Spherical Aberration

For lenses made with spherical surfaces, rays which are parallel to the optic axis but at different distances from the optic axis fail to converge to the same point. For a single lens, spherical aberration can be minimized by bending the lens into its best form. For multiple lenses, spherical aberrations can be canceled by overcorrecting some elements. The use of symmetric doublets greatly reduces spherical aberration.

Meniscus Lenses

The amount of spherical aberration in a lens made from spherical surfaces depends upon its shape.

Coddington Shape Factor

Although spherical aberration cannot be eliminated for a single lens, it can be minimized by the appropriate bending of the lens into its best form. The degree of bending can be characterized by the Coddington shape factor:

The minimum spherical aberration also depends on the object and image distances, so another factor enters, called the Coddington position factor:

The minimum spherical aberration occurs when

The higher the index n, the smaller the aberration for the optimum shape.