An electronic endoscope system has an elongated rigid inserting section and a relay optical system disposed therein. The relay optical system transfers an optical image. A imaging device is disposed at a focus position of the relay optical sysetm in a face-to-face relationship with the imaging surface...http://www.google.de/patents/US4905082?utm_source=gb-gplus-sharePatent US4905082 - Rigid video endoscope having a detachable imaging unit
