Data sheet: Nikkor Ai 20 mm f/3.5

Pekka Buttler, 08/2022 (Updated 03/2023)

Specifications

The table below summarizes the lens’ key specifications:

Brand:	Nikon (Nikkor)	Lens name	20 mm 1:3.5
Focal length 1	20 mm	Angle-of-view 2	94°
Maximum Aperture	f/3.5	In Production	1977–1981
Lens mount	Nikon F	Subfamily (if applicable)	Ai (Automatic indexing)
Length 3	41,1 mm	Diameter 4	63,6 mm
Filter ring diameter	52 mm	Weight	235 grams
Lens element count	11	Lens group count	8
Aperture blades (S/R/C) 5	7 S	Focus throw	100 °
Minimum focusing distance	30 cms	Maximum magnification	1:10,8
Has manual aperture ring	YES	Has Manual focus ring	YES
Aperture mechanism type	Automatic	Aperture click stops 6	3.5-5.6-8-11-16-22

Further notes:
Nikon has two manual lenses with the mainline specifications of 20 mm f/3.5
• The first (referred to as the 20 mm f/3.5 UD ) is an early F-type lens, manufactured between 1967–1974, has a 72 mm filter ring, and is generally quite highly regarded. Also, when introduced, that lens was one of the widest-angle rectilinear SLR lenses in existence.
• The second (this) was an Ai-type lens introduced to replace the earlier 20 mm f/4 lens. This lens was then upgraded to Ai-s specifications (without changes to the optics) in 1981 and remained in production until 1984 when is was replaced by the 20 mm f/2.8 Ai-s lens.
• Nikon’s dedicated lens hood for this lens was the HK-6 clamp-on lens hood.

A brief genealogy of Nikon SLR lens types

Nikon is undoubtedly one of the great names in 35 mm SLR photography. The Nikon F mount has been in continuous production since 1959. During that time, the mount has developed/changed in some detail, however without ever fully sacrificing compatibility.

In short (a longer version is here), the development of Nikon’s SLR lenses can be traced as follows:
• 1959–1977: Pre-Ai. Manual focus lenses that use ‘rabbit ears’ to communicate selected aperture with the camera body.
• 1977–1986: Ai and Ai-s. Manual focus lenses that may have ‘rabbit ears’ for backward compatibility, but are designed to communicate selected aperture with the camera body through indentations in base of aperture control ring.
• 1986–today: AF and AF-D. Autofocus lenses that do not have a focusing motor within the lens, but rely on the focus motor within the camera. All AF and AF-D lenses are simultaneously Ai-s lenses (they are Ai-s lenses extended with AF) ⁷
• 1996–today AF-S and AF-P. Autofocus lenses that have an internal focusing motor and do not rely on the body having a focusing motor.

Adapting

Besides adapting, this lens can be used natively on all current high-end Nikon dSLRs and several earlier medium-to-high-end older Nikon dSLRs ⁸. Likewise, if it still has its rabbit ears, it can be natively used on all Nikon F-mount film cameras ever produced (without the rabbit ears, it is limited to post 1977 bodies).

Thanks to being a fully manual lens (manual aperture, manual focus), the lens can be adapted to all mirrorless cameras using a suitable dumb adapter (and such adapters are easy to find). Moreover, a large range of special adapters (helicoid adapters, tilt/shift adapters, speed boosters) for using Nikon F lenses on most mirrorless systems are available.

Using Nikon F lenses on non-Nikon SLRs and dSLRs is likewise a distinct possibility. Thanks to the relatively generous flange focal distance of the Nikon F mount (46,5 mm), adapter rings for all dSLR mounts are available as well as for a goodly portion of film-era SLR mounts. Such rings may not allow for auto aperture, but even then the lenses can be used in stop-down metering mode.

Footnotes

1. Focal length is (unless stated otherwise) given in absolute terms, and not in Full-frame equivalent. For an understanding of whether the lens is wide/tele, see ‘Angle-of-view’. ↩︎
2. Picture angle is given in degrees and concerns the diagonal picture angle. Rule of thumb:
   > 90 ° ==> Ultra-wide-angle
   70–90 ° ==> Wide-angle
   50–70 ° ==> Moderate wide-angle
   40–50 ° ==> ‘Standard’ or ‘normal’ lens
   20–40 ° ==> Short tele lens
   10-20 ° ==> Tele lens
   5-10 ° ==> Long tele lens
   < 5 ° ==> Ultra-tele lens ↩︎
3. Length is given from the mount flange to the front of lens at infinity. ↩︎
4. Diameter excludes protrusions such as rabbit ears or stop-down levers. ↩︎
5. S=straight; R=rounded; C=(almost)circular at all apertures. ↩︎
6. Numbers equal aperture values on aperture ring; • intermediate click; – no intermediate click.a ↩︎
7. There is a further sub-class of AF-D lenses called AF-I lenses that are otherwise AF-D lenses (meaning, fully Ai-s compatible), but have an internal focus motor. Only long tele lenses were made in AF-I variants. ↩︎
8. As of this writing, the following Nikon dSLRs fully support Aperture priority and manual metered modes on Nikkor Ai lenses: D2, D3, D4, D5, D6, D200, D300, D300s, D500, D600, D610, D700, D750, D780, D800, D800E, D810, D850, D7000, D7100, D7200 ↩︎