In a combination of two lenses(focal length f1 and f2) separated by a distance 'd', there seem to be two formulas to arrive at the effective focal length: 1. f = f1*f2/{(f1+f2)-d} 2. f = (d-f1)*f2/{d-(f1+f2)} Can someone explain the difference? Thanks!

One of them is FFL and other is BFL.

I have given you den=ifnition BUt i wnat you to derive it.

If still doubt lingers tell me.

Front focal length (FFL) or Front focal distance (FFD) is the distance from the front focal point of the system to the vertex of the first optical surface.
    Back focal length (BFL) or Back focal distance (BFD) is the distance from the vertex of the last optical surface of the system to the rear focal point.

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