It is real simple: Total length squared divided by the focal length squared gives you the factor to multiply the exposure by.
Example: If you have a 12" lens and the bellows is out 20" you have 400 divided by 144 for a factor of 3. Being exact with this is not critical.
While the actual measurement is from the rear nodal point of of each lens to the other rear nodal point (and the rear nodal points are not always in the center of the lens, just measure, as Richard Knoppow wrote, from center of lens to center of lens. (This holds with view camera lenses.)
Michael A. Smith ============================================================================================================= To unsubscribe from this list, go to www.freelists.org and logon to your account (the same e-mail address and password you set-up when you subscribed,) and unsubscribe from there.
