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).
|FFL||The distance from the front focal point of the combined lenses to the first lens (dimensionless)|
|f1||Focal length of one of the lenses (m)|
|f2||Focal length of the other of the two lenses (m)|
|d||Distance between the thin lenses (m)|