Spherical aberration

Spherical aberration is an optical defect in which light rays passing through the outer zones of a spherical lens or mirror focus closer to the lens than rays passing near its optical axis.

What causes spherical aberration?

The phenomenon results from the geometry of a spherical surface — the refraction angle of the outer rays differs from what paraxial optics (the small-angle approximation) predicts. The result is a lack of a single, sharp focal point, which reduces image contrast and sharpness, especially at large apertures.

Applications

Correcting spherical aberration — using aspheric lenses, multi-element systems, or an appropriate choice of surface shape (meniscus lenses) — is a key element in designing high-quality photographic lenses, microscopes, and telescopes.

Related terms

MTF

Sequential ray tracing

Numerical aperture