Etymology : New Latin, from Latin, hearth
Pronunciation : fO-k&s
Function : noun
Date : 1644
1. center, hub; area in a dialog box which receives input (Computers). adjust the focus of; make clear, bring into focus; bring to a focus; concentrate. 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. hunt.focus \fo"cus\ , n.; pl. e. focuses (#), l. foci (#). [l. focus hearth, fireplace; perh. akin to e. bake. cf. curfew, fuel, fusil the firearm.].
2. (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.
3. (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.
4. a central point; a point of concentration.
5. 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.
6. 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.
7. A central point; a point of concentration.
8. To bring to a focus; to focalize; as, to focus a camera. maximum clarity or distinctness of an image rendered by an optical system; "in focus"; "out of focus" maximum clarity or distinctness of an idea; "the controversy brought clearly into focus an important difference of opinion" the concentration of attention or energy on something; "the focus of activity shifted to molecular biology"; "he had no direction in his life" a fixed reference point on the concave side of a conic section a point of convergence of light or a point from which it diverges a central point or locus of an infection in an organism; "the focus of infection" put into focus; "Please focus the image; we cannot enjoy the movie" become focussed or come into focus; "The light focused" cause to converge on or toward a central point; "Focus the light on this image".
9. The spellings focusses, focussing, focussed are also used. The plural of the noun can be either foci or focuses.
10. 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.
11. 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.
12. Your focus on something is the special attention that you pay it. IBM has also shifted its focus from mainframes to personal computers.
13. 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.
14. 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.
15. 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.
16. 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.
17. The focus of a number of rays or lines is the point at which they meet.
18. 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.
19. 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.
20. 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.