4, where the lens scored 2,556 lines. But the outer edges of the frame are noticeably fuzzy at f/4, scoring just 568 lines. Stoping down to f/5.6 does very little to improve edge quality (650 lines), but the center-weighted score jumps to 2,934 lines. Edges are better, but still not great, at f/8 (918 lines), and the overall score is the best here at just short of 3,000 lines. At f/11 diffraction sets in, robbing some center sharpness (the center-weighted score drops to 2,749 lines), but edges top 1,300 lines; it's your best aperture to use if you want crisp details from edge to edge.

At the end of the day, the edge issues are a minor quibble for a wide-angle lens. It's rare that an optic can capture such an immersive field of view and deliver edge-to-edge perfection at its maximum aperture. We have not yet been able to test the Leica 18mm to see if it's sharper at the edges than the Zeiss take on the design; I shot a bit with that lens a few years ago and was impressed with its performance, but that was on the crop-sensor M8, not a full-frame camera.

Distortion is a nonissue. The lens shows about 0.4 percent pincushion distortion, which is negligible in field conditions. When paired with the M (Typ 240) and manually coded as the 18mm setting of the Wide-Angle Tri-Elmar there is some slight color shift at the edges of the frame. Instead of using that 18mm setting, you should opt for the M's lens profile for the Leica Elmarit-M 21mm f/2.8 ASPH. It eliminates the color shift, but your images will have embedded data length identifying them with the incorrect focal length.

Not everyone needs a lens that's as wide as the Carl Zeiss Distagon T* 4/18 ZM. It takes a good eye and a lot of skill to effectively capture compelling scenes. It's unfortunate that it shows color shift when paired with the M (Typ 240), and performance at the outer edges of the frame is disappointing unless stopped down to f/11. If you pair the Zeiss with an APS-C or Micro Four Thirds camera, the extreme edges of its coverage range won't be a concern to you. If you can't stomach the price of the Leica Super-Elmar 18mm, the Zeiss is a solid budget alternative. There are a few issues, but it's a good size, shows minimal distortion, and its wide-angle field of view lends itself to the type of photography that requires careful composition and tripod use; in those cases, stopping down to f/11 to get the best image quality possible across the frame is a completely viable option.