Abstract: An objective optical system for an endoscope comprises a retrofocus objective lens assembly consisting of a front-stage diverging lens unit, an aperture diaphragm, a rear-stage converging lens unit, and a prism positioned in this sequence, and a self-scanning solid state image pickup device positioned at the image formation surface of said objective lens assembly. The objective lens assembly satisfies the conditions 0.8<.vertline.f1.vertline./f.ltoreq.1.5, 1<f2/f.ltoreq.1.5, and Np>1.7, where f denotes the resultant focal length of the whole assembly, f1 denotes the resultant focal length of the front-stage diverging lens unit, f2 designates the resultant focal length of the rear-stage converging lens unit, and Np designates the refractive index of the prism.

Abstract: In an optical system including an objective lens to form an image on a focusing plane and a relay lens system to carry the image to another focusing plane, an image rotating prism is inserted in the relay lens system and a field lens is provided between the objective lens and the relay lens. The pupil of the relay lens system is positioned at the center of the optical path of the prism by properly selecting the power of the field lens.