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

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.

Word History and Origins

Origin of focal length¹

First recorded in 1745–55