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( 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,...
If you see in ultra-wide angles the Leica Tri-Elmar-M 16-18-21mm f/4 ASPH. is a solid lens for your rangefinder camera, but it needs to be stopped down for optimal performance.
Ultra-wide field of view; Edge-to-edge sharpness at f/5.6; No barrel distortion; Excellent depth of field scale; Compact; Unique 3-stop zooming design; Focuses to 0.5-meter
Expensive; Soft corners and edges at f/4; Optional add-on viewfinder is huge; Requires special holder for filter use; Manual focus only