Spherical aberration results from a spherical mirror (left) but not from a parabolic mirror (right). Credit: star-optics.com.
Spherical aberration is a defect of spherical mirrors and some lenses and eyepieces that causes parallel light rays striking the element at different distances from the optical axis to be brought to a focus at different points along the axis. There is no place at which all the rays come to a single focus and give a sharp image. Spherical aberration may be reduced by narrowing the lens so that rays do not pass through the edges and by combining lenses to cancel out the defects in each kind of lens.