Carl Zeiss Jena Flektogon 4/20mm

[stevemark]

Reading Marco Krögers excellent publication about the 1950-1970 history of retrofocus wideangles I became somehow interested in the earlier retrofocus designs. During the last few weeks I stumbled upon a few famous German retrofocus wideangles and bought them, e. g. two CZJ 2.8/35mm Flektogons with different computations (both really fogged), a Leitz (Schneider) Super Angulon R 4/21mm and a CZJ Flektogon 4/21mm with exakta mount (also with some fogging and fungus).

Today I was picking up the latter, and even though it's not in the best shape, I took some photos here in Basel / Switzerland to see how it performs. This Flektogon with SN 7191938 probably was made in 1964. According to Marco Kröger it should be the rare 1st computation (max. 7'600 units made) which three negative lenses made out of SSK10 thorium glass. Later versions starting at SN 7'206'500 don't have thorium glass any more.

Well, here's a first impression of it's performance. Be aware that the lens has some fogging and some fungus - not too bad, but still ...



I certainly didn't expect any wonders from the first ever retrofocus superwide - especially not on 24 MP FF cameras!
As expected, the lens is perfectly sharp in the image center even at f4 (wide open), but borders and more so the corners suffer. Surprisingly though, when stopped down to f11, the lens performs rather well: Not too much CAs, and pretty good resolution indeed. ust

Below a few samples just to give you an idea about distortion (quite low at about 2.5%), vignetting (not too bad at f4, and quite OK at f11), contrast and colors. Te latter - contrast and colors - should improve of course once the lens is properly cleaned!

[stevemark]

Here are a few images - re-sized to 1000 x 1500 px, and occasionally complemented with a 100% crop from the same image.
Most images were taken at f11, and no CA correction was applied (unless specifically mentioned). No correction of vignetting or distortion as well, of course.

Here's the entrance to the townhall of Basel, reconstructed after the big earthquake from 1356, and enlarged when Basel joined the Swiss Federation in 1501.




This image was taken at f4, and even in this small 1000 x 1500px version you can see that the corners aren't really sharp:



Inside the courtyard:



100% crop from the image above, albeit with CA correction applied and sharpened:



Alleyway leading from marketplace up to the Münster (cathedrale)



Blaues Haus (blue mansion), built by Lukas Sarasin, a wealthy silk ribbon manufacturer, from 1763 to 1775. Silk and especially the production of related dyes laid the foundations for the chemical (and later pharmaceutical) industry in Basel which these days contributes massively to the Swiss GDP and even more to Swiss exports.

Basel has been a hotspot for science as well for humanism since its University was founded in 1460, supported by Pope Pius II who had his Tuscan birthplace Pienza transformed into a model town for humanism. Sure enough, I feel very close to these ideals, and I'm well connected to both places.



Back to photography - here's a 100% crop from the corner of the above image; this time no CA corrections applied!



Finally a few b/w images, straight out of the camera (apart from re.sizing of course). The Galluspforte (portal of St. Gallus) at the Münster (cathedral) - I think that's a beautiful example of how the lens renders: Not too much contrast (remember the fogging and the fungus), no disturbing distortion, and good detail resolution even close to the corners (see 100% crop below):




Oldest part of the Basler Münster, dating around 1200:



Detail from the northern facade:



And finally an early building by the Swiss master architect Mario Botta - now in his 80s -, who built this huge cylinder originally for the Union Bank of Switzerland (UBS). These days the Bank for International Settlements (BIS, basically the "bank of the 63 most important central banks"), is using the building as its headquarters.


More images will follow once I have cleaned and re-lubed the lens.

