Flange Focal Distance
Flange Focal Distance is distance from a lens mount flange to focal plane, and this distance is the same, irrespective of the actual focal length as a 10mm lens should focus the rays to the same physical location as does a 600mm lens.
But different cameras have different flange focal distance that makes it impossible to make some adapters. For example, you can't make an adapter between Arri PL and OCT-19 mounts. Or you need to use optical elements like in infinity-allowing M42 to Nikon-F adapter.
I have info on FFD and mount diameter of some movie cameras. Wikipedia has more info.
| Camera | FFD, mm | Mount diameter, mm |
1 | 1KSR-1M (Konvas-1M,-7M,-Automat) | 57±0.01 | 47 |
2 | 1KSR-2M (Konvas-2M,-8M, Kinor-35) | 61±0.01 | 68 |
3 | US-3H | 61±0.01 | 68 |
4 | US-2M | 68±0.01 | 68 |
5 | 1-SKL (Temp) | 57±0.01 | 50 |
6 | Arri-2C | 52±0.01 | 41 |
7 | Arri-BL2 | 52±0.01 | 41 |
8 | Arri-BL3 | 52±0.01 | 54 |
9 | Aaton | 40mm ? | ? |