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Stasi
Zeiss
GSK
  
SO-3.5.2 →
  
SO-3.5.1   14606-1
Pinhole lens kit for photocamera

SO-3.5.1 1 is a pinhole lens kit, developed around 1978 by VEB Carl Zeiss Jena in Jena (DDR), especially for the East-German security service – Ministerium für Staatssicherheit (MfS) – better known as the Stasi. The lens came with a set of 1.5×, 2× and 2.5× range extenders, and was intended for use with the Stasi's silent SLR camera, the Geräuscharme Spiegelreflex Kamera (GSK). It could also be used with other surveillance and video cameras, such as Exakta and Robot Star.

The lens is the successor of the SO-3.5, from which it differs by the addition of a diaphragm, that is adjustable with a ring towards the rear.

The lens has a image size of 24 mm and a focal length of 17 mm (ƒ5/17). Both the lens and the range extenders have M28 thread. For use with the Stasi's GSK and other cameras with M42 thread (Praktica) an M28/M42 adapter is needed.

A cone-shaped plastic lens cap with a 1 - 3 mm hole in the tip, should be placed over the lens. A set of suitable caps was supplied with each kit.
  
SO-3.5.1 pinhole lens kit

Note that the lens cap reduces the amount of light that reaches the film. A smaller hole reduces the risk of detection, but also reduces the light throughput, leading to longer exposure times. A possible solution for this was to use a more sensitive film (at the expense of an increased grain).

The bare ƒ5/17 lens can be used directly on the GSK camera, as shown in the image on the right. As the thread of the lens (M28) is different from the thread of the camera (M42), the M28/M42 adapter ringpart of the kit – must be used.

In this configuration, the image has a diameter of approx. 24 mm, which is smaller than a 35 mm frame. As a result, the image – typically a wide-angle view – will be circular as shown here. Zooming-in is possible by adding one or more range extenders between the lens and the camera, at the expense of light reduction.
  
JO=3.5.1 pinhole lens (f5/17) fitted to the GSK

When using a range extender, the image that is projected onto the film will be larger, as a result of which there will be less of a black border. Note that the lens has the best characteristics at the centre of the image. This is also the brightest part of the image. The SO-3.5.1 was typically used by the Stasi for observation of dissidents and for taking compromising pictures in hotel rooms. A similar lens kit – known as SO-3.5.2 was available for use with the video cameras of the Stasi.

Side view of the GSK camera with SO-3.5.1 pinhole lens and three range extenders.

The image above shows the GSK camera with the SO-3.5.1 pinhole lens and all three range extenders (2.5x, 2x and 1.5x) bitted between the lens and the camera body. Note the presence of the M36/M42 adapter ring between the lens assembly and the input port of the camera.

 More about the GSK camera

  1. SO = Sonderoptik (special optics).

Storage case
SO-3.5.1 pinhole lens kit
Inside the storage case
JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
Pinhole lens with aperture cap
Pinhole lens with removed aperture cap
Front lens and first lens group removed
First lens group
The Crypto Museum workshop seen through a 1 mm hole
A view of the street through a 1 mm hole
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Storage case
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SO-3.5.1 pinhole lens kit
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Inside the storage case
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JO=3.5.1 pinhole lens (f5/17) fitted to the GSK
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Pinhole lens with aperture cap
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Pinhole lens with removed aperture cap
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Front lens and first lens group removed
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First lens group
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The Crypto Museum workshop seen through a 1 mm hole
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A view of the street through a 1 mm hole

Parts
The parts of the SO-3.5.1 lens kit

Lens, range extenders and pinhole lens caps
Lens, range extenders and pinhole lens caps
f5/17 pinhole lens with adjustable diaphragm and pinhole lens cap
f5/17 pinhole lens with adjustable diaphragm
GSK with f5/17 pinhole lens
GSK with pinhole lens and 2.5x range extender
GSK with pinhole lens and 2.5x and 2x range extenders
M28/M42 adapter
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Lens, range extenders and pinhole lens caps
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Lens, range extenders and pinhole lens caps
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f5/17 pinhole lens with adjustable diaphragm and pinhole lens cap
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f5/17 pinhole lens with adjustable diaphragm
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GSK with f5/17 pinhole lens
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GSK with pinhole lens and 2.5x range extender
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GSK with pinhole lens and 2.5x and 2x range extenders
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M28/M42 adapter

