| 释义 |
▪ I. focus, n.|ˈfəʊkəs|
Pl. foci; also focuses, in the U.K. often written irregularly focusses.
[a. L. focus hearth, fireplace, in various modern uses.
The Lat. word was first used in sense 1 by Kepler (Astron. pars optica iv. 4, written in 1604); his reason for the choice of the name is not stated, but it is conjectured that the optical sense 2, ‘burning point of a lens or mirror’ (which is easily derived from the lit. sense) must have been already in existence; this would account for Kepler's use, as the ‘burning point’ or ‘focus’ of a parabolic mirror is situate at the geometrical ‘focus’ of its curvature. Sense 4 is from medical Latin. In all senses cf. Fr. foyer:—L. *focārium f. focus.]
1. Geom.
a. In plane geometry: One of the points from which the distances to any point of a given curve are connected by a linear relation.
Also defined as a point from which a pair of isotropic tangents can be drawn to a curve; or as the intersection of tangents from the points in which the line at infinity meets a co-planar circle. (For definitions specially relating to the focus of a conic, see quots. 1881 and 1893.)
1656Hobbes Six Lessons Wks. 1845 VII. 317 The focus of an hyperbole, is in the axis. 1703Moxon Mech. Exerc. 272 Which two points are called the Focusses, or burning points. 1734tr. Maupertuis' Diss. 19 in Keill's Exam. (ed. 2), The Orbits of the Planets are..Ellipses, in whose Focus the Sun is. 1807Hutton Course Math. II. 96 The ellipse and hyperbola have each two foci; but the parabola only one. 1851Nichol Archit. Heav. 199 The second star being in the focus and not the centre of the ellipse. 1881C. Taylor Geom. Conics 1 A conic is a curve traced by a point which moves in a plane containing a fixed point and a fixed straight line in such a way that its distance from the fixed point is in a constant ratio to its perpendicular distance from the fixed straight line. The fixed point is called a focus. 1893J. W. Russell Pure Geom. vii. 67 A focus of a conic is a point at which every two conjugate lines are perpendicular.
b. In solid geometry (see quot.).
1874G. Salmon Analytic Geom. (ed. 3) 109 A point through which can be drawn two lines, each touching the surface and the imaginary circle at infinity and such that the tangent plane to the surface through either also touches the circle at infinity.
2. a. Optics, Heat, etc. The point at which rays meet after being reflected or refracted; also, the point from which the rays appear to proceed (= virtual focus: see 2 b).
1685Boyle Effects Motion ii. 13 Sunbeams refracted or reflected by a burning-glass to a focus. 1704Newton Opticks i. vi. 7 The point from which rays diverge, or to which they converge, may be called their focus. 1831Brewster Optics i. 11 The points that were formerly the radiant points being now the foci. 1853Herschel Pop. Lect. Sc. ii. §24 (1873) 65 A far greater heat than can be produced in the focus of any burning-glass. 1865Livingstone Zambesi ii. 59 This deep trough-like shape caused the sun's rays to converge as into a focus.
b. With various defining words.
conjugate foci: see conjugate a. 6 b; principal focus, the point at which parallel rays meet after passing through a convergent lens; solar focus = prec.; virtual focus, a point at which diverging rays would meet if their directions were reversed; actinic focus or chemical focus (of a lens), the point to which the actinic rays converge.
1706Phillips (ed. Kersey), Virtual Focus, or Point of Divergence (in Dioptricks). 1812–16J. Smith Panorama Sc. & Art I. 426 When parallel rays fall upon a double-convex glass, KG, they will..meet..in a point or principal focus at F. 1874Lommel's Light 90 The lenses of the second group have virtual foci. 1890Woodbury Encycl. Photogr. 293 Unless the lens be rendered achromatic, the actinic or chemical focus does not coincide with the visual focus.
c. transf. and fig.
