Relay lens

Relay lenses are found in refracting telescopes, endoscopes, and periscopes to optically manipulate the light path, extend the length of the whole optical system, and usually serve the purpose of inverting the image.

In modern optical telescopes and telescopic sights with a dual-focal plane design, the objective image is typically already inverted upon reaching the relay lenses, and thus needed to be inverted again back into an erect image (i.e. "erecting" the image) before passing to the eyepiece, so the viewer actually sees an upright target.

The relay lens group is the optical component responsible for that re-inversion, and therefore sometimes collectively called the erector lenses.

Also, for endoscope applications, where small tube diameter is desirable, most of the tube is filled with glass, with thin air gaps to allow for powered surfaces; because marginal ray angle is smaller at a given numerical aperture the higher the index of refraction, this allows the relay to have higher NA for a given diameter.

Karl Storz GmbH licensed the patent for the Hopkins relay lens and introduced endoscopes including such lenses in 1965.