focal length (also focal distance)

distance between the centre of a mirror or a lens and its focus 焦距.

flying fox, flying jump (also flying leap), flying officer, flying picket, flying squad, flying start, flying tackle, flying visit, flyleaf, flyover, flysheet, flyweight, flywheel, FM, FO, foal, foam, foamy, fob, focal, focal point, focsle, focus, focus on, focus...on, fodder, foe, foetal (US fetal), foetus (US fetus), fog, fog-bank, fog-bound, fog-horn, fog-lamp, foggy, fogy (also fogey), foible, foil, foist, fold,