Carl Zeiss Jena SO-3.1 P-Flektogon 35/2.8. Overview of the spy lens from Rodion Eshmakov

Material on the lens especially for Radozhiva prepared Rodion Eshmakov.

A pair of SO-3.1 lenses adapted for modern cameras.

A pair of SO-3.1 lenses adapted for modern cameras. increase.

The non-democratic government of the German Democratic Republic had its own counterpart to the KGB, which left behind not only peculiar East German anecdotes, but also an impressive arsenal of means of maintaining citizens' faith in the party. Known as the "Stasi", the German secret police ("Staatsicherheit") used various special systems developed by Carl Zeiss Jena and VEB Pentacon: for example, a GSK camera with a set of "special lenses" SO-3.1 - SO-3.4 ("sonderoptik") - for surveillance; Dokumator microfilm machine with optics Dokumar – to file photographic materials extracted with the help of GSK into folders; Pentakta projectors with lenses of the same nameso that the microfilm can be viewed.

This article is about a special SO-3.1 35/2.8 lens for the GSK security camera, which was developed by Carl Zeiss Jena in 1973. I got my hands on two copies. Here and below the source of historical information this.


Optical design - 8 lenses in 5 groups, "P-Flektogon";

Schematic diagram and main parameters of the SO-3.1 lens.

Schematic diagram and main parameters of the SO-3.1 lens.

Focal length - 35 mm;
Relative aperture - f / 2.8;
Estimated frame format - 36×24 mm, covered - 44×33 mm;
Field of view angle (on the frame 36×24 mm) – 63°;
The minimum focusing distance is 0.3 m;
Rear focal length - approximately 36mm (lens compatible with SLR cameras, but may hit the mirror on some full-frame models);
Mounting thread to the camera - М36×1;
Features: does not have an iris diaphragm, is radioactive (thorium glass is used).

Design features and adaptation

The SO-3.1 35/2.8 lens was designed by Eberhard Dietzsch for the Stasi Operations and Technical Sector. The key features of this lens are the extended entrance pupil and the small size of the front lens, which allows shooting through narrow openings (for example, in a wall) with minimal vignetting. This feature is also reflected in the name of the optical scheme: the “P” in “P-Flektogon” probably means “Pupil” - “pupil”, and “Flektogon” is the name applied to all retrofocus lenses manufactured by Carl Zeiss Jena, regardless of optical scheme. It must be said that, although archival documents have shed light on the name of this lens, there is no marking on its body, except for the serial number and focusing distance scale, which is typical for special products. By the way, in the USSR there was similar purpose lens with a simpler six-lens scheme.

SO-3.1 and other lenses for the GSK camera.

SO-3.1 and other lenses for the GSK camera.

Thanks to the extended pupil, it becomes possible (but not absolutely correct) to set the aperture in front of the front lens, which greatly simplifies adaptation. Parts for installing the diaphragm were made by 3D printing. To install filters, a frame from a Soviet light filter with a 40.5 mm thread is built into the nose.

The nose of the adapted lens with the aperture set.

The nose of the adapted lens with the aperture set.

The lens helicoid is quite usable and does not require replacement, although the lens block of the lens can be easily unscrewed from it and installed in any other focuser you like. For installation on modern cameras, adapter rings M36x1–M42x1 were made, and the position of the lens block necessary for accurate setting of infinity was fixed by additional adjustment rings.

Adapted lenses SO-3.1.

Adapted lenses SO-3.1.

The lens after adaptation has dimensions smaller than most other wide-angle lenses of the 35 / 2.8 class: this is the merit of the optical design, in which the smallest possible size of the front lens group was a priority.

About thorium glasses, yellowness and radiation

An important feature of the SO-3.1 35/2.8 optical design is the use of thorium glass. Thorium is a radioactive element, an alpha emitter. Being isolated in a glass matrix, it does not pose a threat to human life and health, since its radiation is stopped even by a sheet of paper. However, its more reactive decay products accumulating in the glass create enough background radiation for sensors to detect (but not enough to harm a person), which can be a big problem when trying to transport a lens: on international flights, radiation control is extremely (I would say - too) strict.

In glassmaking, thorium in the form of dioxide (ThO2) was used to produce highly refractive glasses. Thorium glass is easier to manufacture than lanthanum glass, it also has a higher refractive index (up to 1.9-2) in comparison with lanthanum glass (up to 1.8): on the Abbe diagram, it would correspond to a region higher than known lanthanum STCs. For these reasons, it is thorium glasses that can often be found in high-aperture lenses of the middle of the XNUMXth century, among which the most famous are SMC Takumar 50 / 1.4, CZJ Pancolar 50/1.8 (8 petals).

