Flange Focal Distance (FFD) is the distance from a taking lens mount flange to focal plane, and this distance is the same for any given camera, irrespective of the actual lens focal length since a 10mm lens should focus the rays to the same physical location as a 600mm lens.

The FFD should not be confised with the lens focal length or back focal distance. Please refer to the diagram below. Flange focal distance of the OCT-18 mount is 57mm. This distance shold be kept precise up to hunderth of mm in order to allow the lens to focus to infinity and reflect correct value on the distance scale. Preserving precise FFD is one of differences between good and bad lens adapter.

Flamge Focal Distance

Different cameras have different flange focal distance that makes it impossible to make some adapters. This is possible if you can place a lens at such a position where it gives the focused image of a distant object on your camera sensor.

In general case, this requires the lens FFD to be longer than a camera FFD, so these two numbers are the first thing to compare.

Unfortunately, this is not enough. A lens mount may be larger than a camera mount, for example. In this case, you need extra FFD deifference to put the lens completely in front of the camera.

On the other hand, tuny lenses may be recessed into the camera mount is some cases, and an adapter will be possible then even if the lens FFD is shorter than the camera FFD.

General approach to the research of adapting possibility is described in the Buliding an adapter article.

I have info on FFD and mount diameter of some Soviet movie cameras. Wikipedia has more info.

 

CameraFFD, mmMount diameter, mm
1KSR-1M (Konvas-1M,-7M,-Automat)57±0.0147
1KSR-2M (Konvas-2M,-8M, Kinor-35)61±0.0168
US-3H61±0.0168
US-2M68±0.0168
1-SKL (Temp)57±0.0150
Arri-2C52±0.0141
Arri-BL252±0.0141
Arri-BL352±0.0154
Aaton40mm ??