spherical aberration

Spherical aberration occurs when light rays passing through a lens at different distances from its optical axis focus at different points along that axis. Rays striking near the edge of the lens converge at a different distance than rays passing through the center, creating a blurred or hazy image regardless of where you position the focal plane. This is one of the most common optical defects in simple lens systems.

The root cause is geometric: a single curved spherical surface cannot refract all parallel rays to a single point. The curvature that works for paraxial rays (those close to the optical axis) over-focuses the marginal rays at the lens edges. The effect is worst with large apertures and short focal lengths, where ray angles are steepest. A lens might show spherical aberration of 0.5 to 2 diopters depending on its diameter, focal length, and design.

Correction and mitigation

Optical designers correct spherical aberration by combining positive and negative lens elements of different powers and shapes, by using aspheric (non-spherical) surfaces, or by stopping down the aperture to block marginal rays. High-quality camera lenses, microscope objectives, and telescope optics use multiple aspheric or conic surfaces to achieve correction across the visible spectrum. In applications like projectors or simple magnifiers, a small aperture (high f-number) is often the only practical solution.

The term "spherical" refers to the shape of the offending surface, not to the shape of the aberration itself. The effect was first documented in the 17th century during early telescope development and remains unavoidable in single-element lenses. Modern optical simulation software predicts spherical aberration during design, allowing engineers to balance it against other defects like coma, astigmatism, and field curvature rather than eliminate it entirely.

In imaging systems, residual spherical aberration appears as reduced contrast, softness at the image edges, and sensitivity to focus position. Lens testers measure it using interferometry or by analyzing point-spread functions. In precision optical work, spherical aberration budgets are calculated alongside tolerances for manufacturing variation, thermal effects, and assembly errors.