Manual focus Sigma telephoto with YS mount. The Sigma YS swappable mount system (? 1970's - a post on a forum thread "Yamaki System designed 1965", the availability of Nikon AI and pentax K mounts indicates production into the second half of the 1970's ) was a variant on the common t-mount system. The main difference is the dedicated mounts allow the possibility of auto aperture - you can see the M42 mount in the pic has a pin. Ordinary t-mounts can also be used, the thread and registration distance is the same. More info on the YS mount here, and Yu-Lin Chan writes on his lens bubbles blog: " Of all the interchangeable mounts, only the Tamron Adaptall evolved, and eventually survived to became Adaptall-2. But, the YS mount did enjoy a brief period of prosperity.... YS mounts were available for all major cameras of the time....M42 (two versions), Minolta SR and MD, Yashica Contax, Nikon F (there is also an IA version), Konica KR, Pentax K, Leica R ...
The YS adapter derived from the T-mount, but provided auto aperture function, whereas T-Mount lenses invariably have a pre-set aperture. This allowed lenses to have an open aperture metering. ... Changing the mount is like mounting an M42 lens. The mount can be unscrewed, and a new mount can be replaced. "
And wikipedia says: "....Sigma, Upsilon, Accura, Polaris, Sun, Aetna, Beroflex, Petri, Raynox, N.P.S., Formula, Dionar, Universal, Soligor, Yashica, Spiratone. M42×0.75 (not to be confused with M42×1) Same as T-mount but with aperture coupling."
This lens has a double helicoid focus with two rings, the second front ring facilitates additional close focus, but also rotates the front end.
Optics: ?
Filter: 77mm
Aperture: f4-f22, two clicks per stop except f16-22
Iris: 6 blades
Close Focus: 4m on main ring (300°, pentax rotation) then to 1:4 with secondary ring.
Weight: ~ 1kg with M42 mount
Small tripod bush - 1/4".
Pic shows accompanying dedicated hood. The yellow dots on the lens are not original.
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