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]. 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, which is caused by diffraction from the optical system's aperture. Aberrations tend to get worse as the aperture diameter increases, while the Airy circle is smallest for large apertures. ImageMetadata File history File links Download high resolution version (997x717, 893 KB) A section from Ansel Adams The Camera, demonstrating shallow depth of field. ... ImageMetadata File history File links Download high resolution version (997x717, 893 KB) A section from Ansel Adams The Camera, demonstrating shallow depth of field. ... See also list of optical topics. ... Prism splitting light Light is electromagnetic radiation with a wavelength that is visible to the eye, or in a more general sense, any electromagnetic radiation in the range from infrared to ultraviolet. ... This article or section is in need of attention from an expert on the subject. ... A computer generated image of an Airy Disc. ... The intensity pattern formed on a screen by diffraction from a square aperture Diffraction refers to various phenomena associated with wave propagation, such as the bending, spreading and interference of waves passing by an object or aperture that disrupts the wave. ... a big (1) and a small (2) aperture For other uses, see Aperture (disambiguation). ...

An image, or image point or region, is said to be in focus if light from object points is converged about as well as possible in the image; conversely, it is out of focus if light is not well converged. The border between these conditions is sometimes defined via a circle of confusion criterion. The depth of field is the region where the size of the circle of confusion is less than the resolution of the human eye. ...

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. 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.
• 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 mirror is either of two points which have the property that light from one is reflected as if it came from the other.

Focal blur is simulated in this computer generated image of glasses, which was rendered in POV-Ray.

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. 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 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 front of the mirror as in a Cassegrain telescope. A lens. ... A sphere is a perfectly symmetrical geometrical object. ... A parabola A graph showing the reflective property, the directrix (light blue), and the lines connecting the focus and directrix to the parabola (blue) In mathematics, the parabola (from the Greek: παραβολή) (IPA pronunciation: ) is a conic section generated by the intersection of a right circular conical surface and a plane... A mirror, reflecting a vase. ... Collimated light is light whose rays are parallel. ... 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 of its paraxial properties. ... The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror. ... For other uses, see Ellipse (disambiguation). ... In mathematics, a hyperbola (Greek literally overshooting or excess) is a type of conic section defined as the intersection between a right circular conical surface and a plane which cuts through both halves of the cone. ... Image File history File links Download high-resolution version (2048x1536, 2972 KB) copied from English Wikipedia, there uploaded by en:User:Gilles Tran, en:User:Janke and en:User:Veledan. ... Image File history File links Download high-resolution version (2048x1536, 2972 KB) copied from English Wikipedia, there uploaded by en:User:Gilles Tran, en:User:Janke and en:User:Veledan. ... The seawater creature in The Abyss marked CGIs acceptance in the visual effects industry. ... This article or section does not adequately cite its references or sources. ... In geometry, the focus (pl. ... For other uses, see Ellipse (disambiguation). ... In mathematics, a hyperbola (Greek literally overshooting or excess) is a type of conic section defined as the intersection between a right circular conical surface and a plane which cuts through both halves of the cone. ... In geometry, the focus (pl. ... The Forststernwarte Jena 50cm Cassegrain telescope. ... A telescope (from the Greek tele = far and skopein = to look or see; teleskopos = far-seeing) is an instrument designed for the observation of remote objects. ...

See also

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

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 of its paraxial properties. ... 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, as it simply means out of focus. ... In optics, particularly film and photography, the depth of field (DOF) is the distance in front of and beyond the subject that appears to be in focus. ... Depth of focus is a lens optics concept regarding the tolerance of placement of the plane of film in relation to the rear element of the lens. ... In visual optics, 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 is not accommodating. ... In geometry, the focus (pl. ...

References

1. ^ Standard Microscopy Terminology. University of Minnesota Characterization Facility website. Retrieved on 2006-04-21.