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?