Data sheet: Carl Zeiss Jena Flektogon 20 mm f/4

Pekka Buttler, 01/2024


The table below summarizes the lens’ key specifications (measurements are based on the pictured sample):

Brand:Carl Zeiss JenaLens nameFlektogon 4/20
Focal length(s)120 mmAngle-of-view294,5 °
Maximum Aperturef/4In Production1963–1975
Lens mountsExakta, m42Subfamily (if applicable)––
Length348,9 mmDiameter480,1 mm
Filter ring diameter77 mmWeight320 grams
Lens element count10Lens group count6
Aperture blades (S/R/C)56 SFocus throw280 °
Minimum focusing distance16 cmsMaximum magnification1:5,8
Has manual aperture ringYESHas Manual focus ringYES

Further notes:
• The 20/4 Flektogon was – when introduced in 1963 – the widest-angle rectilinear, non-intrusive lens available for SLR cameras. Considering that the optical design was finished in 1961, this feat is all the more remarkable.
• Design-wise, it is clearly a descendant of the Carl Zeiss Jena 25/4 Flektogon [data sheet] and is a typical representative of the first generation of ultra-wide angles (which used to mean anything wider than 80 °). Typical, in that it combines a relatively low maximum aperture basic lens with a preceding negative lens (or several lens elements) in such a way that leads to a front element of significant diameter (hence, also a lens with a large filter ring diameter and an overall trumpet-like shape typical for early ultra-wide-angles).
• This lens was finally replaced in 1975 by the Carl Zeiss Jena 20 mm f/2.8 Flektogon. A [data sheet] of that lens’ Praktica B variant is available.
• My sample of this lens is radioactive, which likely means that rear-earth minerals such as Thorium have been used in the glass mixture. If you’re not familiar with radioactive lenses, I recommend this article.

History of Carl Zeiss Jena

There are few names in camera optics more illustrious than that of Carl Zeiss. The company was founded in the German town of Jena in 1846 by Carl Zeiß (hence: ‘Carl Zeiss Jena‘). During 1846–1945 there are few major developments in lens optics that the company was not involved in. Names that are even today well-known in optics – such as Planar (1896), Tessar (1902), Sonnar (1929), and Biotar (1939) (as well as many names that only optics-buffs know) – were the product of Zeiss’ first century of technological innovation.

After the Second World War Germany was divided into a Soviet zone (subsequently: East Germany) and the west-allied zones (subsequently: West Germany). While the Zeiss works resided in Jena (optics and glasses) and Dresden (cameras), which were in the Soviet sector, a contingent of Zeiss managers decided to move west and ended up setiting up shop in the small town of Oberkochen in the American sector under the name of Opton Optische Werke Oberkochen GmbH. As the relations between the former allied deteriorated and the split into East and West Germany became all the more real, the Oberkochen works changed their name first to Zeiss-Opton and later to Carl Zeiss.

What ensued was a lengthy international trademark dispute with both Zeiss’ (Jena and Oberkochen) laying claim to the name ‘Carl Zeiss’. The resulting stalemate – emblematic of the Cold War in its entirety – resulted in that Carl Zeiss Opton was allowed to use the name Carl Zeiss in the West, but had to use the Opton brand in the East bloc, whereas Carl Zeiss Jena was the only real Carl Zeiss as far as the east bloc was concerned, but could – mostly – not use the Carl Zeiss -name for exports to the West6. The fact remains however, that – starting in 1946 from a shared base – Jena and Oberkochen developed as two independent companies for more than 40 years. After German reunification also Zeiss East and Zeiss West were united again, and have since again been at the undoubtable forefront of lens development.

This lens harkens to the time of the cold-war and the East/West split. During the entire Cold War period, VEB Carl Zeiss Jena was seen by the country’s leadership both as a paragon of the East German technology industry and a showcase of the socialist/communist system as well as a major source of exports (and hence, western currencies). Within the centrally directed economy’s hierarchy, Carl Zeiss Jena therefore had a more prestigious role than other East German optics manufacturers (prominently Meyer-Optik Görlitz and, later, Pentacon), meaning that Carl Zeiss Jena received privileged access to tools and materiel (including the first computer in East Germany) and that its products were always considered the premium alternative.

History of the Flektogon designs

Carl Zeiss Jena was a central player in the development of a class of lenses today known as retro focus lenses (a.k.a. reversed tele focus) – a key approach to allowing the manufacture of wide-angle SLR lenses. While the jury is out on whether Angenieux (France) or Carl Zeiss Jena actually was the first company to bring the concept into production, it is evident that Carl Zeiss Jena was both genuinely innovative and prolific in this niche.

The ‘family name’ of Carl Zeiss Jena’s retro focus wide-angle lenses was ‘Flektogon’ (Carl Zeiss West called its corresponding family ‘Distagon’). The Flektogon family contained lenses with focal lengths of 20, 25 and 35 mm (for 135 film) and 50 and 65 mm (for 6×6 medium format).


Besides going through the customary cosmetic iterations (which are well documented on, it would seem that the optical recipe was recomputed in 1964, meaning that there are (optically) two somewhat different recipes for the 20 mm f/4 Flektogon:
• Version 1 (1963–1964, until Serial 7.206.500) [this lens]
• Version 2 (1964–1975)
Whether version 2 did away with the need for radioactive glass, I cannot say with certainty.


n.B! The following applies this lens in either Exakta or M42 mount.

This lens cannot be used natively on any current SLR or dSLRs. To use it in its native environment, you will need an Exakta or M42-mount film body. Luckily there are a lot of those (especially in M42 mount) available.

Thanks to being a fully manual lens (manual aperture, manual focus), the lens can be adapted to all mirrorless cameras using a suitable adapter. Moreover, both Exakta and M42 lenses are so uncomplicated that a simple ‘dumb adapter’ will do the job perfectly. Moreover, due to the popularity of both mounts, special adapters (helicoid adapters, tilt/shift adapters) are readily available. Alternatively, one can choose to daisy-chain adapters (e.g. M42->Canon EF; Canon EF –> mirrorless) which also opens up a wide range of speed boosters for those photographers that use smaller than full-frame sensors.

Using m42 and Exakta lenses on dSLRs can also be an easy option, depending on which 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 both M42 and Exakta lenses perfectly using a simple adapter ring.
• Minolta / Sony A dSLRs are likewise able to mount M42 lenses using a simple adapter ring, but for Exakta lenses, the difference in flange focal distances is not enough to enable reaching infinity focus without an adapter that uses corrective optics.
• Pentax K dSLRs are likewise able to use M42 lenses using a simple adapter ring, but for Exakta lenses an adapter that uses corrective optics would be needed to allow infinity focus.
• Nikon F dSLRs have a long flange focal distance, meaning that mounting either M42 or Exakta lenses needs an adapter that uses corrective optics to allow anything close to infinity focus.


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 (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

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 Online one can find many lengthy and heated disputes stating that only Jena/Oberkochen is the true Carl Zeiss. While many of these discussions are riddled with misconceptions and a poor grasp of facts and timings, they largely also tend to be tainted by ideologies. Those discussions that focus on claims of one or the other Zeiss not really having rights to using designs developed at pre-war Zeiss are especially ludicrous because after the war practically the entire patent catalogue of pre-war German patents was given freely to everyone (the allied saw this as a form of reparations), hence also kickstarting the Japanese optics industry’s ascendancy (the Japanese optics companies were the most avid users of German optics patents).

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