You lost me here .... the focal length and the distance between the pinhole and the film plane are the same thing. 50 mm is about 1.97 inches - is that 3 a typo?
If designing a camera from scratch, I would pick a film format, then pick an angle of view. From that you could determine the film plane to pinhole distance, and from that work out the pinhole diameter. Ergo, take a look at the "Design Wizard" on the Mr. Pinhole site. In my own case, I was using existing cameras, so there were some limitations on those starting parameters.
Edit: Just tried walking through that "wizard" and its behavior is rather strange. It only takes one parameter, seemingly not giving a chance to set anything else before it coughs up a set of numbers based on mysterious assumptions. The calculator button might be a better tool.
Last edited by DWThomas; 05-11-2010 at 02:10 PM. Click to view previous post history.