Imaging lens
An imaging lens includes a first lens having positive refractive power; a second lens; a third lens; a fourth lens; a fifth lens having positive refractive power; and a sixth lens having negative refractive power, arranged from an object side to an image plane side. The fourth lens is formed in a shape so that a surface on the object side is concave and a surface on the image plane side is convex near an optical axis. The fifth lens is formed in a shape so that surfaces on the object side and the image plane side are convex near an optical axis. The sixth lens is formed in a shape so that surfaces on the object side and the image plane side are concave near an optical axis. The fifth lens and the sixth lens have specific focal lengths, and the fourth lens has a specific Abbe's number.
1. An imaging lens comprising:
a first lens having positive refractive power;
a second lens;
a third lens;
a fourth lens;
a fifth lens having positive refractive power; and
a sixth lens having negative refractive power, arranged in this order from an object side to an image plane side,
wherein said fourth lens is formed in a shape so that a surface thereof on the object side is concave near an optical axis thereof and a surface thereof on the image plane side is convex near the optical axis thereof,
said fifth lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side are convex near an optical axis thereof,
said sixth lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side are concave near an optical axis thereof, and
said fifth lens has a focal length f5, said sixth lens has a focal length f6, and said fourth lens has an Abbe's number νd4 so that the following conditional expression is satisfied:
−2.0< f 5 /f 6<−0.5,
20<ν d 4<40.
2. The imaging lens according to claim 1 , wherein 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:
−1.5 <f 1/ f 2<−0.4.
3. The imaging lens according to claim 1 , wherein said third lens has a focal length f3 so that the following conditional expression is satisfied:
0.5 <f 3 /f <2.0,
where f is a focal length of a whole lens system.
4. The imaging lens according to claim 1 , wherein said second lens is formed in a shape so that a surface thereof on the object side has a curvature radius R 2 f and a surface thereof on the image plane side has a curvature radius R 2 r so that the following conditional expression is satisfied:
0.4 <R 2 r/R 2 f <0.8.
5. The imaging lens according to claim 1 , wherein said fourth lens has a focal length f4 so that the following conditional expression is satisfied:
−5.0 <f 4 /f<− 1.0,
where f is a focal length of a whole lens system.
6. An imaging lens comprising:
a first lens;
a second lens;
a third lens;
a fourth lens;
a fifth lens having positive refractive power; and
a sixth lens having negative refractive power, arranged in this order from an object side to an image plane side,
wherein said sixth lens is formed in a shape so that a surface thereof on the image plane side is concave near an optical axis thereof, and
said third lens has a focal length f3, said fourth lens has a focal length f4, and said second lens is formed in a shape so that a surface thereof on the object side has a curvature radius R 2 f and a surface thereof on the image plane side has a curvature radius R 2 r so that the following conditional expressions are satisfied:
0.5 <f 3 /f <2.0,
−5.0 f 4 /f <−1.0,
0.4 <R 2 r/R 2 f <0.8,
where f is a focal length of a whole lens system.
7. The imaging lens according to claim 6 , wherein 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:
−1.5 <f 1 /f 2<−0.4.
8. The imaging lens according to claim 6 , wherein said fifth lens has a focal length f5 and said sixth lens has a focal length f6 so that the following conditional expression is satisfied:
−2.0 <f 5 /f 6<−0.5.