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/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

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"Focus" Quotes from Famous Books

... had been in courts and camps: of books he knew little or nothing. He had lived in the unquestioning faith of one born and bred in the very focus of Romanism; and thus, at the age of about thirty, his conversion found him. It was a change of life and purpose, not of belief. He presumed not to inquire into the doctrines of the Church. It was for him to enforce those doctrines; and ...

— The Jesuits in North America in the Seventeenth Century • Francis Parkman

... dermis, especially at the top, it may be more easily flattened. After the glass is properly mounted in front of the camera, the lights are placed behind it and light is directed through the skin. The ridge detail is brought into focus on the ground glass. Before the picture is actually taken it is suggested that the ground glass be checked by first using one light and then two lights to see which ...

— The Science of Fingerprints - Classification and Uses • Federal Bureau of Investigation

... ship with. Nor throughout the day did weather conditions or tempers improve. All day long the sky was heavily overcast with dense, low-hanging, dark gray clouds, which, while wholly obscuring the sun, seemed to focus its rays upon us like a vast burning-glass; wherefore it was expedient for the two pajama-clad passengers to keep well within the shelter of the bridge-deck awning. Toward sunset, a dense black wall of cloud settled upon the western ...

— The Red-Blooded Heroes of the Frontier • Edgar Beecher Bronson

... virtue. The young mother with the Madonna face—Lady Newhaven firmly believed that her face, with the crimped fringe drawn down to the eyebrows, resembled that of a Madonna—with her children round her, Lord Newhaven as usual somewhat out of focus in the background; and Hugh, young, handsome, devoted, heartbroken, and ennobled for life by the contemplation ...

— Red Pottage • Mary Cholmondeley

... about his own age, richly dressed, and of distinguished bearing. He appeared chief among his companions, who addressed him with a certain deference, and followed his movements, so that when he turned to look at the newcomer, Ellerey found himself the focus of four pairs of eyes. He met their searching looks with equal inquiry, but experienced a certain attraction toward the man who led the scrutiny. He might be an enemy, but he looked as though he would prove an honest and open one, incapable ...

— Princess Maritza • Percy Brebner

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