The M42 lens mount is a humble screw thread, 42 mm wide with a 1 mm pitch, which is a polite way of saying it twists on and stays put. It was dreamed up in 1938 by Carl Zeiss Jena for Kamerawerk Dresden, then trotted out after the war on cameras like the Contax S and Praktica. It was simple, unpatented, and easy to copy. It earned the nickname "Universal Screw Mount," which sounds either charmingly optimistic or like something you'd find in a hardware store next to the toggle bolts, depending on your temperament.

After the war, everyone invited it to the party. East Germany embraced it, Japan ran with it, and the Soviet bloc produced it by the millions because nothing says "workers' paradise" like standardized threading. Pentax turned it into a global celebrity, Zenit turned it into an industrial output statistic, and engineers kept adding small tricks like auto diaphragms and aperture pins. Compatibility, however, was often more of a suggestion than a promise.

By the late 1970s, faster bayonet mounts pushed the M42 aside with modern efficiency and little sentiment. Still, it refused to disappear, leaving behind mountains of inexpensive, character soaked lenses and adapting cheerfully to modern mirrorless cameras. Today it’s admired for being simple, flexible, and unapologetically old fashioned, which is a nice way of saying it still works and doesn’t pretend otherwise.