Meaning of focal length

fo'cal length"

Pronunciation: [key]
— Optics. Optics.
  1. the distance from a focal point of a lens or mirror to the corresponding principal plane. Symbol: f
  2. the distance between an object lens and its corresponding focal plane in a telescope. Also called
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.