Two convex lenses of focal lengths \(f_1\) and \(f_2\) are separated by a distance \(d\). The…

For two thin lenses separated by a distance $d$, the combined focal length $F$ is given by $\frac1F=\frac1{f_1}+\frac1{f_2}-\frac{d}{f_1f_2}$. The combination behaves like a glass slab, with zero power so that parallel light stays parallel, when $\frac1F=0$, which happens when $d=f_1+f_2$. This is the afocal arrangement of an astronomical telescope; any other separation leaves the system with some converging or diverging power.