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; and a sixth lens having negative refractive power, arranged from an object side to an image plane side. The first lens has an object-side surface and an image plane-side surface having positive curvature radii. The third lens has an object-side surface and an image plane-side surface having positive curvature radii. The fourth lens has an object-side surface and an image plane-side surface having negative curvature radii. The sixth lens is formed so that at least one of an object-side surface and an image plane-side surface is an aspherical surface and has at least one inflexion point. The first lens and the second lens have a specific composite focal length.
1. An imaging lens comprising:
a first lens having positive refractive power;
a second lens;
a third lens;
a fourth lens;
a fifth lens; 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 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,
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 sixth lens is formed in a shape so that at least one of a surface thereof on the object side and a surface thereof on the image plane side is an aspherical surface and has at least one inflexion point at a location off an optical axis thereof, and
said first lens and said second lens have a composite focal length f 12 so that the following conditional expression is satisfied:
0.8< f 12/ f< 1.5
where f is a focal length of a whole lens system.
2. The imaging lens according to claim 1 , wherein said second lens has negative refractive power, and
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.
3. The imaging lens according to claim 1 , wherein said sixth lens is formed in the shape so that a surface thereof on the image plane side has a positive curvature radius.
4. The imaging lens according to claim 1 , wherein said first lens has a focal length f 1 , and said second lens has a focal length f 2 so that the following conditional expression is satisfied:
−0.7< f 1 /f 2<−0.3.
5. The imaging lens according to claim 1 , wherein said fourth lens has a focal length f 4 so that the following conditional expression is satisfied:
−0.3< f/f 4<−0.01.
6. The imaging lens according to claim 1 , wherein said third lens and said fourth lens have a composite focal length f 34 so that the following conditional expression is satisfied:
−20.0< f 34/ f<− 1.0.
7. The imaging lens according to claim 1 , wherein said second lens has a surface on the image plane side situated away from a surface of the third lens on the object side by a distance D 23 on an optical axis thereof, and said first lens has a surface on the object side situated away from a surface of the sixth lens on the image plane side by a distance L 16 on an optical axis thereof so that the following conditional expression is satisfied:
0.05< D 23/ L 16<0.3.
8. The imaging lens according to claim 1 , wherein said first lens has an Abbe's number vd 1 , said fourth lens has an Abbe's number vd 4 , said fifth lens has an Abbe's number vd 5 , and said sixth lens has an Abbe's number vd 6 so that the following conditional expressions are satisfied:
45<vd1<75
45<vd4<75
45<vd5<75
45<vd6<75.
9. The imaging lens according to claim 1 , wherein said second lens has an Abbe's number vd 2 , and said third lens has an Abbe's number vd 3 so that the following conditional expressions are satisfied:
20<vd2<40
20<vd3<40.