aberration   A defect in image formed by a lens or mirror. It can be of the following kinds:

(i) Spherical aberration: In a lens or mirror the paraxial rays (rays near the axis) and the marginal rays (rays away from the axis) are focused at different points. Consequently the lens (or mirror) does not form a point image of an axial point object(see fig.a1)

(ii) Coma: The image of a point object located just off the lens axis appears comet like. It arises because the lateral magnification produced by different zones of the lens are different

(iii) Astigmatism: This defect arises when a point object is situated far off the axis. The rays contained in the vertical plane and the rays contained in the horizontal plane are focused as two lines. These are separated by a distance and are mutually perpendicular.

(iv) Chromatic aberration: A lens (not mirror) forms not only one image of an object, but a series of images, one for each color of light present in the beam. It occurs because the  refractive  index of all transparent media varies with color. The light is thus dispersed into color bands . The defect is corrected by the use of achromatic lenses. It consists of a pair of lenses, one of crown, and the other of flint glass, the latter correcting the dispersion caused by the former. (see fig.a2)