Spherical aberration

Optical aberration

In optics, spherical aberration is a type of aberration found in optical systems that have elements with spherical surfaces. This phenomenon commonly affects lenses and curved mirrors, as these components are often shaped in a spherical manner for ease of manufacturing. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre.

From Wikipedia, under CC BY-SA. More on occurri.