What phenomenon does spherical aberration refer to?

Spherical aberration is a phenomenon that occurs when light rays passing through a spherical lens or reflecting off a spherical mirror do not all converge at the same focal point. Instead, rays that are incident at different distances or angles focus at varying points, leading to a blurred image rather than a sharp one.

In optical systems, particularly those with spherical surfaces, light rays that strike the edges of the lens or mirror converge closer to the optical element than rays that strike near the center. This discrepancy results in different points of convergence based on the distance from which the rays originate. Thus, the correct understanding of spherical aberration encapsulates this effect of varying focus points depending on the angle and position of the incoming light, which can degrade image quality and sharpness.