# Focus (optics)

Image point, is a point where light rays originating from a point on the object converge.

- Focus (optics)

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## Cassegrain reflector

Combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture.

This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system.

## Airy disk

In optics, the Airy disk (or Airy disc) and Airy pattern are descriptions of the best-focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light.

## 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.

## Curved mirror

Mirror with a curved reflecting surface.

Such mirrors always form a virtual image, since the focal point (F) and the centre of curvature (2F) are both imaginary points "inside" the mirror, that cannot be reached.

## Focal length

Optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power.

For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam.

## Defocus aberration

In optics, defocus is the aberration in which an image is simply out of focus.

## Circle of confusion

In optics, a circle of confusion (CoC) 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.

## Geometrical optics

Model of optics that describes light propagation in terms of rays.

For mirrors with parabolic surfaces, parallel rays incident on the mirror produce reflected rays that converge at a common focus.

## Lens

Transmissive optical device which focuses or disperses a light beam by means of refraction.

As mentioned above, a positive or converging lens in air focuses a collimated beam travelling along the lens axis to a spot (known as the focal point) at a distance f from the lens.

## Aperture

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 rays that come to a focus in the image plane.

Hyperleap, 2022