The Leica Tri-Elmar-M 16-18-21mm f/4 ASPH. ($5,450) is both the widest lens and the only zoom lens that Leica offers for its M-mount camera system. It's an expensive lens, especially if you opt to buy the Universal Wide-Angle Viewfinder ($899.95) that goes along with it, but it delivers a field of view that few rangefinder lenses can match. If you only occasionally shoot this wide you may want to consider a low-cost alternative like the Zeiss Distagon T* 4/18 ZM($500.00 at Amazon), but don't expect that lens to deliver crisp edges, even when stopped down.

The Tri-Elmar is one of the larger rangefinder lenses available, but it's quite compact when compared to SLR lenses that cover a similar field of view, especially in terms of diameter. It measures 2.4 by 2.1 inches (HD) and weighs 11.8 ounces. The included hood doesn't add much size, but if you opt to use filters you'll have to get a special adapter ($99.95) to mount large 67mm filters to the front of the lens. Like all Leica M lenses, the barrel is metal and focus is manual. The Tri-Elmar actually focuses down to 0.5-meter, which moves beyond the range at which a rangefinder can confirm focus, but an excellent depth of field scale makes it possible to measure the distance between the camera's sensor plane and your subject for precise focus. If less precision is needed, you can stop the lens down a bit have some leeway in just how much depth of field will be in focus for your shot.

The lens makes it clear when you've moved beyond the limits of rangefinder focusing. The distances marked on the focus ring switch from yellow and white to gray, and there's an extra bit of effort required to twist the lens beyond the 0.7-meter marking. The aforementioned depth of field scale has markings from f/4 all the way down to f/22. The aperture ring covers the same range in half-stop increments.

No digital rangefinder has a viewfinder that covers a 16mm frame, although Voigtlander does make the 35mm Bessa R4M M-mount camera with 21mm frame lines. If you want to precisely frame your images, you'll need an external finder. The Leica Universal Wide-Angle Viewfinder, which is often referred to as the "Frankenfinder" due to its size and how unnatural it looks atop a camera, has adjustable frame lines (16mm, 18mm, 21mm, 24mm, and 28mm) and a control that adjusts based on distance to correct for parallax. If you're using the Tri-Elmar with a body that doesn't support Live View like the M-E (Typ 220)($3,985.00 at Amazon) it will deliver the most precision. If you're fine with working with a bit less precision, you can just go with a standard optical viewfinder; an 18mm finder would be a good compromise as you can estimate the coverage at 16mm and 21mm pretty easily from its frame. Owners of the M (Typ 240)($2,969.00 at Amazon) can opt to use Live View or an external electronic viewfinder.

I used Imatest to check the sharpness that the lens delivers at each of its three set focal lengths; the Tri-Elmar is actually a zoom design, so you can use it in between its set points if desired. At 16mm f/4 it does better than the 1,800 lines per picture height that we use to mark an image as sharp, scoring 2,110 lines on a center-weighted test. The outer third of the frame does score a bit lower, 1,380 lines, but performance improves dramatically across the frame when the lens is stopped down to f/5.6. It scores 2,404 on our center-weighted test there, and the outer edges of the frame show 1,936 lines. Peak resolution is at f/8 for both the average (2,567 lines) and edges (2,071 lines).

Performance is about the same at 18mm. At f/4 the lens shows 2,081 lines with edges that resolve 1,434 lines. Stopping down to f/5.6 bumps the average score to 2,409 lines, with edges that fall just shy of 1,900 lines. Performance is best at f/8 (2,641 lines average, 2,243 at the edges). At 21mm the edges are better at f/4 (1,600 lines), and the center-weighted score is strong at 2,014 lines. Narrowing the aperture to f/5.6 delivers a center-weighted score of 2,321 lines (1,921 lines at the edges), and peak performance is at f/8: The center-weighted score is 2,573 lines and the edges are only slightly softer at 2,342 lines.

Compare this with the Zeiss Distagon T* 4/18 ZM; the Zeiss lens is actually a bit sharper on the center-weighted test (2,556 lines at f/4 and about 3,000 lines at f/8), but the edges of the frame are muddy. At f/4 they show just 568 lines, and only improve to 918 lines at f/8.

The Tri-Elmar doesn't show any barrel or pincushion distortion of note; at its worst (18mm) there's only 0.9 percent pincushion, which isn't noticeable in field conditions. But with any ultra-wide lens you're going to capture a skewed perspective if you're shooting off-kilter. You'll want to shoot as straight on as possible to minimize it, but when working in very tight spaces, stretched perspectives at the edge of the frame may be unavoidable. The above shot of the wedding party riding in the limousine is an example of that; shooting in such a tight space requires a wide lens, but the unfortunate groomsman at the left side of the frame is less than a foot from the camera and falls victim to the perils of wide-angle photography.

When you consider that the only quibble with the Leica Tri-Elmar-M 16-18-21mm f/4 ASPH.'s optical quality is some softness at the edges of the frame at f/4, you know that you've got a pretty special lens for wide-angle photographers. Not everyone needs a lens that covers such an epic field of view, but landscape shooters and documentary photographers who work in tight spaces will appreciate it. The real barrier is the price; you can get most of the performance (at the 18mm focal length) for less money by opting for the Zeiss Distagon T* 4/18, and if you do need to go wider Voigtlander sells its Super Wide Heliar Aspherical II 15mm f/4.5 lens for around $600. Even if you add a pricey 21mm, a trio of lenses that deliver the same coverage as the Tri-Elmar will set you back less money, but you'll lose the convenience of the zoom design, and you'll lose the edge-to-edge image quality that the Tri-Elmar produces at f/5.6. Whether that's worth more than $5,000 is a question that you need to answer for yourself.

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