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Effective Focal Length.

For a two-lens system (such as that in many ERSs):
EFL= (f1*f2)/(f1+f2-d),
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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