LOMO OKS5-18-1 (2.5/18mm)
This lens was designed at NPO "EKRAN". It has extended Back Focal Length so it can be mounted on 35mm film movie cameras with 19mm distance between mirror shutter and film plane.
The lens provides acceptable image quality comparable to Cine Prominar 2/20 mm, and superior to Super Baltar 2/20 mm.
I had ОКС5-18-1 lenses in different mounts (OCT-19, OCT-19 and other).
Specifications of the optical block OKS5-18-1
The lens has only 8 aperture blades, so octagonal shape of the aperture opening is clearly visible.
Optical block has no aperture values marks. Also, there are no click stops. Effective relative aperture may vary a little (I saw T/3.1, T/3.2, and T/3.3 samples).
|Focal length||f'||18.3 mm|
|Angle of view||2ω'||74°08'|
|Effective relative aperture||f:nэ||1:3.2|
|Lens length||Σd||77.2 mm|
|Front focal length||S||-|
|Back focal length||S'||33.6 mm|
|Flange focal distance||Z||-|
|Block length||B||78.4 mm|
|Block max. diameter||D||62.5 mm|
|Rear diameter||D'||33 mm|
|Front element diameter||d1||52.5 mm|
|Rear element diameter||d2||20.0 mm|
|Mount thread||SpM36x0.75 cl 2a|
|Distance from rear end of the housing|
to the focal plane
|Aperture blades quantity||8|
|Frame size||16x22 mm|