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Stasi Zeiss GSK SO-3.5.2 →
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.
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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).
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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 ring
– part 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.
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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.
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
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SO = Sonderoptik (special optics).
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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.
➤ 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.
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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)
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- 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
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Nr. 11 Praktica (M42 thread) 1 Nr. 12 Exakta Nr. 13 Robot Star Nr. 14 C-Mount (TV cameras)
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Range extenders
teleconverters
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The SO-3.5.1 is standard supplied with the following 3 range extenders
(teleconverters):
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Focal length multiplier
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1.5 x
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2.0 x
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2.5 x
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Viewing angle
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51.5°
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39°
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31.6°
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Distance to film
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45 ±0.06 mm
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45 ±0.1 mm
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45 ±0.16 mm
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Visual resolution in Lp/mm (centre)
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110
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90
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70
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Photographic resolution in Lp/mm
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90
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70
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50
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Mount
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M28 × 0.5
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M28 × 0.5
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M28 × 0.5
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Document obtained from BStU [2] and kindly supplied
by Detlev Vreisleben [1].
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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.
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© Crypto Museum. Created: Friday 26 November 2021. Last changed: Friday, 16 December 2022 - 09:15 CET.
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