Imaging lens
An imaging lens includes a first lens having positive refractive power; a second lens having positive refractive power; and a third lens having positive refractive power, arranged in the order from an object side to an image plane side. The first lens and the third lens have object-side surfaces and image plane-side surfaces, curvature radii of which are both positive. When the first lens has refractive power P 1 , the second lens has refractive power P 2 , the third lens has refractive power P 3 , the first lens has a focal length f 1 , and the third lens has a focal length f 3 , the imaging lens satisfies the following conditional expressions: 5.0<( P 1+ P 3)/ P 2<8.5 6< f 3/ f 1<30.
1. An imaging lens comprising:
a first lens having positive refractive power;
a second lens having positive refractive power; and
a third lens having positive refractive power, arranged in this order from an object side to an image plane side,
wherein said first lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side have positive curvature radii,
said third lens is formed in a shape so that a surface thereof on the object side and a surface thereof on the image plane side have positive curvature radii, and
said first lens has a refractive power P 1 , said second lens has a refractive power P 2 , said third lens has a refractive power P 3 , said first lens has a focal length f 1 , and said third lens has a focal length f 3 so that the following conditional expressions are satisfied:
5.0<( P 1+ P 3)/ P 2<8.5
6< f 3/ f 1<30.
2. The imaging lens according to claim 1 , wherein said first lens and said second lens are arranged so that the surface of the first lens on the image plane side is away from a surface of the second lens on the object side by a distance D 12 on an optical axis, and
said second lens and said third lens are arranged so that a surface of the second lens on the image plane side is away from the surface of the third lens on the object side by a distance D 23 on the optical axis so that the following conditional expression is satisfied:
0.5< D 12 /D 23<1.0.
3. The imaging lens according to claim 1 , wherein said second lens has a focal length f 2 so that the following conditional expression is satisfied:
0.1< f 1 /f 2<0.3.
4. The imaging lens according to claim 1 , wherein said second lens has a focal length f 2 so that the following conditional expression is satisfied:
5< f 2 /f< 10
where f is a focal length of a whole lens system.
5. The imaging lens according to claim 1 , wherein said first lens has an Abbe's number νd 1 , said second lens has an Abbe's number νd 2 , and said third lens has an Abbe's number νd 3 so that the following conditional expressions are satisfied:
45<νd1<75
45<νd2<75
45<νd3<75.