A lens mount is the mechanical and electronic interface between a camera body and an interchangeable lens. It defines the flange focal distance (the gap between the mount surface and the sensor), the bayonet shape, the contact pin layout that carries power and data, and the diameter of the throat through which light passes. Together these specs determine which lenses can attach, what they can do once attached, and what adapters are possible between systems.
Modern mirrorless mounts dominate the current market. Sony E (and the full-frame FE variant) has a 46mm throat and an 18mm flange distance. Nikon Z has a 55mm throat and a 16mm flange, the largest throat among major systems and a deliberate engineering choice that allows wide-aperture designs like the Nikkor Z 58mm f/0.95 Noct. Canon RF has a 54mm throat and a 20mm flange. Fujifilm X is APS-C only with a 44mm throat and 17.7mm flange. L-mount, shared by Leica SL, Panasonic S, and Sigma fp, has a 51.6mm throat and 20mm flange.
Legacy DSLR mounts use longer flange distances to leave room for a mirror box. Canon EF is 44mm flange; Nikon F is 46.5mm; Pentax K is 45.46mm. These can be adapted to mirrorless bodies (whose flanges are shorter) using simple spacer adapters that maintain infinity focus. Adapting a shorter-flange lens to a longer-flange body is rarely possible without optical elements, because there is not enough room to push the lens far enough from the sensor.
Leica M is the standout among rangefinder mounts, with a 27.8mm flange distance set in 1954 and unchanged since. Its mechanical simplicity (no autofocus, manual aperture) has allowed every M-mount lens from 1954 to today to mount on every M-mount body. Voigtländer, Zeiss, and dozens of small makers continue to produce M-mount glass for both Leica and adapted mirrorless use.
Autofocus communication, aperture control, image stabilization metadata, and EXIF exchange all travel through the electronic contacts. A native-mount lens can pass full autofocus speed, accurate distance reporting, and stabilization synchronization. An adapted lens passes whatever the adapter implements: sometimes nothing (manual M-mount lenses on E-mount), sometimes everything (Canon EF on RF via Canon’s own EF-EOS R adapter), and often somewhere in between (third-party adapters with limited AF performance).
Choice of mount has long-term consequences. Lens systems are larger investments than bodies and constrain future upgrades. Once a photographer owns three or four lenses in a mount, switching systems becomes a multi-thousand-dollar exercise. This is why mount families like Canon RF and Sony E grow steadily through native primes, zooms, and third-party offerings from Sigma, Tamron, Tokina, and Viltrox, each release deepening commitment to the platform.