How do I identify unknown thread mounts?

Question

A local thrift store is selling a macro bellows with thread (screw) mounts. The mount is not identified. I have a cheap digital caliper. What are the likely mounts that it could be, and how can I distinguish them through measurements?

I know, for example, that there's a nominal 42mm "Pentax" screw mount, but what are the actual measurements that I should expect from measuring male and female mount diameters? Do I need to be concerned about thread pitch?

Answer

At 42mm, the mount could be either M42 (Pentax/Practica/Zeiss) or T-mount. The difference is thread pitch -- the M42 has a 1mm thread pitch (the "wavelength" of the thread, measured from "peak to peak"), and the T-mount has a 0.75mm thread pitch. So, three grooves in three millimeters is M42; four grooves in three millimeters is T-mount (or one of the variations on T, like the Sigma YS).

There's also a slight chance that you might run into an M39 (39mm Leica) mount, but that's vanishingly unlikely on a bellows unit. A bellows is almost useless without TTL focusing (as in an SLR ro a view camera), and the M39 is pretty much a rangefinder-only mount (the exception being the early Leica reflex box that sat between the camera and the lens).

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