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Focus definitions

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Webster's 1828 Dictionary

FO'CUS, n. plu. focuses, or foci. [L. focus, a fire, the hearth.]
1. In optics, a point in which any number of rays of light meet, after being reflected or refracted; as the focus of a lens.
2. In geometry and conic sections, a certain point in the parabola, ellipsis and hyperbola, where rays reflected from all parts of these curves, concur or meet.
The focus of an ellipsis, is a point towards each end of the longer axis, from which two right lines drawn to any point in the circumference, shall together be equal to the longer axis.
The focus of a parabola, is a point in the axis within the figure, and distant from the vertex by the fourth part of the parameter.
The focus of a hyperbola, is a point in the principal axis, within the opposite hyperbolas, from which if any two lines are drawn, meeting in either of the opposite hyperbolas, the difference will be equal to the principal axis.
3. A central point; point of concentration.

WordNet (r) 3.0 (2005)

n
1: the concentration of attention or energy on something; "the focus of activity shifted to molecular biology"; "he had no direction in his life" [syn: focus, focusing, focussing, focal point, direction, centering]
2: maximum clarity or distinctness of an image rendered by an optical system; "in focus"; "out of focus"
3: maximum clarity or distinctness of an idea; "the controversy brought clearly into focus an important difference of opinion"
4: a central point or locus of an infection in an organism; "the focus of infection" [syn: focus, focal point, nidus]
5: special emphasis attached to something; "the stress was more on accuracy than on speed" [syn: stress, focus]
6: a point of convergence of light (or other radiation) or a point from which it diverges [syn: focus, focal point]
7: a fixed reference point on the concave side of a conic section v
1: direct one's attention on something; "Please focus on your studies and not on your hobbies" [syn: concentrate, focus, center, centre, pore, rivet]
2: cause to converge on or toward a central point; "Focus the light on this image" [ant: blur]
3: bring into focus or alignment; to converge or cause to converge; of ideas or emotions [syn: concenter, concentre, focalize, focalise, focus]
4: become focussed or come into focus; "The light focused" [syn: focus, focalize, focalise] [ant: blur, dim, slur]
5: put (an image) into focus; "Please focus the image; we cannot enjoy the movie" [syn: focus, focalize, focalise, sharpen] [ant: blear, blur]

Merriam Webster's

I. noun (plural foci; also focuses) Etymology: New Latin, from Latin, hearth Date: 1644 1. a. a point at which rays (as of light, heat, or sound) converge or from which they diverge or appear to diverge; specifically the point where the geometrical lines or their prolongations conforming to the rays diverging from or converging toward another point intersect and give rise to an image after reflection by a mirror or refraction by a lens or optical system b. a point of convergence of a beam of particles (as electrons) 2. a. focal length b. adjustment for distinct vision; also the area that may be seen distinctly or resolved into a clear image c. a state or condition permitting clear perception or understanding <tried to bring the issues into focus> d. direction 6c <the team lost focus> 3. one of the fixed points that with the corresponding directrix defines a conic section 4. a localized area of disease or the chief site of a generalized disease or infection 5. a. a center of activity, attraction, or attention <the focus of the meeting was drug abuse> b. a point of concentration 6. the place of origin of an earthquake or moonquake 7. directed attention ; emphasis • focusless adjective II. verb (focused; also focussed; focusing; also focussing) Date: 1775 transitive verb 1. a. to bring into focus b. to adjust the focus of (as the eye or a lens) 2. to cause to be concentrated <focused their attention on the most urgent problems> 3. to bring (as light rays) to a focus ; concentrate intransitive verb 1. to come to a focus ; converge 2. to adjust one's eye or a camera to a particular range 3. to concentrate attention or effort • focusable adjective • focuser noun

Oxford Reference Dictionary

n. & v. --n. (pl. focuses or foci) 1 Physics a the point at which rays or waves meet after reflection or refraction. b the point from which diverging rays or waves appear to proceed. Also called focal point. 2 a Optics the point at which an object must be situated for an image of it given by a lens or mirror to be well defined (bring into focus). b the adjustment of the eye or a lens necessary to produce a clear image (the binoculars were not in focus). c a state of clear definition (the photograph was out of focus). 3 the centre of interest or activity (focus of attention). 4 Geom. one of the points from which the distances to any point of a given curve are connected by a linear relation. 5 Med. the principal site of an infection or other disease. 6 Geol. the place of origin of an earthquake. --v. (focused, focusing or focussed, focussing) 1 tr. bring into focus. 2 tr. adjust the focus of (a lens, the eye, etc.). 3 tr. & intr. (often foll. by on) concentrate or be concentrated on. 4 intr. & tr. converge or make converge to a focus. Derivatives: focuser n. Etymology: L, = hearth

Webster's 1913 Dictionary

Focus Fo"cus, n.; pl. E. Focuses, L. Foci. [L. focus hearth, fireplace; perh. akin to E. bake. Cf. Curfew, Fuel, Fusil the firearm.] 1. (Opt.) A point in which the rays of light meet, after being reflected or refrcted, 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 distace 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 R[oe]ntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect. Principal, or Solar, focus (Opt.), the focus for parallel rays.

