With f = focal length and d = distance from film plane to subject, the
displacement from the infinity focus position =
2f / [d/f - 2 + sqrt( (d/f)^2 - 4d/f)]
or if you are conversant in in Excel:
=2*$B$1/(B3/$B$1-2+SQRT((B3/$B$1)^2-4*B3/$B$1))
where B1 is focal length and B3 is subject to film distance - I've tested that; it works.
Please don't ask me to show my work, you sent me back about 40 years in school to work this out.
I provided the displacement from infinity focus because in general the film plane to rear nodal point distance cannot be measured, but you can measure this displacement from any convenient repeatable landmarks on the front and rear standards.
Good Luck - Alan
Oh Yeah, I forgot that there is still an unknown term to be considered when measuring subject to film distances. It is the Principal Point Separation, the distance between the front and rear nodal points. My math assumes it is zero. Schneider publishes this information and perhaps it can be measured accurately enough if you set up your camera with magnification = 1:1. If the actual film to subject distance is different from 4x the focal length then that difference is the Principal Point Separation - assuming that you are using the actual focal length rather than the nominal one.
Again - Good Luck
Bookmarks