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

/ fō′kəl /

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

Example Sentences

My two eyes haven't quite the same focal length and this often puts me out of the straight with a column of figures.

From Project Gutenberg

It contains a telescope made by Fitz, whose focal length is seventeen feet, and its object-glass is twelve and a half inches.

From Project Gutenberg

The distance from the principal focus to the center of the lens is the focal length or principal focal distance of the lens.

From Project Gutenberg

The focal length of double convex lenses of crown glass is about the same as the radius of curvature of either surface.

From Project Gutenberg

I used only the back combination of the lens, which gave a focal length of about twenty-two inches.

From Project Gutenberg