What is aberration?

Distortion of an image from the ideal optical image is called aberration.
An ideal optical image satisfies the following three conditions.
1. A point object creates a point image.
2. A plane object perpendicular to the optical axis creates a plane image.
3. A figure on a plane perpendicular to the optical axis creates an image similar to that figure.
There are the following 5 types of monochromatic aberration, caused by the shape of the lens (these are called the Seidel aberrations, named after the researcher Seidel);
1. Spherical aberration
2. Coma
3. Astigmatism difference
4. Curvature of field
5. Distortion
  . . . and there are the following 2 types of chromatic aberration, caused by the material in the lens.
6. Longitudinal chromatic aberration
7. Lateral chromatic aberration
Types i-iii, vi and vii do not satisfy condition 1) above. Type iv and v aberrations do not satisfy conditions 2) and 3) respectively.

Aberration type		Condition of light ray
Undistorted ideal optical image
Monochromatic aberration	Spherical aberration
	Coma
	Astigmatic difference
	Curvature of field
	Distortion
Chromatic aberration	Longitudinal chromatic aberration
Chromatic aberration	Lateral chromatic aberration

- In a real optical system, the above seven aberrations are mixed, so the image becomes complex.