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