Hi All,
since I am a fan of old glass in front of my SDs, let me bring some light into the dark.
There have always been ambitions to have a standard lens mount for a number of different camera brands.
The oldest and most frequent lens mount really was the M42-screw-mount.
M42 (meaning metric 42 x 1 mm diameter standard thread) used to have a flange to film distance of 45.5mm.
Since many modern DSL-R bajonett-mounts have fewer than 45.5mm flange distance, you now can easily adapt a large number of these old lenses to the cams.
SIGMA-SA has 44.00mm flange distance ... so an adapter is no problem.
NIKON-F for instance is 46.5mm. So no NIKON can take an old M42-lens.
Well. The T2-mount (frequently called T-mount) is a special mount for telephoto lenses. Its flange distance is 55mm.
Although T-mount is based on a 42mm screw mount, it would not fit any M42 mounts. In fact it is an M42 fine-pitch-thread M42 x 0.75.
So It is for telephoto-lenses and astronomical telescopes
only.
Actually, I have quite a number of such "very long" T-mount telephoto-lenses.
Above, the ancient Adaptall System was mentioned. It was the final attempt, to find a standard mount, when all the manufacturers switched to their own bajonetts to protect their brands.
After all, there was another (very rare!) system, put into practice by two lens makers (SOLIGOR and VIVITAR). They called it the T4-mount.
Similar to ADAPTALL, adapters were the link between them and the particular camera bajonett to be fitted.
Even M42-T4 adapters were manufacturerd to fit the lenses to the M42-Standard.
All these ambitions then failed bitterly when the cams came with shutter priority exposure then autofocus and finally by-wire-apperture. Any dreams of a standard mount were ended abruptly.
I am very proud to have two of these very nice T4 lenses (a 35mm 2.8 and a 24mm 2.8 Vivitar) and two of those very rare T4-M42 adapters. Using then an additional M42-SIGMA SA adapter, I nicely can use them with my SIGMA D-SLRs.
See you with nice pictures
Klaus