Spherical mirrors are the most common kind of Concave Mirrors. For a large spherical mirror, only the parallel rays near the principal axis are reflected to converge at F. Instead of a focal point, a disk is formed by parallel rays. This is known as spherical aberration.

In a spherical mirror the focal length, f, is one half of its radius. f = ?r.

Parabolic mirrors cause no such effect. In telescopes, they are used to focus parallel rays from a distant star to a focal point. Conversely, if light sources are placed at the focus of a parabolic mirror, beams of parallel rays will be produced, as in car headlights and flashlights. Notice that light rays go in either direction.