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

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

This also allows Apple to crop the sensor in different ways to deliver true 2x zoom at a 48mm focal length.

From The Verge • Sep. 7, 2022

One has an object-glass of four inches aperture and eight feet six inches focal length, with a field of about 1° 2′, which is very nearly the photographic field of the main telescope.

From Photographs of Nebul? and Clusters Made with the Crossley Reflector by Keeler, James Edward