Webster's 1913 Dictionary

Focus Fo"cus, n.; pl. E. Focuses, L. Foci. [L. focus hearth, fireplace; perh. akin to E. bake. Cf. Curfew, Fuel, Fusil the firearm.] 1. (Opt.) A point in which the rays of light meet, after being reflected or refrcted, 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 distace 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 R[oe]ntgen rays in which the cathode rays are focused upon the anticathode, for intensifying the effect. Principal, or Solar, focus (Opt.), the focus for parallel rays.

Webster's 1913 Dictionary

Focus Fo"cus, v. t. [imp. & p. p. Focused; p. pr. & vb. n. Focusing.] To bring to a focus; to focalize; as, to focus a camera. --R. Hunt.

Collin's Cobuild Dictionary

(foci, focuses, focusing, focused) Frequency: The word is one of the 1500 most common words in English. Note: The spellings 'focusses', 'focussing', 'focussed' are also used. The plural of the noun can be either 'foci' or 'focuses'. 1. If you focus on a particular topic or if your attention is focused on it, you concentrate on it and think about it, discuss it, or deal with it, rather than dealing with other topics. He is currently focusing on assessment and development... Many of the papers focus their attention on the controversy surrounding the Foreign Secretary. = concentrate VERB: V on n, V n on n 2. The focus of something is the main topic or main thing that it is concerned with. The new system is the focus of controversy... Her children are the main focus of her life. N-COUNT: usu sing, usu with supp 3. Your focus on something is the special attention that you pay it. IBM has also shifted its focus from mainframes to personal computers. N-COUNT: usu sing, usu with supp, oft N on n 4. If you say that something has a focus, you mean that you can see a purpose in it. Somehow, though, their latest album has a focus that the others have lacked... N-UNCOUNT 5. If you focus your eyes or if your eyes focus, your eyes adjust so that you can clearly see the thing that you want to look at. If you focus a camera, telescope, or other instrument, you adjust it so that you can see clearly through it. Kelly couldn't focus his eyes well enough to tell if the figure was male or female... His eyes slowly began to focus on what looked like a small dark ball... He found the binoculars and focused them on the boat... Had she kept the camera focused on the river bank she might have captured a vital scene. VERB: V n, V on n, V n on n, V-ed, also V 6. You use focus to refer to the fact of adjusting your eyes or a camera, telescope, or other instrument, and to the degree to which you can see clearly. His focus switched to the little white ball... N-UNCOUNT 7. If you focus rays of light on a particular point, you pass them through a lens or reflect them from a mirror so that they meet at that point. Magnetic coils focus the electron beams into fine spots. VERB: V n prep 8. The focus of a number of rays or lines is the point at which they meet. (TECHNICAL) N-COUNT 9. If an image or a camera, telescope, or other instrument is in focus, the edges of what you see are clear and sharp. Pictures should be in focus, with realistic colours and well composed groups. PHRASE: v-link PHR, PHR after v 10. If something is in focus, it is being discussed or its purpose and nature are clear. This aggression is the real issue the world should be concerned about. We want to keep that in focus... PHRASE: v-link PHR, PHR after v 11. If an image or a camera, telescope, or other instrument is out of focus, the edges of what you see are unclear. In some of the pictures the subjects are out of focus while the background is sharp. PHRASE: v-link PHR, PHR after v

Soule's Dictionary of English Synonyms

n. Point of concentration, point of convergence, converging-point.

Moby Thesaurus

adductor, approach, asymptote, attractant, attrahent, axiom, blurred, blurry, bottleneck, bring into focus, center, center of attraction, center of consciousness, center of interest, clear, collision course, concenter, concentralization, concentralize, concentrate, concentration, concourse, concurrence, confluence, conflux, congress, converge, convergence, converging, core, crossing, cynosure, distinct, elixir, essence, essential, fix, fixate, flower, focal point, focalization, focalize, focus of attention, fundamental, funnel, fuzzy, gist, gravamen, heart, hub, hypostasis, in focus, indistinct, inner essence, kernel, lodestar, lure, magnet, marrow, meat, meet, meeting, mutual approach, narrowing gap, nave, nerve, nub, nucleus, nuts and bolts, out of focus, pinpoint, pith, point of convergence, polestar, postulate, prime focus, principle, put, quid, quiddity, quintessence, radius, rivet, sap, soul, spirit, spokes, spotlight, stuff, substance, tangent, target, the nitty-gritty, unclear, woolly, zero in