FOCUS • DICTIONARY ONE.COM • Definition of Focus

Add this Dictionary
to your website!
See more options...

/fˈoʊkəs/ /fˈoʊkɪs/ Listen

noun

Focus n. (pl. E. focuses, L. foci)

1.

(Opt.) A point in which the rays of light meet, after being reflected or refracted, and at which the image is formed; as, the focus of a lens or mirror.

2.

(Geom.) A point so related to a conic section and certain straight line called the directrix that the ratio of the distance between any point of the curve and the focus to the distance of the same point from the directrix is constant. Note: Thus, in the ellipse FGHKLM, A is the focus and CD the directrix, when the ratios FA:FE, GA:GD, MA:MC, etc., are all equal. So in the hyperbola, A is the focus and CD the directrix when the ratio HA:HK is constant for all points of the curve; and in the parabola, A is the focus and CD the directrix when the ratio BA:BC is constant. In the ellipse this ratio is less than unity, in the parabola equal to unity, and in the hyperbola greater than unity. The ellipse and hyperbola have each two foci, and two corresponding directrixes, and the parabola has one focus and one directrix. In the ellipse the sum of the two lines from any point of the curve to the two foci is constant; that is: AG + GB = AH + HB; and in the hyperbola the difference of the corresponding lines is constant. The diameter which passes through the foci of the ellipse is the major axis. The diameter which being produced passes through the foci of the hyperbola is the transverse axis. The middle point of the major or the transverse axis is the center of the curve. Certain other curves, as the lemniscate and the Cartesian ovals, have points called foci, possessing properties similar to those of the foci of conic sections. In an ellipse, rays of light coming from one focus, and reflected from the curve, proceed in lines directed toward the other; in an hyperbola, in lines directed from the other; in a parabola, rays from the focus, after reflection at the curve, proceed in lines parallel to the axis. Thus rays from A in the ellipse are reflected to B; rays from A in the hyperbola are reflected toward L and M away from B.

3.

A central point; a point of concentration.

Aplanatic focus. (Opt.) See under Aplanatic.

Conjugate focus (Opt.), the focus for rays which have a sensible divergence, as from a near object; so called because the positions of the object and its image are interchangeable.

Focus tube (Phys.), a vacuum tube for Roentgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect.

Principal focus, or Solar focus (Opt.), the focus for parallel rays.

verb

Focus v. t. (past & past part. focused or focussed; pres. part. focusing or focussing) To bring to a focus; to focalize; as, to focus a camera.

See images of 'Focus'

Etymology of 'Focus'

Collaborative International Dictionary of English 0.48

Share |

Add this dictionary
to your browser search bar

"Focus" Quotes from Famous Books

... diffused over the whole surface of the body; but light is focalized in the eye; sound in the ear. The organization of a sense or a pleasure seems diluted and imperfect, unless it is gathered by some machinery into one focus, or local centre. And thus it is that a general state of pleasurable feeling sometimes seems too superficially diffused, and one has a craving to intensify or brighten it by concentration through some sufficient stimulant. ...

— Narrative And Miscellaneous Papers • Thomas De Quincey

... perspective vision receives constant reinforcement from the facts of daily experience. The influence of the above-described changes in experimental conditions is mediated through their effect upon the location of the focus of the limiting and perspective lines of vision. As the plane of the upper boundaries of the enclosing walls was elevated and depressed the intersection of the two systems of lines was correspondingly raised and lowered, and in dependence upon the location of this imaginary point the determination ...

— Harvard Psychological Studies, Volume 1 • Various

... a change, unable to name it, but impossible to escape it. He was different. His eyes were bright, and they looked at her with a focus ...

— The Girl Scouts at Bellaire - Or Maid Mary's Awakening • Lilian C. McNamara Garis

... surface of the sea and then traced their luminous way on the overhanging clouds. Another shift and the shining pathway reached to their very feet, illuminating with its radiance every object within its focus, down to the tiniest shell upon the beach. Esmay, startled, clung to ...

— The Doomsman • Van Tassel Sutphen

... of Miss Mary Blanchet's face. She had not good sight, for all their brightness. When any one talked with her at some little distance across a room, or even across a broad table, he could easily see by the irresponsive look of the eyes—the eyes which never quite found a common focus with his even during the most animated interchange of thought—that Miss Blanchet had short sight. But Miss Blanchet always frankly and firmly declined to put on spectacles. "I have only my eyes to boast of, my ...

— The Galaxy, Volume 23, No. 2, February, 1877 • Various

Copyright © 2025 Dictionary One.com