Fisheye lens system

1. A fisheye lens system comprises a negative front lens group, a diaphragm, and a positive rear lens group, in this order from an object; and said fisheye lens system has a half angle-of-view of 90°,

wherein said negative front lens group comprises at least three negative meniscus lens elements each of which has the convex surface facing toward the object; and

wherein said three negative meniscus lens elements satisfy the following conditions:

0.2<SF (i=1) <0.6

0.8 <SF (i=2) /SF (i=1) <1.5

0.8 <SF (i=3) /SF (i=1) <2.0

wherein

SFi designates the shaping factor of the i th (1≦i≦3) negative meniscus lens element (SFi=(R 1 i −R 2 i )/(R 1 i +R 2 i ));

R 1 i designates the radius of curvature of the object-side surface of the i th (1≦i≦3) negative meniscus lens element; and

R 2 i designates the radius of curvature of the image-side surface of the i th (1≦i≦3) negative meniscus lens element.

2. The fisheye lens system according to claim 1 , further comprising a fourth negative meniscus lens element having the convex surface facing toward the object, and

wherein said fourth negative meniscus lens element is provided on the image side of the third (i=3) negative meniscus lens element.

3. The fisheye lens system according to claim 2 , wherein said fourth negative meniscus lens element satisfies the following condition:

0.8 <SF (i=4) /SF (i=1) <2.5

wherein

SFi designates the shaping factor of the i th negative meniscus lens element (SFi=(R 1 i −R 2 i )/(R 1 i +R 2 i ));

R 1 i designates the radius of curvature of the object-side surface of the i th negative meniscus lens element; and

R 2 i designates the radius of curvature of the image-side surface of the i th negative meniscus lens element.

4. The fisheye lens system according to claim 1 , wherein each of said negative front lens group and said positive rear lens group further comprises at least one set of cemented lens elements constituted by a positive lens element and a negative lens element.

5. The fisheye lens system according to claim 1 , satisfying the following conditions:

−0.5 <f/fF<− 0.1

0.75 <{−Δy (90)/Δθ(90)}/( N OB ·f )<1.5

wherein

fF designates the focal length of said negative front lens group;

f designates the focal length of the entire the fisheye lens system;

Δθ (90) designates an increment (differential amount) of the angle-of-view from the half angle-of-view of 90° (rad);

Δy(90) designates an increment of an image height (differential amount) when the angle-of-view is increased by Δθ (90) from the half angle-of-view of 90°; and

N OB designates the refractive index of air, i.e., on the object side of said negative front lens group.

6. The fisheye lens system according to claim 1 , satisfying the following condition:

3.2 <fB/f< 5.0

wherein

fB designates the back focal distance (the equivalent air thickness from the most image-side surface of said positive rear lens group to the imaging plane);

f designates the focal length of the entire fisheye lens system.