focal length

American  

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.

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focal length British  

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

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focal length Scientific  

/ fō′kəl /

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

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focal length Cultural  

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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Etymology

Origin of focal length

First recorded in 1745–55

Example Sentences

The imagery is convincingly mirror-like—reversed—with eye-like focal length, decent resolution and lowlight sensitivity, making it easy to trust when judging distances, with the help of graphical overlays and warning tones.

From The Wall Street Journal • Apr. 25, 2026

It’s up to me then to define the focal length and the background and the angles, but it’s all based on his design.

From Los Angeles Times • Feb. 15, 2024

"They have a fixed focal length, a fixed size. They are made from materials that give you the lensing property you want, but they don't really have any dynamic characteristics."

From Science Daily • Oct. 23, 2023

The lens was so heavy that he had to screw it into his tripod, and when he aimed at a wall in his apartment, the distance was too short for the focal length.

From New York Times • Mar. 3, 2023

The eyepiece will usually be made of two lenses, or more, and we use its focal length considered as a single lens, in getting the magnifying power.

From Astronomy: The Science of the Heavenly Bodies by Todd, David Peck