Compound lens' focal length (separated in air by some distance )

Description

A lens is a transmissive optical device which affects the focusing of a light beam through refraction. A simple lens consists of a single piece of material, while a compound lens consists of several simple lenses (elements), usually along a common axis.
A compound lens is a collection of simple lenses of different shapes and made of materials of different refractive indices, arranged one after the other with a common axis.
If two thin lenses are separated in air by some distance d, the focal length for the combined system can be calculated by the its one of the focal lengths and the distance between them.