Imaging lens
An imaging lens includes a first lens group having positive refractive power; a second lens group having negative refractive power; and a third lens group having negative refractive power, arranged in this order from an object side to an image plane side. The first lens group includes a first lens having positive refractive power, a second lens having negative refractive power, and a third lens having positive refractive power. The second lens group includes a fourth lens and a fifth lens. The third lens group includes a sixth lens and a seventh lens. The fourth lens is formed in a shape so that a surface on the object side thereof has a negative curvature radius. The fifth lens is formed in a shape so that a surface on the image plane side thereof has a positive curvature radius. The first to third lenses have specific Abbe's numbers.
1. An imaging lens comprising:
a first lens group having positive refractive power;
a second lens group having negative refractive power; and
a third lens group having negative refractive power, arranged in this order from an object side to an image plane side,
wherein said first lens group includes a first lens having positive refractive power, a second lens having negative refractive power, and a third lens having positive refractive power,
said second lens group includes a fourth lens and a fifth lens,
said third lens group includes a sixth lens and a seventh lens,
said fourth lens is formed in a shape so that a surface on the object side thereof has a negative curvature radius,
said fifth lens is formed in a shape so that a surface on the image plane side thereof has a positive curvature radius, and
said first lens has an Abbe's number vd 1 , 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:
40<vd1<75,
20<vd2<35,
40<vd3<75.
2. The imaging lens according to claim 1 , wherein said fourth lens has positive refractive power,
said fifth lens has negative refractive power, and
said fourth lens has an Abbe's number vd 4 and said fifth lens has an Abbe's number vd 5 so that the following conditional expressions are satisfied:
20<vd4<35
20<vd5<35.
3. The imaging lens according to claim 1 , wherein said sixth lens has negative refractive power near an optical axis and positive refractive power increasing toward a periphery thereof, and
said seventh lens has negative refractive power near the optical axis and positive refractive power increasing toward a periphery thereof.
4. The imaging lens according to claim 1 , wherein said third lens is disposed away from the fourth lens by a distance D 34 on an axial axis so that the following conditional expression is satisfied:
0.05 <D 34/ f< 0.2
where f is a focal length of a whole lens system.
5. The imaging lens according to claim 1 , wherein said first lens has a focal length f 1 so that the following conditional expression is satisfied:
0.5 <f 1/ f< 2.0
where f is a focal length of a whole lens system.
6. 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:
−4 <f 2/ f 1<−0.5.
7. The imaging lens according to claim 1 , wherein said first lens has a focal length f 1 and said third lens has a focal length f 3 so that the following conditional expression is satisfied:
0.5 <f 3/ f 1<4.
8. The imaging lens according to claim 1 , wherein said fourth lens and said fifth lens have a composite focal length f 45 , and said sixth lens and said seventh lens have a composite focal length f 67 so that the following conditional expression is satisfied:
0.5 <f 45/ f 67<5.