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?

  • \$\begingroup\$ Followup: I bought the bellows back then (for two whole dollars) and it sat on my shelf since then. But I recently bought a few M42 cameras and lenses at the same thrift store, and they fit, so it was an M42 bellows. M42 adapters for my cameras are on the slow boat from Hong Kong as we speak. \$\endgroup\$
    – coneslayer
    Dec 29, 2013 at 3:25

1 Answer 1


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).

  • \$\begingroup\$ ^^^^^^^^ What he said. I was going to go with the M42 mount too :) \$\endgroup\$
    – Mike
    Jan 24, 2012 at 17:10
  • \$\begingroup\$ +1 - Did they ever make a T-mount bellows? It would seem to be highly unlikely. \$\endgroup\$
    – Joanne C
    Jan 24, 2012 at 18:39
  • \$\begingroup\$ I don't know -- but it would make a certain sort of sense, since the T-mount was the original "Adaptall" (and was apparently Tamron's thing as well). It would mean you could have a single set of lenses, tubes and other accessories and use them on a variety of cameras/mounts using only a single adapter per camera. \$\endgroup\$
    – user2719
    Jan 24, 2012 at 19:00
  • \$\begingroup\$ Some googling finds T-mount bellows, for example: thecuckoofarm.com/cuckoo/photo/camatt/bell/vivbell/vivbell.php \$\endgroup\$
    – coneslayer
    Jan 24, 2012 at 20:10
  • \$\begingroup\$ @coneslayer - Well there you go! I had no idea. I often see the Pentax branded M42 bellows come up for sale but nothing else. \$\endgroup\$
    – Joanne C
    Jan 25, 2012 at 0:53

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