Ok thanks for posting the picture. Looks like it is to go on a normal lens for a 35mm camera as a wide angle conversion. The fixed lens 35mm rangefinders (Konica, Yashica etc) usually had these as add-ons. The one you have may work on other formats also*.
If you don't have a camera with a 49mm thread, can't you just hold it up to the front of the lens and see what it does?
(*That one I'm using on the Bolex also has a 49mm thread.)
I've actually had it on a lens and on a D camera to see what was up with it. I'm thinking, and have yet to prove, that it's giving me a 42mm fov, but I don't know if I'm figuring that out correctly. I'll have to compare it to my 35mm Nikon lens today I guess and see. I was just really wondering how you figure the fov out mathematically.
i'm not too familiar with D cameras, from what I gather from camerapedia they usually have a 68mm equivalent lens, so 68/1.8 is ~37.777. If so for 120mm format you'd get almost twice the field of view as you'd normally would get, for example my Kiev uses a 30mm Zodiak 8 wide-angle lens attachment. They [Dianas] do accept 49 fem adapters don't they?
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