papervision camera formula (was: screen space to 3D space...)

(http://www.nabble.com/screen-space-to-3D-space...-td15355702.html)

Looks like lots of people are confused about papervision cameras and how they compare to real 3D counterparts...
Is the default focal length really 100mm? Anyone got a tutorial out there that explains it?

http://www.nabble.com/Camera-FoV-td11084062.html#a11084062
http://www.nabble.com/Convert-PV3D-zoom---focus-to-Field-of-View---td13530737.html#a13530737
http://www.nabble.com/Understanding-x%2C-y%2C-z%2C-zoom%2C-and-focus-interaction.-td13557494.html#a13557494