Nikon Nikkor UD 20mm f/3.5 was the first retrofocus type 20mm prime for Nikon's now ubiquitous F mount series of cameras. The lens replaced the original Nikkor-O 21mm f/4 rangefinder prime (which users could still use on SLR cameras with mirror lockup - somewhat of a hassle in its own right). The lens was produced from 1967 throug 1974 and was eventually succeeded by a slightly slower, but more compact 20mm prime. The Nikkor UD 20mm f/3.5 is a non Ai lens and does not offer automatic indexing. This also means that the lens cannot be mounted on many of newer bodies without modification - the metal metering prong has to either be removed or shimmed sufficiently so it does not hit the body prism during mounting. The Nikkor UD 20mm is commonly available on used markets and typically sells for ~US$200 (as of March 2009).
The optical construction of the lens consists of 11 elements in 9 groups. The lens carries an UD designation, but this does not really mean that the lens incorporates a UD element. Nikon used letter codes in 60s to designate the number of elements in the lens design, and UD actually means Unis +Deci, or 11 elements (thanks Roland for clarification). The build quality of the lens is superb, as is the case for majority of older Nikon lenses - the barrel is metal as are the aperture and focusing rings. The focusing ring is pretty smooth and the aperture ring is snappy, moving in one full f-stop increments. The minimum aperture setting is f/22. The lens is slightly larger then the new versions of Nikkor 20mm, measuring 75 x 70mm (2.9 x 2.7in) and weighing 390g (13.8oz). The minimum focusing distance is 30cm (1ft) and the lens accepts 72mm screw-in filters.
The lens can in theory be adapted to alternative mount cameras. There are some caveats through. The lens can be used (with an appropriate adapter obviously) on any Four Thirds camera, as well as APS-C type Canon bodies. However, you should be extra careful when trying to use this lens on any full frame Canon bodies since the protruding fin on the back of the lens can hit the mirror box of the camera. You should really consider cutting or shaving it off if you plan to use the lens on a Canon FF body - the operation of the lens will not be affected and you would still be able to use the lens on Nikon bodies (assumiing of course you also converted it to Ai).
|Lens Composition||11 elements in 9 groups|
|Angular Field||94 degrees|
|f-stop Scale||f/3.5-f/22, manual|
|Lens Hood||Metal screw-in (included)
|Lens Case||No. 5 (included)|