1762Goldsm. Cit. W. lxxi, A box where they might see and be seen; one, as they expressed it, in the very focus of public view. 1781Cowper Conversation 239 Centering in a focus round and neat, Let all your rays of information meet. 1824Carlyle in Froude Life (1882) I. 260, I am meditating..on the great focus of all purposes—the arranging of my future life. 1874J. T. Micklethwaite Mod. Par. Churches 106 If it [a picture] be placed over the altar, it is in the very focus of the building.
†d. Theatr. The best-illuminated part of the stage. Obs.
1881Era Almanack 97 He tried to keep me out of the focus! 1885Eng. Illustr. Mag. 647/1 Every body tried to get into what was called the focus—the ‘blaze of publicity’ furnished by the ‘float’ or footlights.
e. That point or position at which an object must be situated, in order that the image produced by the lens may be clear and well-defined. Hence in focus, or out of focus, lit. and fig. depth of focus (of a lens): the power of giving a ‘sharp’ image of objects not in the same plane; now usu. expressed as a distance, and (a) used as a synonym of depth of field (see field n. 16 c); (b) distinguished from this and used with the sense: the distance between the two extreme axial points behind a lens at which an image is judged to be in focus.
1713Derham Phys.-Theol. iv. ii. 89 note, If the paper..be..in the focus of the glass, [the image will be] distinct. 1727Swift Petit. Colliers &c. Wks. 1755 III. i. 129 To know the due distances of the said focus's. 1799Med. Jrnl. II. 228 At the nearest focus of distinct vision. 1840Dickens Barn. Rudge i, John gradually concentrated the whole power of his eyes into one focus. 1858A B C of Photogr. (ed. 10) 48 The focus of a portrait lens is very limited in depth. 1860C. Waldack Treat. Photogr. ii. 22 The most essential conditions required in a portrait tube, are, rapidity of action, sharpness, and depth of focus... It is for want of depth of focus, that in so many portraits the hands..are out of shape. 1890Woodbury Encycl. Photogr. 295 After a certain distance all objects will be in focus. 1894H. Drummond Ascent of Man vi, Evolution was given to the modern world out of focus. 1911,1939[see depth of field s.v. field n. 16 c]. 1940Mortimer & Sowerby Wall's Dict. Photogr. (ed. 15) 183 Depth of focus. Synonyms: Depth of Field, Depth of Definition. 1948A. L. M. Sowerby Dict. Photogr. (ed. 17) 179 Depth of focus, strictly, means range of movement of lens that is permissible before image of object focused upon becomes noticeably unsharp. More usually applied to range of distances over which an object can move without becoming noticeably unsharp... In this sense..synonyms are ‘Depth of Field’ and ‘Depth of Definition’. 1958M. L. Hall Newnes Compl. Amat. Photogr. 48 Another type of depth of focus scale is engraved upon the lens barrel. 1961G. Millerson Technique Telev. Prod. iii. 35 The near and far distances..give us the depth of field (often erroneously called depth of focus—a different matter entirely). 1966McGraw-Hill Encycl. Sci. & Technol. II. 431/1 There may be some latitude in focusing in which there is no apparent change in sharpness. This distance is known as the depth of focus.
f. to bring, etc., in, into, to a focus; lit. and fig.
1788Franklin Autobiog. Wks. 1840 I. 122 The bringing all these scattered counsels into a focus. 1860Tyndall Glac. ii. xxiv. 354 The place where the rays were brought to a focus behind the lens. 1875Jevons Money (1878) 252 The transactions of many different individuals..are brought to a focus.
g. The focal length (of a lens); also, the adjustment (of the eye, or an eyeglass) necessary to produce a clear image.
1693E. Halley in Phil. Trans. No. 205. 960 This dioptric problem, is that of finding the focus of any sort of lens. 1757Ellis ibid. L. 287 A lens of about one inch and half focus. 1837Brewster Magnet. 332 The focus of the lens being suited to the distance of the needle. 1861Palgrave Gold. Treas. Pref., Rapid alteration of the eye's focus in looking at the landscape. 1871Tyndall Fragm. Sc. (1879) I. ii. 50 The focus was attained, first by the pupil and afterwards by the retina.
