Little Help- Aperture Calculation for Eurynar

[patrickjames]

I barely ever ask questions here, but I know you guys can help me with this.

I have a Rodenstock 150mm f/3.5 Eurynar from 1928 ish that I have noticed is convertible when I take off the front elements. Ignoring the fact for a moment that the image quality may not be fantastic with the lens this way, how can I figure out the aperture values so I can give it a go? My measurements indicate that the aperture blades are roughly 276mm from the focal plane when the lens is converted (rear cells) at infinity. So, is there a way that I can translate the aperture values on the shutter (via a formula), or should I just figure out new ones? I would prefer a way to translate because I don't have calipers with me, but I guess I could get some if that is the only way to go.

Any ideas?

Patrick

[Paul Fitzgerald]

you could focus the complete lens at inf. and view an evenly lighted wall much closer and take a meter reading of the GG wide open than do the same for the converted lens. the difference should put you in the ball park.