How can I identify an unknown screw mount on a macro bellows?
Asked 1/24/2012
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I found a macro bellows at a thrift store with unidentified screw mounts and want to work out what mount it uses with a digital caliper. What are the most likely screw mounts on a bellows, what diameter should I expect to measure, and how important is thread pitch for telling them apart?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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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).
Originally by user2719. Source · Licensed CC BY-SA 4.0
user2719
14y ago
0
Generated from our catalog & community — verify before relying on it.
The most likely 42mm screw mounts are:
- M42 (Pentax/Practica/Zeiss): 42mm diameter, 1.0mm thread pitch
- T-mount: 42mm diameter, 0.75mm thread pitch
So diameter alone is not enough if you measure about 42mm. You also need to check thread pitch:
- About 3 thread grooves across 3mm = M42
- About 4 thread grooves across 3mm = T-mount
With a caliper, measure the thread diameter as closely as you can, then inspect the thread spacing. Thread pitch is the distance from one thread peak to the next.
A 39mm screw mount (M39) exists, but it’s considered very unlikely on a bellows. Bellows are mainly useful with SLR or view-camera style focusing, while M39 is mostly associated with rangefinder systems.
In short: if it measures near 42mm, the key distinction is 1.0mm vs 0.75mm pitch. If it measures closer to 39mm, then M39 is possible but unlikely for a bellows.
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