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Webster 1913 Edition


Focus

Fo′cus

(fō′kŭs)
,
Noun.
;
pl. E.
Focuses
(fō′kŭs-ĕz)
, L.
Foci
(fō′sī)
.
[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 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.
☞ 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 Rœntgen 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.

Fo′cus

,
Verb.
T.
[
imp. & p. p.
Focused
;
p. pr. & vb. n.
Focusing
.]
To bring to a focus; to focalize; as, to focus a camera.
R. Hunt.

Webster 1828 Edition


Focus

FO'CUS

,
Noun.
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.

Definition 2022


focus

focus

English

Noun

focus (countable and uncountable, plural foci or focuses)

  1. (countable, optics) A point at which reflected or refracted rays of light converge.
    The heat of sunlight at the focus of a magnifying glass can easily set dry leaves on fire.
  2. (countable, geometry) A point of a conic at which rays reflected from a curve or surface converge.
  3. (uncountable, photography, cinematography) The fact of the convergence of light on the photographic medium.
    • 2013 July-August, Catherine Clabby, Focus on Everything”, in American Scientist:
      Not long ago, it was difficult to produce photographs of tiny creatures with every part in focus. [] A photo processing technique called focus stacking has changed that. Developed as a tool to electronically combine the sharpest bits of multiple digital images, focus stacking is a boon to biologists seeking full focus on a micron scale.
    Unfortunately, the license plate is out of focus in this image.
  4. (uncountable, photography, cinematography) The quality of the convergence of light on the photographic medium.
    During this scene, the boy’s face shifts subtly from soft focus into sharp focus.
  5. (uncountable) Concentration of attention.
    I believe I can bring the high degree of focus required for this important job.
  6. (countable, seismology) The exact point of where an earthquake occurs, in three dimensions (underneath the epicentre).
    The earthquake's focus was at exactly 37 degrees north, 18 degrees south, seventy five meters below the ground.
  7. (computing, graphical user interface) The indicator of the currently active element in a user interface.
    Text entered at the keyboard or pasted from a clipboard is sent to the component which currently has the focus.
  8. (linguistics) The most important word or phrase in a sentence or passage, or the one that imparts information.

Translations

Verb

focus (third-person singular simple present focuses or focusses, present participle focusing or focussing, simple past and past participle focused or focussed)

  1. (transitive) To cause (rays of light, etc) to converge at a single point.
  2. (transitive) To adjust (a lens, an optical instrument) in order to position an image with respect to the focal plane.
    You'll need to focus the microscope carefully in order to capture the full detail of this surface.
  3. (transitive, followed by on or upon) To concentrate one's attention.
    Focus on passing the test.
  4. (intransitive) To concentrate one’s attention.
    If you're going to beat your competitors, you need to focus.
  5. (computing, graphical user interface, transitive) To transfer the input focus to (a visual element), so that it receives subsequent input.
    The text box won't receive the user's keystrokes unless you explicitly focus it.

Usage notes

The spellings focusses, focussing, focussed are more common in Commonwealth English than in American English, but in both varieties they are less common than the spellings focuses, focusing, focused.

Derived terms

Related terms

Translations


Catalan

Noun

focus m (plural focus)

  1. focus

Italian

Etymology

Borrowed from Latin focus, whence also Italian fuoco (an inherited doublet).

Pronunciation

  • IPA(key): /ˈfɔkus/, [ˈfɔː.kus̪]
  • Hyphenation: fò‧cus

Noun

focus m (invariable)

  1. focus (all senses)

Latin

Etymology

  • The origin is uncertain. Usually connected with Old Armenian բոց (bocʿ).
  • Some connect this along with faciēs, facētus, fax, from Proto-Indo-European *bʰeh₂- (to shine). Cognate with Ancient Greek φαίνω (phaínō, to shine).
  • In explaining how Kepler discovered the elliptical orbits, Nicholas Mee provides this explanation:

"One of the interesting properties of an ellipse is that if there were a light bulb at one focus, then all the light that it emits would reflect off the ellipse and converge at the other focus. This is why Kepler originally used the name focus for these points." (Gravity, 2014, p.74) "Fos" means light in Greek.

Pronunciation

Noun

focus m (genitive focī); second declension

  1. hearth, fireplace

Inflection

Second declension.

Case Singular Plural
nominative focus focī
genitive focī focōrum
dative focō focīs
accusative focum focōs
ablative focō focīs
vocative foce focī

Derived terms

Related terms

Descendants

  • Aragonese: fuego
  • Aromanian: foc
  • Asturian: fueu, fuau, fuebu, fuegu, fuibo, fuibu, fuíu, fuogu, fuou, ḥuebu, ḥuegu, ḥuego, ḥueu, ḥuibu, ḥuíu, ḥuöu, fou, fougu
  • Catalan: foc
  • Corsican: focu
  • Dalmatian: fuc
  • Emilian: fûg
  • English: focus (borrowing), foo fighter (via French)
  • Fala: fogu
  • Franco-Provençal: fuè
  • Friulian: fûc
  • Galician: fogo
  • German: Fokus
  • Greek: φουφού (foufoú)
  • Guinea-Bissau Creole: fugu
  • Istriot: fògo
  • Istro-Romanian: foc
  • Italian: fuoco, focus (borrowing)
  • Karipúna Creole French: djife
  • Korlai Creole Portuguese: fog
  • Kristang: fogu
  • Ladin: fech, fesc
  • Lombard: féch, feegh, fögh
  • Macanese: fogo
  • Megleno-Romanian: foc
  • Middle French: feu
  • Mirandese: fuogo
  • Mozarabic: [script needed] (fóco)
  • Neapolitan fuoco
  • Occitan: fuòc, huec
  • Old French: feu, fou, fu
  • Old Portuguese: fogo

References