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focal length

noun

, Optics.
  1. the distance from a focal point of a lens or mirror to the corresponding principal plane. : f
  2. the distance between an object lens and its corresponding focal plane in a telescope.

focal length

noun

  1. the distance from the focal point of a lens or mirror to the reflecting surface of the mirror or the centre point of the lens

focal length

/ fō′kəl /

  1. The distance between the optical center of a lens or mirror to its focal point .

focal length

  1. The distance between the focal point of a lens (that is, the point at which the lens will focus parallel rays of light) and the lens itself.

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Word History and Origins

Origin of focal length1

First recorded in 1745–55

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