Flange Focal Distance
The distance from a camera's lens mounting flange to the sensor or film plane.
Measured in
mm
In depth
This single measurement determines what can be adapted onto what: a lens can only be adapted to a body whose flange distance is shorter, with the adapter making up the difference. SLRs need 40 to 46 mm to clear the mirror, while mirrorless mounts sit near 16 to 20 mm — which is why mirrorless bodies can take almost any legacy lens and DSLRs can take almost none.
Examples in practice
An 18 mm mirrorless flange distance leaves 26 mm of space to build an adapter for a 44 mm SLR lens.