3. Similarly in Acoustics. The point or space towards which the sound waves converge.
1644Evelyn Diary 8 Feb., Standing at one of the focuses, which is under a tree..the voice seems to descend from the clouds. 1843J. Martineau Chr. Life xliii. (1876) 499 Could we only find the focus of those stray tones. 1857Thoreau Maine W. (1894) 362 We were exactly in the focus of many echoes.
4. Of a disease: The principal seat (in the body); also, a point where its activity is manifest.
1684tr. Bonet's Merc. Compit. vi. 183 That the focus of burning fevers is in the Head Hippocrates seems to assert. 1796H. Hunter tr. St. Pierre's Stud. Nat. (1799) III. 231 She informed me that the focus of my disorder was in the nerves. 1854Jones & Siev. Pathol. Anat. (1874) 304 Sclerosis of particular regions or isolated foci occurs in the Cord as in the brain.
5. a. The centre of activity, or area of greatest energy, of a storm, volcanic eruption, etc.
1796H. Hunter tr. St. Pierre's Stud. Nat. (1799) I. p. lx, The focus of the tides is removing farther and farther from our coasts. 1804C. B. Brown tr. Volney's View 98 One of the great layers of the country, where earthquakes have their principal focus. 1832Lyell Princ. Geol. II. 127 The original isle was the primitive focus, or centre, of a certain type of vegetation. 1862Scrope Volcanos 266 Any..focus or reservoir of lava from which an eruption has proceeded. 1869Phillips Vesuv. ii. 13 Vesuvius was seen to be the focus of the eruption. 1875Bedford Sailor's Pocket-bk. iv. (ed. 2) 78 The centre or focus of the West Indian hurricanes.
b. fig. A centre or ‘hotbed’ (of intrigue, sedition, etc.); a centre of activity or energy.
1808Wellington in Gurw. Desp. II. 440 Poonah is the focus of his intrigues. 1830R. Knox Béclard's Anat. Life 8 Central Schools..had been established in the departments as so many foci of knowledge. 1837W. Irving Capt. Bonneville (1849) p. xv, New-York, the great focus of American enterprise. 1870Huxley Lay Serm. i. (ed. 5) 5 The principal focus of scientific activity.
6. nonce-uses.
a. In Lat. sense: A fireplace or furnace; in quot. fig.
b. A centre of radiant heat.
1779J. Moore View Soc. Fr. I. xx. 175, I consider these men as the enemies of their country, and that place as a focus for consuming freedom. 1794J. Hutton Philos. Light, etc. 174 Surrounding particles receiving that addition of heat from the focus, are made to burn; and in burning, these coals return that heating species of light to the focus, for the increase of its burning.
7. attrib., as focus point, focus error.
1891Pall Mall G. 31 Aug. 2/1 That all railway servants shall have..not more than an agreed amount of focus error.
▸ focus group n. Marketing and Polit. a representative group of people (in quot. 1938: politicians) brought together so that their attitudes and opinions can be assessed through guided discussion, as part of a process of forming future policy or strategy; freq. attrib.
1938H. Nicolson Let. 2 Mar. (1966) I. 327, I went to such an odd luncheon yesterday. It is called ‘The *Focus Group’, and is one of Winston [Churchill]'s things. 1965N.Y. Times 8 Aug. 24 (advt.) Leading manufacturer of consumer products seeks a professional marketing researcher... Should..be familiar with store audits, focus group interviews and questionnaire design. 1984Fortune 5 Mar. 39/3 H-P's personal computer division even ran focus groups—in which potential customers sit around and talk about their needs and interests—to see what computer buyers wanted. 2000N. Fairclough New Labour, New Lang. iv. 98 There is a certain irony about..Blair's reliance on focus group research in determining how to move politically.
▸ focus-group v. Marketing and Polit. (orig. U.S.) trans. to use a focus group to evaluate or discuss (a product, etc.); to consult (people of a particular type) as a focus group; also intr.
