Add <mount> element only for mounts for interchangeable lenses. If the lens is fixed (mostly a compact camera), a <mount> element for it may be declared locally by the user (see Adding mount adapters: lensfun‑add‑adapter), but it should not be in the core database.

Possible embedded elements:

<name>string</name>

This gives the name of the mount. Mount names for fixed-lens cameras – and only they – must start with a lower case letter.

<compat>string</compat>

Declares that this mount is compatible with another one. Usually this means that either they are directly compatible (e.g. the Pentax KAF2 mount is compatible with Pentax K so you can insert K lenses into a KAF2 camera) or there exists mount adapters which permit to install lenses with such mounts into this mount. Note that the compatibility is unidirectional, e.g. in the above example it doesn't say that you can insert Pentax KAF2 lenses into a Pentax K camera; if you need a two-way compatibility, declare it both ways. Also the “compatibility” is restricted in the sense that if mount A is compatible with mount B (e.g. you can put B lenses on an A camera), and mount B is compatible with mount C, this does not neccessarily mean that mount A is compatible with mount C. Recursion doesn't work here, this is a design decision. If you need to make mount A compatible with mount C, declare it so explicitely.

Things are slightly complicated with mirrorless systems because they are adaptable to virtually any non-mirrorless system. In order to avoid too many useless entries in the lens selection lists for mirrorless mounts, only important (e.g. because only few native lenses yet exist) compatibilities should be included into Lensfun's official database (e.g. Canon EF on Canon M). The user can override this easily in their local database files, adding compatibilities for all mounts they have adaptors for, see Adding mount adapters: lensfun‑add‑adapter.