## First a little Math

The general formulas to compute the effective focal length (F) and back focal length (BFL) of two lenses are:

F = (f1 x f2) / (f1 + f2 - d)

BFL = (f2 x (d - f1)) / (d - (f1 + f2))

where f1 is the focal length of lens #1, f2 is the focal length of lens #2, and d is the distance between the lenses.

*Note that f1 and f2 can be either positive or negative.*