Afocal system

As afocal is called a lens system which is neither gathering nor scattering. That is parallel incoming beams are indeed broken convergent or divergent within the system, further lenses or mirrors in the optical path cause, however, that the light emerges collimated again. In other words, an optical axis parallel to the incoming beam leaves parallel to the axis, the system again. This applies strictly only in the paraxial optics, otherwise the spherical aberration can cause a deviation.

An afocal system, such as a double-sided telecentric lens does not have focus, or it is infinite. It also has no primary and nodes.

Oblique incident to the optical axis ray bundles occur at a different angle to the optical axis from back when they originally arrived. This causes a change of magnification when the afocal system is used as a prefix before any other imaging system. This can be about the eye, which a telescope is set before.

Some zoom lenses included as a module an afocal system.