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Focus   /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.





Collaborative International Dictionary of English 0.48


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"Focus" Quotes from Famous Books



... sir," replied the officer, with a chuckle; and he did watch, with close attention and great interest, while Thorndyke laid the glass slips on the microscope stage and proceeded to focus. ...
— The Red Thumb Mark • R. Austin Freeman
... Harold realized that here was his opportunity: in one stroke, one easy shot, he could turn the day's ignominy into triumph. He could focus Virginia's admiration upon himself. But the impulse had even deeper significances. It was not the way of sportsmen, wandering in file on mountain trails, to clamor for the first shot at game. Whatever is said is usually in solicitation to a companion to shoot; and Virginia felt oddly ...
— The Snowshoe Trail • Edison Marshall
... shop on the corner is the political and social focus of the neighbourhood. I shall never forget the pallid and ghastly countenance of the newsdealer when the rumour first went the rounds that "Hampy" was elected. Every evening a little gathering of local sages meets in the shop; on tilted chairs, in a haze of tobacco, ...
— Pipefuls • Christopher Morley
... recalled her to the game, and she made an effort to focus her attention on the cards. But it was quite useless. Her play grew wilder and more erratic with each hand that was dealt, until at last a good no-trump call, completely thrown away by her disastrous tactics, brought the rubber ...
— The Moon out of Reach • Margaret Pedler
... of thought or thought of having a thought. He not only puts a microscope to his eyes to know with, but his eyes have ingrown microscopes. The microscope has become a part of his eyes. He cannot see anything without putting it on a slide, and when his microscope will not focus it, and it cannot be reduced and explained, he explains that ...
— The Lost Art of Reading • Gerald Stanley Lee


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