John Shriver , Mar 26, 2009; 10:03 p.m.
Core formula is:
1 / f = 1 / u + 1 / v
Where f is the focal length of the lens, u is lens to subject distance, and v is lens to film distance.
More properly, v and u are measured from the focal point of the lens. For a symmetric lens, that's usually smack-dab in the middle of the lens. For a telephoto lens, it's way in front of the lens -- that's the definition of telephoto. For a retrofocus lens, it's way behind the lens.
Magnification is v / u.