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In
geometrical optics Geometrical optics, or ray optics, is a model of optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural sci ...
, a focus, also called an image point, is a point where light rays originating from a point on the object
converge Converge may refer to: * Converge (band), American hardcore punk band * Converge (Baptist denomination), American national evangelical Baptist body * Limit (mathematics) * Converge ICT, internet service provider in the Philippines See also

...
. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by aberrations of the imaging optics. In the absence of significant aberrations, the smallest possible blur circle is the
Airy disc A computer-generated Airy disk from diffracted white light ( D65 spectrum). Note that the red component is diffracted more than the blue, so that the center appears slightly bluish. In optics Optics is the branch of physics Physics (f ...
, which is caused by
diffraction Diffraction refers to various phenomena that occur when a wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which r ...

diffraction
from the optical system's
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of ray (optics), rays that come to a focus (optics), focus ...

aperture
. Aberrations tend to worsen as the aperture diameter increases, while the Airy circle is smallest for large apertures. An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The border between these is sometimes defined using a "
circle of confusion In optics Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (phy ...
" criterion. A principal focus or focal point is a special focus: * For a
lens A lens is a transmissive optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, s ...

lens
, or a spherical or parabolic mirror, it is a point onto which
collimated light A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no beam divergence, divergence, would not disperse with distance. Howe ...
parallel to the axis is focused. Since light can pass through a lens in either direction, a lens has two focal points – one on each side. The distance in air from the lens or mirror's principal plane to the focus is called the ''
focal length The focal length of an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

focal length
''. *
Elliptical
Elliptical
mirrors have two focal points: light that passes through one of these before striking the mirror is reflected such that it passes through the other. * The focus of a
hyperbolic Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry. The following phenomena are described as ''hyperbolic'' because they ...

hyperbolic
mirror A mirror is an object that reflects an image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers t ...

mirror
is either of two points which have the property that light from one is reflected as if it came from the other. Diverging (negative) lenses and
convex mirrors
convex mirrors
do not focus a collimated beam to a point. Instead, the focus is the point from which the light appears to be emanating, after it travels through the lens or reflects from the mirror. A convex parabolic mirror will reflect a beam of collimated light to make it appear as if it were radiating from the focal point, or conversely, reflect rays directed toward the
focus Focus, FOCUS, or foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe The Adelaide Fringe, formerly Adelaide Fringe Festival, is the world's second-largest annual arts festival (after the Edinburgh Festival ...
as a collimated beam. A convex elliptical mirror will reflect light directed towards one focus as if it were radiating from the other focus, both of which are behind the mirror. A convex
hyperbolic Hyperbolic is an adjective describing something that resembles or pertains to a hyperbola (a curve), to hyperbole (an overstatement or exaggeration), or to hyperbolic geometry. The following phenomena are described as ''hyperbolic'' because they ...

hyperbolic
mirror A mirror is an object that reflects an image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers t ...

mirror
will reflect rays emanating from the focal point in front of the mirror as if they were emanating from the focal point behind the mirror. Conversely, it can focus rays directed at the focal point that is behind the mirror towards the focal point that is in front of the mirror as in a Cassegrain telescope.


See also

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Autofocus An autofocus (or AF) optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and ...
*
Cardinal point (optics) In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the focal points, the principal points, and the nodal points. For ''ideal'' sys ...
*
Defocus aberration Image:Bokeh Example.jpg, A photograph of Christmas lights with significant defocus aberration. In optics, defocus is the aberration in optical systems, aberration in which an image is simply out of focus (optics), focus. This aberration is famil ...
*
Depth of field For many cameras, depth of field (DOF) is the distance between the nearest and the farthest objects that are in acceptably sharp focus in an image. The depth of field can be calculated based on focal length The focal length of an optical ...

Depth of field
*
Depth of focus Depth of focus is a Lens (optics), lens optics concept that measures the tolerance of placement of the image plane (the film plane in a camera) in relation to the lens. In a camera, depth of focus indicates the tolerance of the film's displacement ...
*
Far point In visual perception Visual perception is the ability to interpret the surrounding environment using light in the visible spectrum reflected by the objects in the environment (biophysical), environment. This is different from visual acuity, wh ...
*
Focus (geometry) In geometry Geometry (from the grc, γεωμετρία; ' "earth", ' "measurement") is, with , one of the oldest branches of . It is concerned with properties of space that are related with distance, shape, size, and relative position of ...
*
Fixed focus A photographic lens A camera lens (also known as photographic lens or photographic objective) is an optical Optics is the branch of physics Physics is the natural science that studies matter, its Elementary particle, fundamental con ...
*
Bokeh In photography, bokeh ( or ; ) is the aesthetic quality of the blur produced in defocus aberration, out-of-focus parts of an image. Bokeh has also been defined as "the way the lens renders out-of-focus points of light". Differences in opt ...

Bokeh
*
Focus stacking 250px, Focus stacking (for extended depth of field) in Bright field microscopy, bright field light microscopy. This example is of a diatom microfossil in diatomaceous earth. Top left are the three source images captured at three different focus di ...

Focus stacking
*
Focal plane In Gaussian optics, the cardinal points consist of three pairs of points located on the optical axis of a rotationally symmetric, focal, optical system. These are the focal points, the principal points, and the nodal points. For ''ideal'' sys ...

Focal plane
*
Manual focus{{unreferenced, date=May 2007 In the field of photography, a manual focus camera is one in which the user has to adjust the Focal length, focus of the photographic lens, lens by hand. Before the advent of autofocus, all cameras had manually adjuste ...


References

{{Authority control Geometrical optics Science of photography