Abbe diagram for optical glass of the LZOS catalog. The Izyum plant in Ukraine offers the same set of glass grades, except for the low-dispersion fluorophosphate crown OK-4.

Abbe diagram for optical glass of the LZOS catalog. The Izyum plant in Ukraine offers the same set of glass grades, except for the low-dispersion fluorophosphate crown OK-4.

In addition to the problems associated with the isolation and purification of natural thorium from highly active impurities, there is another one - the radiation resistance of glass. It is well known that ionizing radiation leads to the breaking of chemical bonds. In solids, this leads to the formation of ions in unusual charge states and/or free electrons localized in structural defects, which imparts color to the material. For example, table salt irradiated with radiation acquires a purple color due to the expulsion of chlorine anions from the structure in the form of chlorine atoms. At the same time, an electron remains in place of the chloride ion - the so-called F-center (color center, “farbe” - with German “color”) is formed, which intensively absorbs the visible radiation of the orange and green ranges.

In glass, radiation-induced defects are usually ions in atypical charge states, for example, lead +3 instead of +2. Extra electrons or their lack relative to the normal state of ions in glass change the nature of the chemical bond and the excitation energy of the electrons of these bonds, i.e. the band structure of the material, which can lead to the appearance of absorption of light in the visible wavelength range, i.e., the appearance of color in the glass. As a rule, colorless optical glasses turn yellow/brown when exposed to ionizing radiation. Glasses intended for operation under conditions of ionizing radiation contain so-called "charge traps": ions of an element with a variable formal charge, both charge forms of which have low absorption in the visible region. Usually, cerium acts as such an element: the addition of only 0.1-1% of its oxide increases the radiation resistance of glass by orders of magnitude, but this addition is enough for the glass itself to acquire a yellowish tint due to cerium.

Fortunately, the unusual electronic states that are formed under the action of radiation in glass are not very stable: each ion tends to return to its previous state, and the electron tends to occupy the level with the lowest energy. For this to happen, it is necessary to help overcome a certain potential barrier of defect relaxation by imparting additional energy in the form of heat or light. Only a minute heating up to 150 degrees leads to the relaxation of the overwhelming number of colored defects in the glass, but this method is of little use for already assembled lenses. Long-term exposure to light with a wavelength equal to the wavelength absorbed by defects is a good, but slow, way to discolor the glass of old lenses damaged by radiation. For this reason, it is recommended to irradiate yellowed lenses with ultraviolet light or expose them to the sun.

A couple of my SO-3.1 35 / 2.8 lenses differ in light transmission: one of them is very yellow, the other is practically not. This is probably due to the storage conditions of their former owner - one could lie in the dark or in a cold place, and the other - in the light in a very warm place. Below are photographs of two lenses that allow you to visually compare the appearance of glass damaged by radiation and relaxed.

The important thing is that these lenses are absolutely identical, including the optical design. The difference is only in the conditions of storage / use in the past.

Even more clearly the difference between yellowed and colorless thorium glass is demonstrated by the light transmission spectra obtained from both lenses. It can be noted that in the IR range, the light transmission of the lenses is exactly the same, and in the visible region, the yellowed lens demonstrates a strong absorption of blue-violet rays.

Transmission spectra for yellowed (Yellow) and colorless (Colorless) SO-3.1 objectives, recorded under equal conditions on a Varian Carry 300 spectrophotometer.

Transmission spectra for yellowed (Yellow) and colorless (Colorless) SO-3.1 objectives, recorded under equal conditions on a Varian Carry 300 spectrophotometer.

Since the spectra were recorded under equal conditions for lenses of the same design, by subtracting another from one spectrum, one can obtain the light absorption spectrum (negative decimal logarithm of the light transmission value, taken in fractions of unity) by radiation-induced colored glass defects.

The absorption spectrum of light by radiation-induced glass defects is the difference between the spectra of yellowed and colorless lenses.

The absorption spectrum of light by radiation-induced glass defects is the difference between the spectra of yellowed and colorless lenses.

It can be noted that there is no difference between the lenses in the IR region, but also in the region of wavelengths shorter than 350 nm, but this is only due to the fact that the glass itself does not transmit UV rays, regardless of the presence or absence of colored defects. The graph shows that for wavelengths of 380–400 nm, the light transmission of a yellowed lens is ~10 times less than that of a colorless one. In addition, the radiation of this particular range is suitable for "treating" the yellowing of glass in thorium lenses, since it is absorbed most strongly by defects and is transmitted by defect-free glass.

From a photographic point of view, the yellow thorium glass acts as a light filter, lowering the color temperature and reducing the light transmission of the lens by up to 1-1.5 stops. exposure.

