M42 is a type of threaded mount used to attach a lens to a camera. The camera end uses an internal thread, the lens' end uses an external thread. The diameter if the thread is 42mm and the thread pitch is 1 mm. More than 40 camera models and a dozen brands of
cameras used this mount at some point. The register (or distance from the edge of the mount to the film plane) is 45.46 mm

M42 is also the name of a Messier Object numbered 42 known as the great Nebula and NGC1976. It is located in the constellation of Orion in the vicinity of his sword.

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