Effective Focal Length.
For a two-lens system (such as that in many ERSs):
EFL=Effective Focal Length,
f1=Focal Length of Lens1,
f2=Focal Length of Lens2,
d=Distance between Lenses.
In fixed-beam ERS fixtures, d is a constant, but is a variable in zoom ellipsoidals.
For more, see the entry Gullstrand's Equation.
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