Leica's APS-C mirrorless camera system is rather young, but it already has a lens library that covers the gamut of focal lengths from ultra-wide to telephoto using a trio of zoom lenses. The widest, the Super-Vario-Elmar-T 11-23mm f/3.5-4.5 ASPH. ($1,895), comes with the premium price you expect from a Leica lens, but its optics and all-metal build live up to that pedigree. If you shoot with the T (Typ 701) or are interested in capturing ultra-wide 4K video with the SL (Typ 601), rest assured that the 11-23mm is an excellent performer.

Design
The 11-23mm is about the average size for a mirrorless zoom lens—3 by 2.9 inches (HD)—and weighty, but not overly so, at 13 ounces. It's available only in black, and matches the look of other Leica rangefinder lenses with its focal range printed on the barrel in yellow and a red dot on the side as a visual aid to line up with the mounting point on your camera's lens mount.

The lens is designed for use with the APS-C Leica T. It covers a field of view equivalent to a 17-35mm zoom on a full-frame system, placing it squarely in the ultra-wide category at its widest angle. Owners of the full-frame SL may look to the lens as a zoom for video recording, as that camera only uses an APS-C sensor area when rolling 4K footage.

The metal barrel gives the lens a premium feel. It has two control rings, both knurled metal. The wider ring sits closer to the base and adjusts the zoom, with focal lengths marked at 11, 14, 18, and 23mm. The lens extends slightly when zoomed. A lens hood is included, and the front element accepts 67mm filters.

The manual focus ring is at the front of the barrel. It's narrower than the zoom ring, but still comfortable to grip and turn. The 11-23mm's manual focus is achieved by wire—turning the focus ring activates a motor that adjusts the lens elements. That's in stark contrast to most SLR lenses, which have a mechanically coupled focus ring that moves the elements physically. The 11-23mm's focus ring does have a good amount of tension, so turning it feels very similar to turning a mechanical ring. That's a big step up from mirrorless lenses with focus rings that turn freely.

The lens focuses on subjects as close as 7.9 inches, measured from the image sensor. It's not a macro, but it does let you get pretty close to subjects, framing them with a vast background behind when shooting at 11mm. The 7.9-inch distance is maintained throughout the zoom range, capturing images at up to 1:9.7 magnification at 11mm and 1:5.4 at 23mm.

Image Quality
I used Imatest to check the 11-23mm's optical performance when paired with the 16-megapixel Leica T (Typ 701). At 11mm f/3.5 it scores 1,902 lines per picture height on a center-weighted sharpness test. That's better than the 1,800 lines we like to see in an image. Image quality is strong through most of the frame, with just a bit of softness at the periphery.

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There's modest improvement as you stop down. At f/4 the lens shows 1,934 lines. It peaks at f/5.6 (2,008 lines), and continues to deliver strong performance at f/8 (1,986 lines) and f/11 (1,821 lines). Diffraction takes its toll at f/16, cutting the resolution to 1,577 lines.

Image quality remains strong at 14mm. At the maximum aperture, f/3.9, the lens manages 1,910 lines, and peaks at f/5.6 (1,965 lines). You can comfortably shoot at f/8 (1,934 lines) and f/11 (1,800 lines), but try to avoid f/16 (1,594 lines).

Test results are similar enough at 18mm and 23mm to lump them together. At 18mm f/4.2 and 23mm f/4.5 the lens shows about 2,000 lines. Stopping down to f/5.6 bumps the score to 2,050 lines, and there's a slight drop to 2,030 lines at f/8. Diffraction cuts detail slightly at f/11 (1,910 lines) and f/16 (1,690 lines).

There's some barrel distortion at the wide end of the zoom range—3.6 percent at 11mm and 2.2 percent at 14mm. You'll notice that straight lines are rendered with an outward curve at these focal lengths. If that effect is distracting, it's easily remedied in the Lightroom software package that is included with the T. Distortion isn't a problem at 18mm or 23mm.

I used an ExpoDisc to shoot flat gray frames with the 11-23mm and Imatest's Uniformity tool to evaluate how even the illumination is across the frame. At 11mm f/3.5 the corners lag behind the center by 1.6EV, which gives them a noticeably dim look. They pull within one stop of the center at f/5.6, which is noticeable if you're looking for it, and within a negligible half-stop at f/8. At longer focal lengths, illumination is even across the frame at all f-stops. If dim corners are distracting, they can be brightened in Lightroom.

Conclusions
The Leica Super-Vario-Elmar-T 11-23mm f/3.5-4.5 ASPH. is well-built, sharp, and compact, especially when you consider its ultra-wide angle of view. If you shoot with the Leica T, or often work with 4K video with the full-frame SL, it's an excellent choice for those times when you want to shoot wide. It's not without some faults, notably some barrel distortion, but that's not atypical for a lens of this type, and it's something that can be compensated for using software correction. It comes at a premium price when compared with other lenses, but that's true of almost any product with the Leica badge, and the 11-23mm offers up strong optical performance to match its asking price.