{HEAD ~ An observation optical system} for {TAIL ~ non flexible endoscopes} 2 or 6, wherein said at least another relay lens module which comprises negative lens elements comprises, in the vicinities of 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 
{ HEAD ~ An observation optical system } for { TAIL ~ non flexible endoscopes } 2 or 6 , wherein said at least another relay lens module which comprises negative lens elements comprises , in the vicinities of 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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