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; a fifth lens; and a sixth lens, arranged in this order from an object side to an image plane side. The fifth lens is formed in a shape so that a surface thereof on the image plane side has a positive curvature radius. The fifth lens and the sixth lens have a specific composite focal length. The first lens is disposed away from the second lens by a specific distance on an optical axis thereof. The second lens is disposed away from the third lens by a specific distance.
1. An imaging lens comprising:
an aperture stop;
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 negative refractive power;
a fifth lens; and
a sixth lens, arranged in this order from an object side to an image plane side,
wherein said fourth lens is formed in a shape so that a surface thereof on the object side has a negative curvature radius,
said fifth lens is formed in a shape so that a surface thereof on the image plane side has a positive curvature radius, and
said fifth lens and said sixth lens have a composite focal length f 56 , said first lens is disposed away from the second lens by a distance D 12 on an optical axis thereof, and said second lens is disposed away from the third lens by a distance D 23 on an optical axis thereof so that the following conditional expressions are satisfied:
f 56<0
2< D 23/ D 12<20
0.3<( D 12/ f )×100≦1.03
2<( D 23/ f )×100<10
where f is a focal length of a whole lens system.
2. The imaging lens according to claim 1 , wherein said first lens, said second lens, and said third lens have a composite focal length f 123 so that the following conditional expression is satisfied:
0.5< f 123/ f< 1.5.
3. The imaging lens according to claim 1 , wherein said second lens has a focal length f 2 , and said third lens has a focal length f 3 so that the following conditional expression is satisfied:
−1.3< f 2/ f 3<−0.3.
4. The imaging lens according to claim 1 , wherein said third lens has a focal length f 3 so that the following conditional expression is satisfied:
0.5< f 3/ f< 3.0.
5. The imaging lens according to claim 1 , wherein said third lens has a focal length f 3 , and said fourth lens has a focal length f 4 so that the following conditional expression is satisfied:
−1.0< f 3/ f 4<−0.01.
6. The imaging lens according to claim 1 , wherein said third lens is formed in a shape such that a surface thereof on the object side has a curvature radius R 3 f and a surface thereof on the image plane side has a curvature radius R 3 r so that the following conditional expression is satisfied:
−2.0< R 3 f/R 3 r< 0.6.
7. The imaging lens according to claim 1 , wherein said third lens is disposed away from the fourth lens by a distance D 34 on the optical axis, and said fourth lens is disposed away from the fifth lens by a distance D 45 on the optical axis so that the following conditional expression is satisfied:
0.2< D 34/ D 45<2.0.
8. The imaging lens according to claim 1 , wherein said fourth lens has negative refractive power, and
said fourth lens has an Abbe's number νd 4 so that the following conditional expression is satisfied:
40<ν d 4<75.
9. The imaging lens according to claim 1 , wherein said sixth lens has negative refractive power, and
said sixth lens has a focal length f 6 , and said fourth lens and said fifth lens have a composite focal length f 45 so that the following conditional expression is satisfied:
2.5< f 45/ f 6<7.0.
10. The imaging lens according to claim 1 , wherein said sixth lens has positive refractive power, and
said sixth lens has a focal length f 6 , and said fourth lens and said fifth lens have a composite focal length f 45 so that the following conditional expression is satisfied:
−0.5< f 45/ f 6<−0.1.