Editing Optical aberration (section)

==Overview==
[[File:Abbildungsfehler am Hohlspiegel (Katakaustik).svg|thumb|upright=1.5|Reflection from a spherical mirror. Incident rays (red) away from the center of the mirror produce reflected rays (green) that miss the focal point, F. This is due to [[spherical aberration]].]]

With an ideal [[Lens (optics)|lens]], light from any given point on an object would pass through the lens and come together at a single point in the ''image plane'' (or, more generally, the ''image surface''). Real lenses, even when they are perfectly made, do not however focus light exactly to a single point. These deviations from the idealized lens performance are called ''aberrations'' of the lens.

Aberrations fall into two classes: ''monochromatic'' and ''chromatic''. Monochromatic aberrations are caused by the geometry of the lens or mirror and occur both when light is reflected and when it is refracted. They appear even when using [[monochromatic light]], hence the name.

[[Chromatic aberration]]s are caused by [[dispersion (optics)|dispersion]], the variation of a lens's [[refractive index]] with [[wavelength]]. Because of dispersion, different wavelengths of light come to focus at different points. Chromatic aberration does not appear when monochromatic light is used.

===Monochromatic aberrations===

The most common monochromatic aberrations are:

*[[Defocus aberration|Defocus]]
*[[Spherical aberration]]
*[[Coma (optics)|Coma]]
*[[Astigmatism (optical systems)|Astigmatism]]
*[[Field curvature]]
*[[Image distortion]]

Although defocus is technically the lowest-order of the optical aberrations, it is usually not considered as a lens aberration, since it can be corrected by moving the lens (or the image plane) to bring the image plane to the optical focus of the lens.

In addition to these aberrations, [[Piston (optics)|piston]] and [[Tilt (optics)|tilt]] are effects which shift the position of the focal point.  Piston and tilt are not true optical aberrations, since when an otherwise perfect wavefront is altered by piston and tilt, it will still form a perfect, aberration-free image, only shifted to a different position.

===Chromatic aberrations===
{{comparison_axial_lateral_chromatic_aberration.svg}}
{{Main|Chromatic aberration}}

Chromatic aberration occurs when different wavelengths are not focussed to the same point.  Types of chromatic aberration are:

*Axial (or "longitudinal") chromatic aberration
*Lateral (or "transverse") chromatic aberration