1985Adweek 22 July 18/2 They probed and interviewed and analyzed and *focus-grouped and one-on-oned..in the most exhaustive and expensive consumer research project even undertaken in the industry. 1989S. Estrich in D. R. Runkel Campaign for President iv. 157 We were focus grouping their ads probably as much as they were. 1998Newsweek 18 May 69/2 The era before movies were focus-grouped and special-effected to death. 2002N.Y. Times Mag. 10 Mar. 43/2 Coke focus-grouped teenagers in Tokyo and asked why they drink Coke Classic.
▸ focus-grouped adj. Marketing and Polit. (orig. U.S.) (of a group of people) consulted in a focus group; (of a product or issue) evaluated or discussed in a focus group.
1992Washington Post 3 Nov. a1/4 Today, the most polled and *focus-grouped voters in history reveal what they really think. 2003Daily Tel. 4 July 25/5 They have assembled a scrupulously contrived and focus-grouped product that is as charmless as a verruca.
▸ focus-grouping n. (a) the use of small working groups within a school (rare); (b) Marketing and Polit. (orig. U.S.) the use of focus groups.
1968C. James Young Lives at Stake ix. 239 One of the advantages of *focus-grouping is that he can seek advice and help quite naturally from people working so closely with him. 1991Daily Herald (Chicago) 21 Jan. iii. 1/5 We are really interested in the focus grouping idea. 2002N.Y. Times Mag. 10 Mar. 41/2 Beyond focus-grouping, what makes for a hit beverage?
▪ II. focus, v.|ˈfəʊkəs|
Pples. focused, -ing; in the U.K. commonly, but irregularly, written focussed, -ing.
[f. prec. n.]
1. trans. To draw to a focus; to cause to converge to or as to a focus.
1875Bedford Sailor's Pocket-bk. v. (ed. 2) 132 The catoptric system..requiring less delicacy in putting up and focussing the light. 1881A. G. Bell Sound by Radiant Energy 2 A beam of sunlight was focussed into one end of an open tube. 1895Jrnl. R. Inst. Brit. Archit. 28 Mar. 353 Focussing and concentrating the sound in one or more points in his building.
b. fig.
1807Uti Possidetis xxx, All the Talent of the Nation Focuss'd in Cab'net concentration. 1862W. M. Rossetti in Fraser's Mag. Aug. 195 Focussing our observation to a single point. 1863J. Brown Horæ Subs. (ed. 3) 80 Inferior to my father..in power of—so to speak—focussing himself. 1888Burgon Lives 12 Gd. Men I. iii. 331 He could..instantly focus his thoughts.
c. intr. for refl. To converge to or as to a focus.
1863Thornbury True as Steel I. 148 Light flashing and focussing on armour. 1888Harper's Mag. Apr. 764 The eyes..flamed as if the life of the man had centralized and focussed within them.
2. To adjust the focus of (the eye, a lens, etc.).
1814W. Taylor in Monthly Mag. XXXVIII. 214 Accordingly as the eye is supposed to be focussed for seeing the foreground of the distance. c1865J. Wylde in Circ. Sc. I. 66/2 Arrangements..for focussing the lenses. 1881Tylor Anthropol. ii. 47 The eagle's eyes are focussed to see small objects far beyond man's range. 1885Illustr. Lond. News 9 May 491/3 Austell focussed his eye-glass on his wife.
absol.1853Hand-bk. Photogr. 13 It is well in focusing to make the first adjustment by placing the lens midway. 1890Woodbury Encycl. Photogr. 294 Focus upon some distinct object about 150 or 200 yards away.
3. To bring into focus; to bring (the image, etc.) to the proper focus.
1775S. J. Pratt Lib. Opin. (1783) III. 8 Sir Charles Crazy focuss'd her through an opera-glass. 1858A B C of Photogr. (ed. 10) 57 Having focussed the right-hand view, half of the sensitive plate is first impressed. c1865J. Wylde in Circ. Sc. I. 143/1 The image..is focussed..by..adjusting the lens. 1879Cassell's Techn. Educ. III. 97 Unless the picture be accurately focussed it will appear blurred. |