15: Lens L1 and L2 is beam expander. L1 has a front focal length FFL of... | Download Scientific Diagram
optics - How can I increase the flange focal distance (back focal length) of DSLR lens? - Photography Stack Exchange
Focal points and focal planes, Gaussian optics
Ball Lenses and Half-Ball Lenses
Is flange focal distance of a camera system measured from the front edge of the lens mount to the film plane, or the back edge of the lens mount to the film
Overview of various methods for measuring a lens focal length