Optical properties

The lens uses an eight-lens optical design using thorium glass - and this helps it a lot in achieving high image quality despite such compromises as a remote entrance pupil and a large back focal length. In the center of the frame, the lens creates a sharp image from an open aperture, at the edges of the frame the sharpness is limited by coma and chromatic aberrations, but in general the lens behaves much better than the conventional Mir-1 or even than the new Zenitar 35/2 (which I still experienced a little ). On covered apertures, the resolution also improves across the field, but due to the use of a pre-lens aperture, vignetting also increases. A feature of the lens is also the presence of a noticeable barrel-shaped distortion.

Image contrast under normal lighting conditions is at the level of good optics with single-coated lenses. In the backlight, veiling and glare appear in the form of "sunny rain" and bunnies.

The color rendition of a colorless lens is with some greenery, a yellowed lens gives the effect of a warm light filter.

Unusual for class 35 / 2.8 lenses is SO-3.1 bokeh: the lens does not create bright “scales” or pronounced “twisting”, which is reminiscent of high-quality modern optics. Nevertheless, SO-3.1 definitely has its own zest in blurring the background.

The lens covers a frame of 44×33 mm, with the “correct” aperture setting it can be used with medium format cameras without any problems. I used SO-3.1 on F/2.8-F/4 with shift adapter on Sony A7s for "medium format" shots.

Below are sample photos on a full frame camera Sony A7s without using a shift adapter.

Next - "medium format" photos - "shiftorams" taken on Sony A7s using the Fotodiox Pro EOS-NEX Shift adapter.


Carl Zeiss Jena P-Flektogon SO-3.1 35 / 2.8 is a wide-angle lens with an unusual optical design and a pattern atypical for old optics of this class. Eight lenses and thorium glass helped the Jena opticians achieve optical quality that was not achieved in conventional consumer Flektogon 35 / 2.8 Harry Zollner, and which, apart from dubious experts at that time, unfortunately, no one could evaluate.

You will find more reviews from readers of Radozhiva here... All Rodion reviews in one place here.

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Comments: 14, on the topic: Carl Zeiss Jena SO-3.1 P-Flektogon 35/2.8. Overview of the spy lens from Rodion Eshmakov

  • Sergei

    I wonder which of the eight lenses is thorium? If the latter, then the lens should decently emit alpha radiation onto the film immediately after birth.
    The main risks of radiation arise much later - as thorium naturally decays, gamma radiation (which is difficult to shield) begins to manifest itself significantly. Moreover, different thorium-containing lenses emit differently (possibly influenced by both the mass of the thorium glass itself and other radioactive impurities.
    It is interesting to compare both instances with each other.

    • Rodion

      I think that thorium is in the rear lens. Maybe it is not even in one of the lenses - there are enough candidates (positive lenses) in the lens.
      The range of alpha particles is only a couple of centimeters, the film will not suffer. Daughter isotopes have an activity several times greater than that of thorium itself, while beta radiation is effectively shielded by the body and lenses, and gamma rays will have to be put up with, yes. Fortunately, this is not cesium-137 and not cobalt-60.
      The radioactivity of glass depends, of course, on the proportion of thorium in it and on the composition of the matrix: if it contains elements that absorb radiation, then the glass can emit less. The radioactivity of the lens will depend on the location of the thorium lens in the scheme and on its geometric parameters.
      By the way, taking into account the fact that the “secular equilibrium” in the thorium series is achieved in several decades, then the activity of glass with thorium can be considered equivalent to the activity of natural thorium.
      In the photo review on a colorless sample.

  • mr.swar

    I wonder which of the eight lenses is thorium? -
    The penultimate one in gluing. Only an idiot can put a thorium or lanthanum optical component last (to the photographic material, to the eye, etc.)

    If the latter, then the lens should decently emit alpha radiation on the film immediately after birth.-
    The lens already decently glows, not only in alpha, but also in beta.
    Thorium 232 - releases Alpha - becomes Radium 228, then Radium 228 - releases Beta - becomes Actinium 228 and so on. The above decay chain shows that the optical components contain thorium and its daughter elements: radium, actinium, radon, polonium, bismuth, thallium and lead in minute amounts as a result of the slow decay of thorium.
    On a digital camera, at a shutter speed of 60-120 seconds, hot pixels are clearly visible. The smaller the pixel size, the more hot pixels.

    The main risks of radiation arise much later - as thorium naturally decays, gamma radiation (which is difficult to shield) begins to manifest itself significantly.
    This is the purest truth, only no one knows about this and not Gamma, but Beta radiation.
    The destruction of the optical component will not release radiation or toxic elements remaining in the structure of the glass lattice. However, this will contaminate the premises and make it easier to swallow or inhale small particles of radioactive dust.
    Disassemble a lens with thorium optical components only with gloves and on a plastic film. After disassembly and assembly, hand over the gloves and polyethylene film to the appropriate point for receiving radioactive materials. Wipe the tool with a damp cloth and wrap in the above-mentioned polyethylene film.
    It is strictly forbidden to break, rub, scratch thorium optical components to avoid getting radioactive particles into the body.

