It does not take long to notice that spherical mirrors do not produce a precise central focus. This imprecise focus occurs even when the mirror has a perfectly circular curve. Therefore a spherical shape is not ideal for a converging mirror. Spherical mirrors produce spherical aberrations: on a spherical mirror, the outer incident rays do not converge at the focus.

The ideal shape for a converging mirror is that of a parabola. Parabolic mirrors do not produce spherical aberrations; all incident rays parallel to the principal axis converge at the focus.