| 摘要 |
An objective optical system has at least three lens units having positive, negative, and positive optical powers. A fourth lens unit can have either negative or positive optical power. The system satisfies at least one of:(A) 0.15<=beta4W/FLWobj<=0.28, where beta4W represents image forming magnification of fourth lens unit at wide angle end, and FLWobj represents focal length of the system at wide angle end, and(B) the third lens unit being a biconvex lens element which satisfies the relationship Nd3>=1.6, where Nd3 represents d-line refractive index of lens material of the biconvex lens element. The system can also satisfy the relationship 0.038<=1/FL1<=0.068 where FL1 represents focal length of first lens unit. The third lens unit can have at least one aspherical surface which, relative to a height Y in an optional direction perpendicular to the optical axis such that 0.7Ymax<Y<Ymax, satisfies the relationship -0.07<phi3.(N'-N).(d/dy){x(y)-x0(y)}<0 where phi3 represents refracting power of third lens unit, N represents d-line refractive index of medium of third lens unit for an aspherical surface on the object side, N' represents d-line refractive index of the medium for an aspherical surface on the image side, x(y) represents a shape of the aspherical surface, and x0(y) represents a reference spherical surface shape of the aspherical surface.
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