# focus

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## focus,

in optics, the point at which rays converge after reflection by a concave mirrormirror,
in optics, a reflecting surface that forms an image of an object when light rays coming from that object fall upon it (see reflection). Usually mirrors are made of plate glass, one side of which is coated with metal or some special preparation to serve as a reflecting
or refraction by a convex lenslens,
device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved.
, also known as a real focus. The point from which rays appear to diverge after reflection by a convex mirror or refraction by a concave lens is known as a virtual focus. See imageimage,
in optics, likeness or counterpart of an object produced when rays of light coming from that object are reflected from a mirror or are refracted by a lens. An image of an object is also formed when this light passes through a very small opening like that of a pinhole
.

## focus

(focal point) See focal length.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006

## Focus

A center of interest or activity drawing attention to the most important aspect of a design scheme, such as the main space, materials, scale, lighting, or orientation.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

## Focus

the area in which a subterranean shock occurs deep in the earth’s crust or in, the upper mantle, resulting in an earthquake.

## Focus

in mathematics:

(1) The focus of a curve of degree 2—an ellipse, a hyperbola, or a parabola—is a point F lying in the plane of the curve and possessing the property that the ratio of the distance between any point on the curve and F to the distance to the directrix is a constant equal to the eccentricity.

(2) One of the types of critical points of ordinary differential equations. All integral curves that pass through points in the immediate vicinity of such a critical point are spirals with infinitely many turns that approach the critical point without restriction as they wind around it.

## Focus

in optics, the point at which the rays of a parallel beam (or their apparent extension, if the system converts a parallel beam into a divergent beam) intersect after passing through an optical system. If the rays pass parallel to the optical axis of the system, the focus is found on this axis and is called the principal focus. In an ideal optical system, all foci are located on a plane perpendicular to the axis of the system called the focal plane. In a real system, foci are located on some surface called the focal surface.

## focus

[′fō·kəs]
(electronics)
To control convergence or divergence of the electron paths within one or more beams, usually by adjusting a voltage or current in a circuit that controls the electric or magnetic fields through which the beams pass, in order to obtain a desired image or a desired current density within the beam.
(geophysics)
The center of an earthquake and the origin of its elastic waves within the earth.
(mathematics)
A point in the plane which together with a line (directrix) defines a conic section.
(nucleonics)
To guide particles along a desired path in a particle accelerator by means of electric or magnetic fields.
(optics)
The point or small region at which rays converge or from which they appear to diverge.
To move an optical lens toward or away from a screen or film to obtain the sharpest possible image of a desired object.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

## focus

1. a point of convergence of light or other electromagnetic radiation, particles, sound waves, etc., or a point from which they appear to diverge
2. another name for focal point focal length
3. Optics the state of an optical image when it is distinct and clearly defined or the state of an instrument producing this image
4. Geometry a fixed reference point on the concave side of a conic section, used when defining its eccentricity
5. the point beneath the earth's surface at which an earthquake or underground nuclear explosion originates
6. Pathol the main site of an infection or a localized region of diseased tissue
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

## FOCUS

(database, language)
A hierarchical database language from Information Builders, Inc.

## FOCUS

(1) (focus) In software, the current window, menu or dialog box that is affected by a key stroke or mouse movement. For example, after you click from one window to another, the second one is said "to have the focus."

(2) (Federation On Computing in the United States, www.acm.org/focus) The U.S. representative of the International Federation for Information Processing (IFIP), www.ifip.or.at. FOCUS was founded in 1991 by the ACM and the IEEE Computer Society (IEEE-CS).

(3) A DBMS from Information Builders that runs on more than 35 different platforms. FOCUS has been widely known for its 4GL and report writing capabilities and is the product that built the company. It included a hierarchical database in its first release in 1975 and has evolved to support more than 80 database and file types including Information Builders' own multidimensional database (FOCUS Fusion). See EDA, WebFOCUS and FOCUS Fusion.
Copyright © 1981-2019 by The Computer Language Company Inc. All Rights reserved. THIS DEFINITION IS FOR PERSONAL USE ONLY. All other reproduction is strictly prohibited without permission from the publisher.
References in periodicals archive ?
As good as the focuser is, its primary drawback is that it provides no accommodation for dropping in a filter between the telescope and camera.
The scope's focuser will take 2-inch eyepieces, but a 1.25-inch 2x Barlow lens and two eyepieces--9 mm (133x, 266x) and 32 mm (38x, 76x)--come standard, as does an 8 x 50 finder.
An 8 x 50 finder, 2-inch focuser, 2-inch star diagonal, and 1.25-inch adapter ensures the widest choice of eyepieces; a 25-mm (33x) is supplied.
The other thing I had concerns with was the focuser's visual back.
Caption: The large size of the KAF-16200 CCD detector may push the capabilities of fast focal-ratio telescopes with 2-inch focusers.
I could turn the knob and nothing would move at first, then it would grab, even with the focuser locks off.
Caption: The shield mounted on the focuser. Note that you can access the eyepiece set screw through the gap in the shield.
The focuser's relatively short travel was also an issue that cropped up when I was shooting with a DSLR camera.
The Feather Touch focuser was superlative with absolutely no image shift or slop when focusing.
To check the optical collimation, I slipped a high-power eyepiece into the focuser and examined the image of a bright star.
However, the camera and coma corrector are held in the focuser with two small setscrews that press directly on the barrel of the coma corrector.
Caption: Doug's homemade focusers are equally simple and elegant.

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