Imaging lens
An imaging lens includes a first lens having positive refractive power; a second lens having negative refractive power; a third lens having positive refractive power; a fourth lens having positive refractive power; a fifth lens having negative refractive power; and a sixth lens having negative refractive power. The first lens has an object-side with a positive curvature radius. The second lens has object-side and image plane-side surfaces with positive curvature radii. The third lens has an image plane-side surface with a negative curvature radius. The fourth lens has object-side and image plane-side surfaces with negative curvature radii. The fifth lens has object-side and image plane-side surfaces with negative curvature radii. The sixth lens has strong positive refractive power close to a periphery, and an aspheric image plane-side surface having an inflexion point. The first to sixth lenses have specific Abbe's numbers to satisfy specific conditions.
1. An imaging lens comprising:
a first lens having positive refractive power;
a second lens having negative refractive power;
a third lens having positive refractive power;
a fourth lens having positive refractive power;
a fifth lens having negative 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 first lens is formed in a shape so that a surface thereof on the object side has a positive curvature radius,
said second 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 image plane side has a negative curvature radius,
said fourth 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 negative curvature radii,
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 have negative curvature radii,
said sixth lens is formed to have negative refractive power near an optical axis and positive refractive power increasing toward a periphery portion thereof,
said sixth lens is formed in a shape so that a surface thereof on the image plane side has an aspheric shape having an inflexion point, and
said first lens has an Abbe's number νd1, said second lens has an Abbe's number νd2, said third lens has an Abbe's number νd3, said fourth lens has an Abbe's number νd4, said fifth lens has an Abbe's number νd5, and said sixth lens has an Abbe's number νd6 so that the following conditional expressions are satisfied:
45<ν d 1<75
20<ν d 2<40
45<ν d 3<75
45<ν d 4<75
20<ν d 5<40
45<ν d 6<75.
2. The imaging lens according to claim 1 , wherein said fifth lens and said sixth lens have a composite focal length f56 so that the following conditional expression is satisfied:
−1.0< f 56 /f<− 0.3
where f is a focal length of a whole lens system.
3. The imaging lens according to claim 1 , wherein said first lens and said second lens have a composite focal length f12 so that the following conditional expression is satisfied:
1.1< f 12 /f< 3.0
where f is a focal length of a whole lens system.
4. 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:
−0.8< f 1/ f 2<−0.3.
5. The imaging lens according to claim 1 , wherein said third lens has a focal length f3 so that the following conditional expression is satisfied:
1.0< f 3 /f< 5.0
where f is a focal length of a whole lens system.
6. The imaging lens according to claim 1 , wherein said fourth lens has a focal length f4 and said fifth lens has a focal length f5 so that the following conditional expression is satisfied:
−0.4< f 4/ f 5<−0.1.