Four-group endoscope objective lens

Claim 1

An endoscope objective lens formed of only four lens groups, in order along an optical axis from the object side, as follows: a first lens group having negative refractive power; a second lens group having positive refractive power; a diaphragm; a third lens group having positive refractive power; and a fourth lens group having positive refractive power; wherein said first lens group includes a first lens element having negative refractive power and the image side lens surface of said first lens element is concave; said second lens group includes a lens component that includes second and third lens elements; said third lens group includes a fourth lens element having positive refractive power and said fourth lens element has its lens surface of smaller radius of curvature on its image side; said fourth lens group includes a lens component that includes a fifth lens element having positive refractive power and a sixth lens element that is a meniscus lens element having negative refractive power; and the following conditions are satisfied: 1.50 <|d/f 1 |<2.50 0.96 <f/h <1.04 2.00 <f 2+3 /f <25.0 15.0<|ν 2 –ν 3 | 10.0<( f× (ν 5 –ν 6 ))/(| R|×Bf ) where d is the distance along the optical axis between the concave lens surface of said first lens element and said diaphragm; f 1 is the focal length of said first lens group; f is the focal length of the endoscope objective lens; h is the maximum height of an image formed by the endoscope objective lens; f 2+3 is the focal length of said second lens group; ν 2 is the Abbe number of said second lens element; ν 3 is the Abbe number of said third lens element; ν 5 is the Abbe number of said fifth lens element; ν 6 is the Abbe number of said sixth lens element; |R| is the absolute value of the radius of curvature of the adjoining lens surfaces of said fifth lens element and said sixth lens element; and Bf is the back focal length of the endoscope objective lens.