Guys,

What is considered the back flange on a lens? The backmost piece of metal in the mount, or the backmost piece of glass?

I found this diagram illustrating two kinds of back flange, optical and mechanical. Which is it when we talk about M42 lenses having a back flange to GG distance of 45.5mm?

http://www.isorainbow.com/images/tech/flange.gif

Thanks,
Josh

So far as I know that diagram illustrates the focal length of a lens.

"Back flange" may be easyspeak for "flange to focal plane". This measurement is from the rear face (back) of the flange which fits against a matching face on a mount on the camera, to the emulsion surface of the film in the camera, or the imaging surface of the CCD or in the case of the AGUS/ALDU, the groundglass rear-projection screen.

The focal length of a lens is not often the same distance as the flange to focal plane distance. It can be more or less depending on the field of view of the lens. There's better brains than mine can take it from here.

Bob, thanks, that helps. In doing some more research, I found that it is asy ou say, the back of the lens body, not counting the threads or bayonet.