Patent ID: 8879173

Claim:
An imaging lens comprising: a first lens having negative refractive power; a second lens having positive refractive power; a third lens having negative refractive power; and a fourth lens having positive refractive power, arranged in this order from an object side to an image plane side, wherein said second lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are both positive, said third lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are both negative, said fourth lens is formed in a shape so that a curvature radius of a surface thereof on the object side and a curvature radius of a surface thereof on the image plane side are both positive, and said first lens has a focal length f1 and said second lens has a focal length f2 so that the following conditional expression is satisfied: −0.6 <f 2 /f 1≦−0.29, said fourth lens has a focal length f4 so that the following conditional expression is satisfied: 0.7 <f 2 /f 4<1.0, said third lens has the surface on the image plane side having the curvature radius R 3 r and said fourth lens has the surface on the object side having the curvature radius R 4 f so that the following conditional expression is satisfied: −1.8 <R 3 r/R 4 f<− 0.8, and said second lens has the surface on the object side having the curvature radius R 2 f so that the following conditional expression is satisfied: 0.15< R 2 f/f <0.35 where f is a focal length of a whole lens system.