    Moreover, different thorium-containing lenses emit differently (perhaps both the mass of the thorium glass itself and other radioactive impurities) .-
    Not only the mass of the optical glass containing thorium affects, but also the purity of thorium oxide, which was added at the time of glass melting.

    It is interesting to compare both instances with each other.-
    To unscrew the lenses in both copies and compare them, there is a suspicion that the optical components may have different thorium content and purity.
    Alpha and Beta radiation creates F-centers in glass because radioactive decay displaces electrons, causing the glass to turn yellow or brown.
    UV light can remove some of the yellowing. It may take up to 7 days of exposure to the sun or UV light sources to reduce yellowing.
    The safe distance in the air is 1-1,5 meters from the thorium optical components.
    The aluminum case is a good shielding material from alpha and beta radiation.
    Thorium optical components have gradually been replaced by optical components containing lanthanum oxide having similar optical properties. Lanthanum itself is very weakly radioactive, but the radiation is detectable by a sensitive instrument, the level is negligible compared to the background radiation.

    • Rodion

      A detailed comment, but only to the point.
      It is hardly an idiot to place the lens last - it is very doubtful that this would have such a negative effect on film. You yourself write about 120 seconds for a digital matrix, which is clearly more sensitive to radiation. Contradiction.
      Further, thorium glass is crown glass. In lenses of the double Gauss type, crown glasses are used in positive lenses, and flints are used in negative ones. And this lens is one way or another - the development of double Gauss. So it's unlikely that the thorium lens is second from last.
      You are also mistaken about the breakup.
      In the lens for 50 years, a secular equilibrium of isotopes has been established, therefore there is alpha, beta, and gamma radiation. And real issues with shielding arise only for gamma rays, the range of which in soft tissues is tens of centimeters. Beta rays are effectively blocked by aluminum foil, and hence by body parts. Another mistake, bad luck?
      Well, I don’t even want to comment on this whole technically safe frenzy. I can imagine what you would do if you were to precipitate sodium uranyl acetate under a microscope with sophomores in analytics.
      The purity of the thorium dioxide certainly has an effect. But then again - there can hardly be anything besides its daughter isotopes. Conditional radium-226 will not be there - they and thorium are so heterogeneous in properties that there is no chance to isolate them together.
      Different content of thorium in lenses? How can you even write this nonsense. In the same lens, produced at the same time, having the same properties, it cannot be that different glass is used. The amount of thorium cannot float, just like any other optical glass component. Illiterate conclusion.
      F centers are hardly formed in glass, since there are many charge traps. The formation of F centers is characteristic of ionic crystals, where the cation has a constant oxidation state. In addition, F-centers, that is, localized electrons, in many known compounds give a blue or purple color, not yellow.
      In glass, the main type of defects are cations in atypical oxidation states.
      The safe distance is not clear where it comes from and what it is in general is also unclear.
      Finally, lanthanum oxide is not at all equivalent to thorium dioxide in terms of optical properties: it has a lower refractive index, and the technological process for the production of lanthanum glasses is more confused.

  • Sergei

    There is a good overview of the radioactivity of certain lenses (which also included optics with four lanthanum lenses - Mitakon speedmaster 50mm / 0,95 of the first version).
    But only two really shine strongly and dangerously - Canon FD 55mm f / 1.2 SSC Aspherical (Measured at 46532 CPM and Fujica Fujinon 50mm f / 1.4 non-EBC early style = non-uniformly segmented focusing ring (measured at 35137 CPM @ back element )

    • Rodion Eshmakov

      Counts per Minute (CPM) - bad indicators of the danger of a particular lens. Yes, in radiochemistry there are certain difficulties in the analysis in the translation of parrots into understandable physical quantities. But without this, this is all garbage and nothing - it will not be possible to judge danger or safety from these data.

  • Sergei

    And there, by the way, there are references to radioactive front or rear optical elements. Probably at that time it was possible to put thorium and lanthanum there.

  • i is glorious

    The yellowed lens gives a very interesting effect, which is difficult to do programmatically.

  • Detlev

    The SO-3.1 is not a spy lens, it is an observation lens. It was not used in walls, but in briefcases.

    • Rodion

      Thank you for the information. By the way, taking photos from briefcase is a kind of spying. Also it explains great the fact that this lens has extended eye pupil.

      • Detlev

        I am a co-author of two books about spycameras.

      • Detlev

        Here is the second one

  • Detlev

    The image circle diameter is 40 mm

    • Rodion

      But for 36×24 image circle diameter must be nore than 43 mm. Also this lens covers 44×33 mm, so you have useless data.

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