Spherical Aberration

Spherical aberration is a type of aberration found in optical systems that use elements with spherical surfaces. Lenses and curved mirrors are most often made with surfaces that are spherical, because this shape is easier to form than non-spherical curved surfaces. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre. This deviation reduces the quality of images produced by optical systems.

Astrophysics:

In a real optical system there is a tradeoff between image brightness and image quality.

It arises from the fact that spherical curves are the easiest to grind on glass surfaces. These spherical shapes are close to the shapes required for proper image formation, but differ slightly, so the images are imperfect.