The effective focal length of a lens includes some tolerances. To guarantee plug and play performance these tolerances need to be compensated for. Therefore the Optoskand external optic modules are prealigned during assembly. The alignment process is made with a reference fiber and calibrated measurement equipment. The lens is moved to the position to collimate the beam then fixed.
The Effective Focal Length (EFL) is related to the imaging properties of the lens and is what we normally call the focal length of the lens.
The Design Focal Length (DFL) of the focusing unit is related to the working distance of the focusing unit. The DFL is chosen close to the EFL and defines the distance from our mechanical interface to the focal spot position of a parallel incoming beam. The optics is prealigned, which means that variations due to tolerance in EFL and/or Back Focal Length will be adjusted for.
The Back Focal Length (BFL) is the distance from the exit side of a lens on the optical axis to the focal point assuming a parallel incoming beam. This distance is not used in the description of the focusing optics, but defines the position of the lens seat within the holder.
The lens above is a doublet lens (it consists of two lens elements).