Glossary
Flange focal distance
Flange focal distance is the gap between a camera's lens mounting flange and its sensor or film plane, and the difference between two mounts is exactly the thickness an adapter must add.
Also called register, flange distance, flange back distance
How it works
Every mount fixes this number. M42 is 45.46mm, Canon FD is 42mm, Nikon F is 46.5mm. Mirrorless mounts are far shorter because there is no mirror box: Sony E is 18mm, Fuji X is 17.7mm, Canon RF is 20mm, Micro Four Thirds is 19.25mm.
Adapting works when the source mount is longer than the destination. The adapter is a spacer that makes up the difference, so the lens sits at the distance it was designed for and still reaches infinity focus. No glass is involved.
Adapting in the other direction, from a short mount to a longer one, requires corrective optics that degrade image quality, or you lose infinity focus. This is why vintage SLR glass adapts beautifully to mirrorless and why the reverse is a compromise.
Why it matters when buying
Check that your lens mount has a larger flange distance than your camera mount. Every vintage SLR mount in this catalog does for every mirrorless system listed, which is why a plain mechanical adapter is all you need.