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

See Examples For:

focal length

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

For many months most of the observations and photographs have been made with the Cassegrain combination of mirrors, giving an equivalent focal length of 134 feet and involving three reflections of light.

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