Thread mounts (also called screw mounts) are characterised by that the lens mount basically is a giant (hollow) screw which is screwed into the camera’s corresponding ‘nut’. Mounting thread mount lenses typically requires several revolutions, and changing lenses is therefore significantly slower (than with bayonet or breech-lock lenses). Also, it has been known to happen that a combination of an incorrect mounting angle (lens optical axis not perpendicular to film plane) and the use of force has damaged the threads (on either lens or camera).
While no recent lens mounts use thread mounts, some of the most iconic mounts of photographic history (such as M42 and the Leica thread mount) use thread mounts.
Thread mounts are characterised by two metrics: the outer diameter of the thread, and the thread pitch (spacing). Trying to match incompatible thread mounts may result in damage to the threads.
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