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 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.

&fist; 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.

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.

• (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.
  
• (countable) (mathematics) a point at which rays reflected from a curve or surface converge.
  
• (uncountable) (photography, cinematography) The fact of the convergence of light on the photographic medium.
       Unfortunately, the license plate is out of focus in this image.
  
• (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.
  
• (uncountable) concentration of attention.
        I believe I can bring the high degree of focus required for this important job.
  
• (transitive) To adjust (a lens) 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.
  
• (transitive) (followed by on or upon) To concentrate one's attention.
       Focus on passing the test.
  
• (intransitive) To concentrate one's attention.
       If you're going to beat your competitors, you need to focus.
  
  Latin
• hearth, fireplace