Spherical Aberration
Definition
Blurring caused by a spherical lens or mirror focusing edge rays and center rays at different points.
Technical definition
A monochromatic aberration: rays through the periphery of a spherical surface focus closer than paraxial rays, producing a blurred caustic instead of a sharp point. Corrected with aspheric surfaces, aperture stops, or parabolic mirrors.
Example
The Hubble Space Telescope's original flawed primary mirror suffered spherical aberration — its images were blurry until corrective optics were installed in 1993.
Common confusion
Spherical aberration affects all colors equally (it is geometric, not dispersive) — unlike chromatic aberration. Stopping down the aperture (using only the center) reduces it, which is why pinhole images look sharp.