Pekka Buttler, 09/2026
Specifications
The table below summarizes the lens’ key specifications (measurements based on pictured, last version of the lens):
| Brand: | Tokyo Kogaku (Topcon) | Lens name | RE.AUTO-TOPCOR 1:4 f=20mm |
| Focal length(s)1 | 20 mm | Angle-of-view2 | 94 ° |
| Maximum Aperture | f/4 | In Production | 1969–1978 |
| Lens mount (this lens) | Topcon/Exakta | Generation | RE. Auto-TOPCOR3 |
| Length4 | 38,5 mm | Diameter5 | 65,0 mm |
| Filter ring diameter | 62 mm (see notes) | Weight | 216 grams |
| Lens element count | 8 | Lens group count | 6 |
| Aperture blades (S/R/C)6 | 6 S | Focus throw | 160 ° |
| Minimum focusing distance (measured) | 31 cms | Maximum magnification (measured) | 1:11,1 |
| Has manual aperture ring | YES | Has Manual focus ring | YES |
| Aperture mechanism type | auto | Aperture click stops 7 | 4•5.6•8•11•16•22 |
Further notes:
• At a focal length of 20 mm this is the widest serially manufactured8 lens in Topcon’s lens lineup for its professional level cameras. This lens was introduced in 1969, unseating the 25/3.5 [data sheet] as the widest Topcon lens.
• In an era when most lenses were uniformly black (and when even silver rings were going out of fashion), Topcon initially went in the opposite direction in the styling of their Topcon RE. Auto-TOPCOR series of lenses. These lenses were initially matte silver, with black rubber grips giving a contrast.
• However, after the 1971 introduction of the Super D, Topcon also started offering most of its lenses in black, however this lens was only ever offered in silver.
• As this lens was a very late addition to the Topcon lens lineup and was quite expensive to boot, it never was manufactured in large numbers. While some sources say as few as 1500 copies would have been produced, JAPB sleuthwork indicates 2000 copies would be a more accurate number. In either case, a very rare lens today.
• While the lens offers a 62 mm filter thread, the lens was intended to be used with a clamp-on filter holder for using IX-standard9 front filters. This filter holder was originally supplied with every lens, but they are today in short supply.
• Alternatively you CAN use a regular 62 mm low profile filter without it producing hard vignetting, but stacked filters or thick filters are a no-go zone.
Versions and variations
The 20 mm f/4 Topcon lens is an ultra-wide-angle for the Topcon RE system and was introduced in 1969. Only one cosmetic/ergonomic variant can be identified. That variant is silver-coloured, has its focal length reported in millimetres and both the lens name and “made in” text are in all-caps.
No black or ‘Navy’ versions have ever been sighted.
History of Topcon
Unless you know your camera lore, you might not know that Topcon was once – between 1957 and 1976 – one of the foremost camera companies in the world.
You can read more details in the Topcon company profile.
Adapting
When intending to adapt Topcon/Exakta lenses keep in mind that most Topcon/Exakta lenses have control pins in the lens flange that vanilla Exakta lenses do not have. Hence, there is a theoretical possibility to that some old-time Exakta adapters have been manufactured that do not accommodate these pins. When procuring an adapter, make sure that it explicitly is compatible with Topcon lenses, or visually inspect the adapter regarding whether it makes room for the Topcon control pins.
With that caveat, the following applies to all Topcon/Exakta mount lenses.
This lens cannot be used natively on any current SLR or dSLRs. To use it in its native environment, you will need a Topcon RE Super film body or one of the subsequent Topcon cameras with the Topcon Exakta mount. While these were never produced in immense numbers, they seem to have stood the test of time reasonably well, and can still be found in perfectly serviceable condition.
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’. Moreover, due to the popularity of the Exakta mount, special adapters (helicoid adapters, tilt/shift adapters) are readily available.
Using Exakta mount lenses on dSLRs can also be an easy option, depending on the marque of dSLR.
• Canon EF has the shortest flange focal distance among full-frame dSLR’s and Canon’s wide range of dSLRs are able to mount Topcon/Exakta lenses perfectly using a simple adapter ring.
• Minolta / Sony A dSLRs, Pentax K and Nikon F mount dSLRs do not have a short enough flange focal distance to enable reaching infinity focus without an adapter that uses corrective optics.
Footnotes
- 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’. ↩︎
- Picture angle is given in degrees (based on manufacturers’ specs) 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 ↩︎ - See more about Topcon lens generations in Topcon Company profile. ↩︎
- Length is given from the mount flange to the front of lens at infinity. ↩︎
- Diameter excludes protrusions such as rabbit ears or stop-down levers. ↩︎
- S=straight; R=rounded; C=(almost)circular at all apertures. ↩︎
- Numbers equal aperture values on aperture ring; • intermediate click; – no intermediate click. ↩︎
- Topcon is known for having designed and prototyped many interesting lenses that sadly never made it into serial production. ↩︎
- This filter standard is referred to as both the IX series or series 9 of filters and filters are easily available second hand. ↩︎