Gullstrand's Equation

In simplified terms, a method for calculation of the EFL (Effective Focal Length) of a lens train.

For stage purposes, For a two-lens system (such as that in many ERSs):

EFL= (f1*f2)/(f1+f2-d),
where
EFL=Effective Focal Length,
f1=Focal Length of Lens1,
f2=Focal Length of Lens2,
d=Distance between Lenses.

It's the d in the equation that allows zoom optics to function and provide variable EFLs.
For more complete explanation, see the site Gullstrand's Equation.