Imaging lens
An imaging lens includes a first lens having negative refractive power; a second lens having positive refractive power; a third lens; and a fourth lens arranged in the order from an object side to an image plane side. The first lens has an image plane-side surface having a positive curvature radius. The first lens has a specific focal length to satisfy a specific conditional expression.
1. An imaging lens comprising:
a first lens having negative refractive power;
a second lens having positive refractive power;
a third lens; and
a fourth lens, 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 image plane side has a positive curvature radius, and
said first lens has a focal length f 1 , and said first lens is situated away from the second lens by a distance dA on an optical axis thereof so that the following conditional expressions are satisfied:
−75 <f 1/ f≦− 13.818
0.3< dA/f< 1.0
where f is a focal length of a whole lens system.
2. The imaging lens according to claim 1 , wherein said fourth lens has positive refractive power.
3. The imaging lens according to claim 1 , wherein said first lens has an Abbe's number νd 1 so that the following conditional expression is satisfied:
45<νd1 <70.
4. 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:
1.0< f 4 /f < 2.5
where f is the focal length of the whole lens system.
5. The imaging lens according to claim 1 , wherein said third lens has an Abbe's number νd 3 so that the following conditional expression is satisfied:
20 <νd3 <40.
6. The imaging lens according to claim 1 , wherein said fourth lens has an Abbe's number νd 4 so that the following conditional expression is satisfied:
45<νd4 <70.
7. An imaging lens comprising:
a first lens having negative refractive power;
a second lens having positive refractive power;
a third lens; and
a fourth lens, arranged in this order from an object side to an image plane side,
wherein each of said first lens, said second lens, said third lens, and said fourth lens is formed of one single lens,
said second lens is arranged to face the third lens,
said second lens is formed in a shape so that a surface thereof on the object side has a negative curvature radius, and
said first lens is situated away from the second lens by a distance dA on an optical axis thereof, said first lens has an Abbe's number νd 1 , and said third lens has an Abbe's number νd 3 so that the following conditional expressions are satisfied:
0.3< dA/f< 1.0
45<νd1<70
20<νd3<40
where f is a focal length of a whole lens system.
8. The imaging lens according to claim 7 , wherein said fourth lens has positive refractive power.
9. The imaging lens according to claim 7 , wherein said fourth lens has a focal length f 4 so that the following conditional expression is satisfied:
1.0< f 4/ f< 2.5
where f is the focal length of the whole lens system.
10. The imaging lens according to claim 7 , wherein said fourth lens has an Abbe's number νd 4 so that the following conditional expression is satisfied:
45<νd4<70.
11. An imaging lens comprising:
a first lens having negative refractive power;
a second lens having positive refractive power;
a third lens; and
a fourth lens, arranged in this order from an object side to an image plane side,
wherein each of said first lens, said second lens, said third lens, and said fourth lens is formed of one single lens,
said second lens is arranged to face the third lens,
said second lens is formed in a shape so that a surface thereof on the object side has a negative curvature radius, and
said first lens is situated away from the second lens by a distance dA on an optical axis thereof, said third lens has an Abbe's number νd 3 , and said fourth lens has an Abbe's number νd 4 so that the following conditional expressions are satisfied:
0.3< dA/f< 1.0
20<νd3<40
45<νd4<70
where f is a focal length of a whole lens system.
12. The imaging lens according to claim 11 , wherein said fourth lens has positive refractive power.
13. The imaging lens according to claim 11 , wherein said first lens has an Abbe's number νd 1 so that the following conditional expression is satisfied:
45<νd1<70.
14. The imaging lens according to claim 11 , wherein said fourth lens has a focal length f 4 so that the following conditional expression is satisfied:
1.0< f 4/ f< 2.5
where f is the focal length of the whole lens system.