An observation optical system for non flexible endoscopes 2 or 6, wherein said at least another relay lens module which comprises negative lens elements comprises, in {HEAD ~ the vicinities} of {TAIL ~ the center} of said relay lens module, at least two negative lens elements made of a glass material having an Abbes number not exceeding 40 
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An observation optical system for non flexible endoscopes 2 or 6 , wherein said at least another relay lens module which comprises negative lens elements comprises , in { HEAD ~ the vicinities } of { TAIL ~ the center } of said relay lens module , at least two negative lens elements made of a glass material having an Abbes number not exceeding 40
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