focal plane

American  

noun

Optics.

1. a plane through a focal point and normal to the axis of a lens, mirror, or other optical system.

2. the transverse plane in a telescope where the real image of a distant view is in focus.

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focal plane British  

noun

1. the plane that is perpendicular to the axis of a lens or mirror and passes through the focal point

2. the plane in a telescope, camera, or other optical instrument in which a real image is in focus

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Etymology

Origin of focal plane

First recorded in 1890–95

Example Sentences

The process was time consuming in instances when a worm left the focal plane of the camera, blurring the image, in which case Ran needed to click by hand to record its position.

From Science Daily • Oct. 15, 2025

Tiny robot arms position the ends of 5000 optical fibers to match the locations of galaxies on the telescope’s focal plane, and quickly reposition them when the telescope moves to different part of the sky.

From Science Magazine • Apr. 4, 2024

A four-story tower was built on top of the foundation, and the focal plane, or the height of the light, is 63 feet.

From Washington Post • Sep. 2, 2022

Its traditional 12-megapixel sensor has a larger focal plane than the Ultra.

From The Verge • Mar. 11, 2020

With telescopic sights, a fine hair in the focal plane of the telescope provides a far more accurate guide to the precise alignment of a star.

From "The Scientists" by John Gribbin