Lens mounts
Reference specifications for every mount covered here. The flange focal distance is the number that decides what adapts onto what; the third-party position decides whether you can buy anything other than the camera maker’s own lenses.
The one rule worth knowing
Adapting works in exactly one direction: from a mount with a longer flange focal distance onto a body with a shorter one. An adapter can only add distance between lens and sensor, never remove it. A Canon EF lens expects to sit 44 mm from the sensor, so on an L-Mount body whose own mount sits at 20 mm there is 24 mm spare to build the adapter into. Reverse it and the lens would have to be recessed inside the camera, which is why no adapter exists to put a mirrorless lens on an SLR.
Read the mounts below as a ladder, shortest flange first: Nikon Z at 16 mm, Sony E and Canon EF-M at 18 mm, Canon RF and L-Mount at 20 mm, Leica M at 27.8 mm, then the SLR mounts from Canon FD at 42 mm up to Nikon F at 46.5 mm. Anything lower on that ladder adapts onto anything above it, given a couple of millimetres to work with. Nikon F adapts onto almost everything; Nikon Z adapts onto nothing.
Flange focal distance explained covers the arithmetic, and why mirrorless lenses don’t fit DSLRs covers the consequence.
Fitting is not the same as working
Clearance settles only whether the lens reaches infinity focus. Whether autofocus, aperture, stabilisation and EXIF survive the trip is a separate question, decided by electronics rather than geometry, and it has a different answer for every combination.
A fully mechanical lens always works, because there is nothing to translate — an M42 or Leica M lens on a good adapter behaves exactly as it always did. A modern electronic lens depends on the adapter relaying a conversation between two systems that were never designed to speak to each other, and the result ranges from indistinguishable from native to single- shot only. Each mount page below lists both directions, and every adaptation has its own page stating what actually happens.