Superwide-angle lens system
A superwide-angle lens system includes a negative first lens group, an aperture diaphragm, and a positive second lens group. The first lens group includes two negative meniscus lens elements, and a positive lens element. The second lens group includes a cemented lens having negative and positive lens elements; and a positive lens element. The following conditions (1) and (2) are satisfied: −1.45<f 12 /f<−1.15 (1) 1.4<SF 2<2.4 (2) f 12 : the combined focal length of the two the negative meniscus lens elements in the first lens group, f: the focal length of the lens system, SF 2 : the shaping factor of the negative meniscus lens element on the image side in the first lens group.
1. A superwide-angle lens system comprising a negative first lens group, an aperture diaphragm, and a positive second lens group, in that order from the object side,
wherein said first lens group is provided with a negative meniscus lens element having a convex surface on the object side, a negative meniscus lens element having a convex surface on the object side, and a positive lens element, in that order from the object side,
wherein said second lens group is provided with a cemented lens having a negative lens element and a positive lens element; and a positive lens element, in that order from the object side, and
wherein the following conditions (1) and (2) are satisfied:
−1.45 <f 12 /f<− 1.15 (1), and
1.4 <SF 2<2.4 (2), wherein
f 12 designates the combined focal length of the two said negative meniscus lens elements provided in said first lens group,
f designates the focal length of the entire said superwide-angle lens system,
SF 2 designates the shaping factor of the negative meniscus lens element provided on the image side of the two said negative meniscus lens elements provided in said first lens group,
SF 2=( r 21 +r 22)/( r 21 −r 22),
r 21 designates the radius of curvature of the surface on the object side of said negative meniscus lens element provided on the image side, and
r 22 designates the radius of curvature of the surface on the image side of said negative meniscus lens element provided on the image side.
2. The superwide-angle lens system according to claim 1 , wherein the following condition (3) is satisfied:
1.75 <nd 1 (3), wherein
nd1 designates the refractive index at the d-line of the negative meniscus lens element provided on the object side of the two said negative meniscus lens elements.
3. The superwide-angle lens system according to claim 1 , wherein the following condition (4) is satisfied:
−0.5 <SF 6<0.1 (4), wherein
SF 6 designates the shaping factor of the positive lens element provided on the image side of said second lens group,
SF 6=( r 61 +r 62)/( r 61 −r 62),
r 61 designates the radius of curvature of the surface on the object side of said positive lens element provided on the image side of said second lens group, and
r 62 designates the radius of curvature of the surface on the image side of said positive lens element provided on the image side of said second lens group.
4. The superwide-angle lens system according to claim 1 , wherein the following condition (5) is satisfied:
0.9 <fG 1 /fG 2<1.8 (5), wherein
fG 1 designates the focal length of said first lens group, and
fG 2 designates the focal length of said second lens group.
5. The superwide-angle lens system according to claim 1 , wherein said superwide-angle lens system comprises a fisheye lens system.