Lens configuration
The SO-3.5.1 is a so-called front stop lens [3] in which the (virtual) entrance pupil (EP) [4] is located approx. 5 mm in front of the first lens element. The EP must coincide with the physical aperture that is placed in front of the lens, which is why the plastic cap has its conical shape: it ensures that the aperture (i.e. the pinhole) is placed exactly 5 mm in front of the lens. As a result of this lens design, the image is barely cropped by the pinhole aperture in front of the lens cap.

Simplified lens configuration with light path.


 Check out the full lens simulation

The drawing above shows the various lenses and groups of lenses inside the SO-3.5.1. Move over the image to see the light ray path, or click the drawing to see a full simulation. Below is an x-ray image of the complete kit. The actual SO-3.5.1. lens is at the bottom left, to its right is the 2.5x range extender. At the top are the 1.5x and 2x range extenders and the M36/M42 adapter ring.

X-ray image of the contents of the SO-3.5.1 kit. Click to enlarge.


Specifications
  • Focal length
    17 mm
  • Focal ratio
    1:5
  • Angle of view
    80°
  • Pinhole
    1 - 3 mm
  • Pupil distance
    5 mm
  • Image size
    24 mm Ø
  • Resolution
    120 L/mm (centre), 80 L/mm (edge)
  • Thread
    M28
  • Diameter
    30.1 mm (without diaphragm)
Checklist
  • Wooden storage case
  • Lens ƒ5/17 with adjustable diaphragm
  • Range extender × 1.5
  • Range extender × 2
  • Range extender × 2.5
  • Adapter Nr 11: M28/M42 mount
  • Various pinhole lens caps with different openings
Adapters   optional
  • Nr. 11
    Praktica (M42 thread) 1
  • Nr. 12
    Exakta
  • Nr. 13
    Robot Star
  • Nr. 14
    C-Mount (TV cameras)
  1. Standard in the kit.

Range extenders   teleconverters
The SO-3.5.1 is standard supplied with the following 3 range extenders (teleconverters):

Focal length multiplier 1.5 x 2.0 x 2.5 x
Viewing angle 51.5° 39° 31.6°
Distance to film 45 ±0.06 mm 45 ±0.1 mm 45 ±0.16 mm
Visual resolution in Lp/mm (centre) 110 90 70
Photographic resolution in Lp/mm 90 70 50
Mount M28 × 0.5 M28 × 0.5 M28 × 0.5
 Teleconvertors datasheet


Documentation
  1. Sonderobjectiv SO-3.5 - datasheet (German) 1
    MfS. Undated.

  2. Sonderobjectiv SO-3.5.1 - datasheet (German) 1
    MfS. Undated.

  3. Telekonverter für die Nadelöhrobjektive SO-3.5.1 1
    MfS. Undated.

  4. Lens simulation 1
    Unknown author, 10 August 2018.
  1. Document obtained from BStU [2] and kindly supplied by Detlev Vreisleben [1].

References
  1. Detlev Vreisleben, Personal Correspondence
    November 2021.

  2. Bundesbeauftragte für die Stasi-Unterlagen (BStU) 1
    Federal Commissioner for the Stasi-Records.

  3. Eberhard Dietzsch, Front stop photo lenses
    SPIE Vol. 1780 Lens and Optical Systems Design, 1992. pp.721-726.

  4. Wikipedia, Entrance Pupil
    Retrieved November 2021.
  1. Full name: Bundesbeauftragte für die Unterlagen des Staatssicherheitsdienstes der ehemaligen Deutschen Demokratischen Republik (DDR) — Federal Commissioner for the Records of the State Security Service of the former German Democratic Republic (GDR) — officially abbreviated to BStU.

Further information
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© Crypto Museum. Created: Friday 26 November 2021. Last changed: Saturday, 11 December 2021 - 13:43 CET.
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