# Location of lens having effective focal length

We know a/c to Gullstrand's equation that the effective focal length of two lenses separated by a distance $$d$$ is given as $$\frac{1}{f_{eq}}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{d}{f_1f_2},$$ but the equation doesn't clarify on the position of the lens having this effective focal length, how do i calculate that?

What you can do is make an equivalent lens system: a box containing the effective lens at a given distance from the front $$d_f$$ and the back $$d_b$$ of the box. The length of this box is in general $$d_f + d_b \neq d$$, and so, when you replace your two lenses by this box, you would have to move all the optics you had after the pair of lenses. Note that if the two lenses are near a telescope arrangement, then this equivalent box would have near infinity length.