Spherical Aberration

As light rays pass through a lens or are reflected by a mirror, they change direction. The extent of the change of direction is different at the edges compared to at the centre of these devices. This results in something called spherical aberration. Incoming parallel light rays are not all focused at the exact same point. The mirror of the NASA/ESA Hubble Space Telescope initially suffered from spherical aberration, and a correcting optical device had to be installed to bring the images into focus.

Top: Perfect lens. Bottom: Spherical aberration.