Patent ID: 7502182

Claim:
An objective lens comprising: in order from an object side of the objective lens, a front-group divergent lens system comprising a first lens, the first lens having a concave surface on an image side thereof; an aperture stop; and a rear-group convergent lens system comprising a second lens, a third lens and a fourth lens in this order from the object side, wherein the second lens is a positive meniscus lens having a convex surface on the image side thereof, the third and fourth lenses comprises a cemented lens, one of the third and fourth lenses is a positive lens, and the other of the third and fourth lenses is a negative lens, wherein the objective lens has a back focal length longer than 2.5 times a combined focal length of the objective lens, and the objective lens satisfies formula (1): | v 3 −v 4 |×f 2 /{|R A |·( Bf+D A /n 4 )}≧9 wherein f represents the combined focal length of the objective lens, Bf represents the back focal length of the objective lens, v 3 represents an Abbe number of the third lens, v 4 represents an Abbe number of the fourth lens, R A represents a radius of curvature of an interface between the third and fourth lenses, D A represents a center thickness of the fourth lens, and n 4 represents a refractive index of the fourth lens.