Imperfections in image formation: astigmatism, coma, and distortion

by Quirino Sugon Jr.

There are several imperfections or aberrations if you use lenses to form images:

1.  Astigmatism

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning “without” and στίγμα (stigma), “a mark, spot, puncture”. (Read more in Wikipedia)

2.  Coma

Coma is an inherent property of telescopes using parabolic mirrors. Light from a point source (such as a star) in the center of the field is perfectly focused at the focal point of the mirror (unlike a spherical mirror, where light from the outer part of the mirror focuses closer to the mirror than light from the center–spherical aberration). However, when the light source is off-center (off-axis), the different parts of the mirror do not reflect the light to the same point. This results in a point of light that is not in the center of the field looking wedge-shaped. The further off-axis, the worse this effect is. This causes stars to appear to have a cometary coma, hence the name. (Read more in Wikipedia)

3.  Distortion

Distortion can be thought of as stretching the image non-uniformly, or, equivalently, as a variation in magnification across the field. While “distortion” can include arbitrary deformation of an image, the most pronounced modes of distortion produced by conventional imaging optics is “barrel distortion”, in which the center of the image is magnified more than the perimeter (figure 7a). The reverse, in which the perimeter is magnified more than the center, is known as “pincushion distortion” (figure 7b). (Read more in Wikipedia)