Patent ID: 7486449

Claim:
An objective lens for endoscope, comprising, in order from an object side: a front-group divergent lens system; an aperture diaphragm; and a rear-group convergent lens system, wherein the front-group divergent lens system comprises first and second lenses which are negative lenses, the rear-group convergent lens system is configured in order from an object side by: a positive third lens in which an object-side surface is larger in absolute value of a radius of curvature than an image-side surface; and a cemented lens configured by a positive fourth lens in which an object-side surface is larger in absolute value of a radius of curvature than an image-side surface and a fifth lens having a negative refractive power and a meniscus shape, the cemented lens having a positive refractive power as a whole, and the objective lens satisfies following conditional expressions: | d x /f F |≧ 3.0 (1) ( f/f 3 )×ν 3 <23 (2) f 2 ×(ν 5 −ν 4 )/{ R A ×( Bf+d 5 /n 5 )}>7 (3) where d x : distance between a rear principal point of the front-group divergent lens system and a front principal point of the rear-group convergent lens system, f F : composite focal length of the front-group divergent lens system, f: composite focal length of the objective lens, f 3 : focal length of the third lens, ν 3 : Abbe number of the third lens, ν 4 : Abbe number of the fourth lens, ν 5 : Abbe number of the fifth lens, R A : radius of curvature of a cementing surface between the fourth lens and the fifth lens, B f : back focus of the objective lens, d 5 : center thickness of the fifth lens, and n 5 : refractive index of the fifth lens with respect to the d-line.