An image that is partially in focus, but mostly out of focus in varying degrees.

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge ^[1]. In Optics, a ray is an idealized narrow Beam of light. Rays are used to model the propagation of Light through an optical system by dividing the real light 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. Aberrations are departures of the performance of an optical system from the predictions of Paraxial optics. In the absence of significant aberrations, the smallest possible blur circle is the Airy disc, which is caused by diffraction from the optical system's aperture. The Airy disk (or Airy disc) is a phenomenon in Optics. Owing to the wave nature of light, light passing through an Aperture is diffracted Diffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle Aberrations tend to get worse 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 criterion. In Optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source

A principal focus or focal point is a special focus:

• For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused. A lens is an optical device with perfect or approximate Axial symmetry which transmits and refracts Light, converging or diverging "Globose" redirects here See also Globose nucleus. A sphere (from Greek σφαίρα - sphaira, "globe In Mathematics, the parabola (pəˈræbələ from the Greek παραβολή) is a Conic section, the intersection of a right circular A mirror is an object with a surface that has good Specular reflection; that is it is smooth enough to form an Image. Collimated light is Light whose rays are nearly parallel and therefore will spread slowly as it propagates 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 cardinal points and the associated cardinal planes are a set of special points and planes in an optical system which help in the analysis The focal length of an optical system is a measure of how strongly it converges (focuses or diverges (diffuses Light.
• 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. In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a
• The focus of a hyperbolic mirror is either of two points which have the property that light from one is reflected as if it came from the other. In Geometry, a hyperbola ( Greek, "over-thrown" has several equivalent definitions

Focal blur is simulated in this computer generated image of glasses, which was rendered in POV-Ray. Computer animation Computer-generated imagery (also known as CGI) is the application of the field of Computer graphics or more specifically 3D computer graphics The Persistence of Vision Raytracer, or POV-Ray, is a Ray tracing program available for a variety of computer platforms

A diverging (negative) lens, or a convex mirror does 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 as a collimated beam. In Geometry, the foci (singular focus) are a pair of special points used in describing Conic sections The four types of conic sections are the Circle 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. In Mathematics, an ellipse (from the Greek ἔλλειψις literally absence) is a Conic section, the locus of points in a A convex hyperbolic 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. In Geometry, a hyperbola ( Greek, "over-thrown" has several equivalent definitions In Geometry, the foci (singular focus) are a pair of special points used in describing Conic sections The four types of conic sections are the Circle Conversely, it can focus rays directed at the focal point that is behind the mirror towards the focal point that is front of the mirror as in a Cassegrain telescope. The Cassegrain reflector is a combination of two coaxial reflectors used in Cassegrain telescopes and radio antennas First developed in 1672 by Laurent Cassegrain A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation.

See also

• Cardinal point (optics)
• Defocus aberration
• Depth of field
• Depth of focus
• Far point
• Focus (geometry)
• Fixed focus
• Bokeh

References

1. ^ Standard Microscopy Terminology. The cardinal points and the associated cardinal planes are a set of special points and planes in an optical system which help in the analysis In Optics, defocus is the one aberration familiar to nearly everyone who has ever needed eyeglasses or used a camera videocamera microscope telescope or binoculars In Optics, particularly as it relates to Film and Photography, the depth of field (DOF is the portion of a scene that appears sharp in the image Depth of focus is a 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 Visual perception, the far point is the point at which an object must be placed along the optical axis for its image to be focused on the Retina when the Eye In Geometry, the foci (singular focus) are a pair of special points used in describing Conic sections The four types of conic sections are the Circle A fixed-focus lens is a lens of which the focus is not adjustable This article is about the photographic usage of the term "Bokeh" University of Minnesota Characterization Facility website. Retrieved on 2006-04-21. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 753 BC - Romulus and Remus found Rome ( traditional date)