You may find yourself wanting to use a lens belonging to a particular photographic system on a camera made by another manufacturer and then not compatible (in the digital world it happens quite often, especially with mirrorless cameras); this is the typycal situation in which help comes from the adapter rings, which are today quite popular and promise interchangeability between systems, almost complete at least among the major ones.
The adapter ring has two main functions: the first is the physical connection, means securing connection between mounts (lens and camera body) which differ between them in diameter and often in electrical / mechanical contacts. Often the automatics present on a system cannot be replicated on the other and are lost, while still allowing you to work in manual mode. The second function is to adapt the lens register distance to make them compatible. Lens register distance or registration means in photography the distance between the focal plane of the film (or sensor) and the plane of the lens mount on the camera body and is typical of any system (SLR and otherwise) in trade now or in the past. Here lies a major problem: as the register is characteristics of each lens mount, this will be designed to focus on that particular floor and no one else thus making it impossible to use on bodies with different registration distance. The adapter rings to make up for this in two ways: if the register of the body is lower than the lens one, the difference will be offset by the sheer size of the adapter while in the opposite case, there are additional lenses that will correct the focus mistake due to insifficient registration distance, at the expense of quality which rapidly decays.
The table below summarizes the dimensional characteristics of the main mounts&bayonets systems for 16mm, 35mm, medium format and movie formats: all of them, in some way, can be used in both mirrorless and in modern digital SLR but, it is not unusual to find even adapters rings for film cameras even for less known mounts.
|MANUFACTURER||MOUNT||DIAMETER (mm)||REGISTER (mm)|