Front focal length (FFL)
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.
The distance from the front focal point of the combined lenses to the first lens is called the front focal length (FFL).
|The distance from the front focal point of the combined lenses to the first lens (dimensionless)
|Focal length of one of the lenses (m)
|Focal length of the other of the two lenses (m)
|Distance between the thin lenses (m)