Breech-lock refers to a type of lens mount which is based on that the lens is inserted straight (not rotated, as with bayonet mounts) into the camera, whereafter a locking ring is rotated to tighten the link. Breech-lock mounts are relatively rare, but especially legacy lens enthusiasts encounter them regularly.
The tightening ring of the breech-lock can be either mounted on the lens (e.g. Canon FL/FD) or on the camera body (Praktina, Pentacon Six). Most Breech-lock mounts typically are based purely on friction, meaning that the tightening ring may loosen in action, increasing the risk of dropping a lens.
Other types of lens mounts:
- thread/screw mounts
- bayonet mounts
Left picture: lens aligned with body before docking (notice red dots on lens and body aligned)
Right picture: Breech-lock ring tightened (notice position of red dot on breech-lock ring)