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