Register distance

The register distance, also referred to as the flange focal distance, is the distance between the lens mount, and the film plane. Modern cameras don't use film anymore, so the film plane is in fact the sensor plane.

SLR cameras, with a moving reflex mirror behind the lens mount, need a fairly long register distance. The reason why Micro Four Thirds cameras can be made more compact than SLR cameras, is that they don't need the reflex mirror, and can have a shorter register distance.

A lens mount with a short register distance can be considered a universal mount, since it is possible to create adapters for mounts with longer distance. Micro Four Thirds is such a mount. There are adapters for a number of different formats, e.g., Nikon F and Pentax K.

For comparison, here is the register distance of various formats:

Samsung NX Mini	Very short
Pentax Q	9.20mm
Nikon 1	17.00mm
C mount	17.526mm
Fujifilm X	17.70mm
Sony E (NEX)	18.00mm
Sony FZ	19.00mm
Leica L (T and SL)	19.00mm
Micro Four Thirds	19.25mm
Hasselblad XCD	20.00mm
Samsung NX	25.50mm
Fujifilm G	26.70mm
Leica M	27.80mm
Konica Hexar RF	28.00mm
Olympus Pen F	28.95mm
Contax G	29.00mm
Contax RF	34.85mm
Four Thirds	38.67mm
Konica AR	40.70mm
Canon FD	42.00mm
Minolta MD	43.72mm
Sigma SA	44.00mm
Canon EF	44.00mm
Minolta AF/Sony Alpha	44.00mm
Pentax K	45.46mm
M42 screw	45.46mm
Contax/Yashica	45.50mm
Olympus OM	46.00mm
Nikon F	46.50mm
Leica R	47.00mm
Contax N	47.00mm
Arri PL	52.00mm
Oct-19	61.00mm
Mamiya 645	63.30mm
Pentax 645	70.87mm
Hasselblad 500	74.90mm
Pentax 6x7	84.95mm

The new Samsung mirrorless NX format has a longer register distance than Micro Four Thirds. However, the Samung format still has a shorter distance than most relevant formats, e.g., Leica M, so that it is possible to create adapters just as easily as for Micro Four Thirds.

On the other hand, the Samsung NX has a register distance about 2mm shorter than Leica M, which may be why Novoflex has not yet announced an adapter for it. 2mm may be too short to make a quality adapter.