EF 50mm f/1.0L USM
The discontinued EF 50mm f/1.0L
USM is a professional L series autofocus lens. On the used market sells for as much as double the original retail value. It was the fastest
SLR lens in production during its lifetime.
 This lens has a metal body and mount, and plastic extremities. It also features a wide rubber focus ring that is
damped, a distance window with
infrared index, and the ability to set the focus range from 0.6m to infinity, or 1m to infinity. In common with the
EF 85mm f/1.2L USM it uses an electronic "focus by wire" system and requires power from the camera in order to manual focus. The 8-blade diaphragm and maximum
aperture of f/1.0 give this lens the ability to create extremely shallow
depth of field effects and to support low light situations. The optical construction of this lens contains 11 lens elements, including two ground and polished
aspherical lens elements. This lens uses a floating front extension
focusing system, powered by a ring-type USM motor. The front of the lens does not rotate, but does extend when focusing.
Despite its price and large maximum aperture, the 1.0L was not a particularly sharp lens at any aperture, and the two cheaper 50mm options offered far better sharpness when stopped down beyond about f/2.8.
 This, combined with the high production cost and low sales volume, led to it being discontinued in 2000 and eventually superseded by the f/1.2 edition.