Effective Focal Length Calculator

The calculators below perform a rough determination of the Effective Focal Length (EFL) of simple 2- and 3-element eyepieces. You need only specify the focal length of the individual elements and the spacing between the elements. The formulas employed by these calculators are approximations.

The calculators are also available as an Excel spreadsheet. Click to download Click to download.

2-Element Focal Length Calculator
2-element eyepiece Enter the focal lengths of the lenses (f1 and f2), and the spacing (d) between the principal planes of the elements. Then click Calculate EFL to perform the calculation.

The lens types shown in the diagram are for illustration only; many different arrangements are possible. The calculator does not care what type of lens is used in each position.

f1
f2
d
EFL
2-element equation

3-Element Focal Length Calculator
3-element eyepiece Enter the focal lengths of the lenses (f1, f2 and f3), and the spacing (d1-2 and d2-3) between the principal planes of adjacent elements. Then click Calculate EFL to perform the calculation.

The lens types shown in the diagram are for illustration only; many different arrangements are possible. The calculator does not care what type of lens is used in each position.

f1
f2
d1-2
f3
d2-3
      EFL
3-element equation
 

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