Imaging lens

Abstract

An imaging lens includes a first lens group having negative refractive power; a stop; and a second lens group having positive refractive power, arranged in the order from the object side to the image plane side. The first lens group includes a first lens having a strong concave surface facing the image plane side and negative refractive power; and a second lens having negative refractive power and a biconcave lens shape near the optical axis. The first lens group has a focal length F1, the second lens group has a focal length F2, the first lens has a focal length f1, and the second lens has a focal length f2 so that the following conditional expressions are satisfied: −1.0< F 1/ F 2<−0.5 0.2< f 1/ f 2<1.5.

What is claimed is: 
     
         1 . An imaging lens comprising:
 a first lens group having negative refractive power;   a stop; and   a second lens group having positive 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 negative refractive power and a second lens having negative refractive power, and   said first lens group has a focal length F1, said second lens group has a focal length F2, said first lens has a focal length f1, and said second lens has a focal length f2 so that the following conditional expressions are satisfied:
   −1.0 <F 1 /F 2<−0.5
 
   0.2 <f 1 /f 2<1.5. 
   
     
     
         2 . The imaging lens according to  claim 1 , wherein said first lens is formed in a shape having a concave surface facing the image plane side, and
 said second lens is formed in a shape having concave surfaces near an optical axis thereof.   
     
     
         3 . The imaging lens according to  claim 1 , wherein said second lens group includes a third lens having positive refractive power and a lens group, and
 said lens group is composed of a lens having positive refractive power and a lens having negative refractive power, and   said lens group has negative refractive power as a whole.   
     
     
         4 . The imaging lens according to  claim 3 , wherein said third lens has a focal length f3 so that the following conditional expression is satisfied:
   1.0 <f 3 /f< 2.5   where f is a focal length of a whole lens system.   
     
     
         5 . The imaging lens according to  claim 3 , wherein said lens having positive refractive power of the lens group of the second lens group has a focal length fp and said lens having negative refractive power of the lens group of the second lens group has a focal length fn so that the following conditional expression is satisfied:
   0.7 <|fp/fn|< 1.5   
     
     
         6 . An imaging lens comprising:
 a first lens group having negative refractive power;   a stop; and   a second lens group having positive 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 negative refractive power and a second lens having negative refractive power,   said second lens group includes a third lens having positive refractive power and a lens group, and   said lens group is composed of a lens having positive refractive power and a lens having negative refractive power,   said lens group has negative refractive power as a whole, and   said first lens group has a focal length F1, said second lens group has a focal length F2, said lens having positive refractive power of the lens group of the second lens group has a focal length fp, and said lens having negative refractive power of the lens group of the second lens group has a focal length fn so that the following conditional expressions are satisfied:
   −1.0 <F 1 /F 2<− 0 . 5 
 
   0.7 <|fp/fn| <1.5. 
   
     
     
         7 . The imaging lens according to  claim 6 , wherein said first lens is formed in a shape having a concave surface facing the image plane side, and
 said second lens is formed in a shape having concave surfaces near an optical axis thereof.   
     
     
         8 . The imaging lens according to  claim 6 , 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:
   0.2 <f 1 /f 2<1.5.   
     
     
         9 . The imaging lens according to  claim 6 , wherein said third lens has a focal length f3 so that the following conditional expression is satisfied:
   1.0 <f 3 /f< 2.5   where f is a focal length of a whole lens system.