How Do You Correct Spherical Aberration?

correct-spherical-aberration Credit: rayand/CC-BY-2.0

Spherical aberration in mirrors is usually corrected by using a mirror which is not a sphere. A parabolic mirror is often used instead of a spherical mirror. In lenses, the simplest method for minimizing spherical aberration is to use an aperture in front of the lens. Symmetric doublets, such as the orthoscopic doublet, are also efficient in greatly reducing the spherical aberration in lenses.

Spherical aberration happens because the light rays fail to converge at a single point when using a spherical mirror or a lens. In lenses, the light waves that pass through the outer edges of a spherical lens are refracted more than the ones that pass through the center. Spherical aberration cannot be eliminated completely from a single lens, but it can reduced by bending the lens in its "best form." Coddington shape factor is used for characterizing the degree of bending.

Similarly, the light waves reflected from the outer edges of a spherical mirror focus closer to the mirror than the rays that reflect from the central part of the mirror. A parabolic mirror's outer edges differ significantly from a spherical mirror, and they form clear, sharp images that lack the blurriness associated with the images formed by spherical mirrors.