Gullstrand's Equation CalculationsGullstrand's equation can be used to calculate the effective focal length of a thick lens or two separated lenses with respect to the second principal plane. It is often useful to calculate the front and back vertex powers or focal lengths. Since Gullstrand powers and focal lengths are measured with respect to a hypothetical plane, it is often more useful to deal with front and back vertex focal lengths. 
Index Lens concepts Thick lens concepts  

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