afocal

An afocal optical system is one in which parallel light rays entering the system emerge as parallel rays, with the effective focal point pushed to infinity. This happens when the system's optical power (the reciprocal of focal length) equals zero. Unlike a conventional lens or mirror, an afocal system does not converge or diverge light; instead it redirects it while preserving the collimated beam structure. The most common practical example is a telescope in normal adjustment, where an objective lens of long focal length and an eyepiece of short focal length are spaced such that their focal points coincide.

Afocal systems are built from combinations of lenses or mirrors whose individual powers cancel out. A simple afocal magnifier consists of two positive lenses separated by the sum of their focal lengths. When spaced this way, a parallel beam in produces a parallel beam out, but the output beam diameter differs from the input, creating angular magnification without focusing action. Binoculars and rifle scopes use afocal designs for this reason: they magnify the angular size of distant objects while keeping the exit pupil at a fixed distance from the eye.

The key advantage of an afocal system is that it can magnify or redirect light without forming an intermediate image. This matters when you need to relay a collimated beam across a distance, or when you want magnification without the eye accommodation demand that a real focal point would impose. In laser systems and beam projection, afocal telescopes are used to expand or contract beam diameter while keeping the beam parallel, essential for controlling divergence and spot size at a distance.

The design constraint is strict: the separation distance between optical elements must be exact. A small error in spacing or alignment will break the afocal condition and introduce unwanted focus or aberration. Temperature and mechanical vibration can shift spacing enough to degrade performance. Quality afocal instruments use precision mechanical tubes and sometimes active adjustment mechanisms to maintain the condition under field use.

The term comes from the Greek prefix a- (without) and the Latin focalis (relating to a hearth or center point). It is contrasted with focal systems, which form real images at finite distances. In optical design documents and ray-tracing software, afocal systems are identified by having zero or near-zero effective focal length and back focal length parameters that extend to infinity.