The Primary Optical device is the first lens or mirror that light touches. The diameter of the Primary Optical Device is also (generally) the Aperture Diameter. The physical or effective distance between the Primary Optical Device and the focal point is called the focal length, measured along the optical axis.

Given a point-like object sufficiently far away that its light rays impinge the primary optical device as light rays parallel to each other and parallel to the optical axis, the optical system will focus the rays to the focal point. The plane perpendicular to the line containing the optical axis and the focal point is the focal plane. It is on this plane that all images are formed.

Optical systems can use lenses, mirrors, or a combination of both to focus light.

Refractor telescopes make use of lenses to focus light.