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[stevemark]

And here's an image of the lens itself, mounted on an Exakta Vaerx IIa with prism finder:



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[Ultrapix]

thanks for sharing, I thougth this veteran was worse than it looks, and I don't exclude to buy it to complete my Carl Zeiss Jena outfit. I just got a Nikkor Ais 20 3.5, and when it comes to the edges IQ it is not all that better... the workaround is to stop down and go beyond infinity, if your adapter allows for that (generally they do)

[Alun Thomas]

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This is an open aperture 100% corner crop from my copy of the same lens (serial 6894863). It has been edited for exposure only, as the corner was quite dark. The lens does vignette at open aperture, but not so much that it's greatly noticeable. The corner resolution seems decent for a 196X lens, because I rarely buy camera makers (top shelf) lenses I mostly compare this to the various third party efforts over the next 10-15 years, and it holds up well in comparison.

I haven't taken many shots that could be used to judge the full frame performance of the lens, I did take it on a weekend away last year, most pictures I took were of fairly undemanding subjects resolution-wise, and generally have darker corners anyway. I was happy with the shots though, and intend to take the lens on an upcoming trip away to use as a street lens, it should be good to show everything happening in a fairly busy location. (These shots have also been edited to bring up the shadows, I generally expose to prevent highlight blowouts and bring up the shadows as needed) (All shots at F/

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[stevemark]

[caspert79]

Quite an accomplishment for such an old lens.

[Ultrapix]

[Ultrapix]

[stevemark]

[stevemark]

Lens diassembled (and partly already reassembled) for cleaning and re-lubricating:



A detailed description will be published later on in the repair section.

Fungus was limited to the rear of the large front lens and luckily came off without any traces of permanent damage.
Fogging was a bit more spread out - but again I was lucky since - in addition to the front lens - only the front side of the second lens as well as the inner surface of the rear triplet were affected. The fogging on the inner surface of the rear triplet was difficult to remove. At first I even thought that the lens surface was corroded, but some acetone - after a few minutes of letting it soak - was dissolving the layer of hardened oil (?) after some time. Still there was a fair amount of rubbing and pressure needed to remove the fogging. That was the first tiime I've seen such a persistent "dirt" on a lens surface. It's, however, not completely surprising since the old 1950s / 1960s CZJ grease from the focuing threads often is very difficult to remove.

Tomorrow, the lens will be re-lubed and reassembled. I'm pretty curious to see its real performance !!

S

[stevemark]

Well, everything put together again - looks (and feels) much better now, and I'm looking forward to examine its performance in more detail soon:



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[Ernst Dinkla]

I see no mention of a (UV) light treatment on the 3 Thorium elements. It can enhance the optical performance of the lens.

[blotafton]

Beautiful architecture in those shots. That old lens seems to manage well when stopped down.

[stevemark]

[Ultrapix]

Pretty impressive that even wide open, letting away the glow, you can distinctly read 1568 on the extreme upper right corner

[stevemark]

[stevemark]

Vignetting wide open and the the unique rendering are resulting in some pretty interesing images, such as thus one:




At f11 the contrast and the intensity of the colors are really good (well, this sample was completely dismantled and cleaned - most samples will have some degree of haze though, and less bright colors):




Distortion is visible only with critical subjects (such as modern buildings); as soon as there are some ornaments or if the architecture is slightly more "alive", you won't notice it at all:



Tomorrow - if the weather is OK - I'll go to the mountains, trying to take a few nice landscape shots at an altitude of 2500-3000 meters above sea level. Let's see how the lens is behaving in strong and harsh daylight

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[Alsatian2017]

I'm using this lens for quite a while. My sample has the serial number 8418124 so it has the revised optical formula established in 1964. I'm using the lens on a 42 megapixel full frame sensor. I can definetely say that Stephan has been quite conservative in affirming that the Flektogon is right at home at f/11 and 24 megapixels. At least my sample (no haze, clean optics) performs very well and I'm quite impressed by its performance, given that its design dates back to 1961.Naturally, given its single coating, the lens flares quite badly.


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[stevemark]

Thanks for those images - your sample of the later computation seems to perform rather well, too!!

Here's a link to my repair / clean / relubricate thread for the early Carl Zeiss Jena Flektogon 4/20mm:

https://forum.mflenses.com/viewtopic,p,1597676.html#1597676

S