The Leica 21mm f/1.4 Summilux-M Aspherical Lens is a super-wide angle lens designed for high-speed performance. Its bright f/1.4 aperture allows for an extremely shallow depth of field (DOF), and at a medium f/stop, the focal range is so great that entire scenes can be reproduced sharply. Because of its floating element design, the very high resolution is retained even in the close-up range.
The lens makes highly dramatic effects possible, like monumental foregrounds, strongly receding backgrounds and a wide horizon. With this lens, focusing and metering are performed by looking through the camera's viewfinder, while the actual framing is checked with an accessory viewfinder that mounts in the camera's hot-shoe.
This lens features "6-bit" coding, which allows the digital M camera to read this information optically and to identify which lens is being used. The camera can then (optionally) apply a "final stage" software based vignetting correction (for RAW images the lens used is simply recorded, no change is made).