Optical: systems and elements – Lens – Reverse telephoto
Reexamination Certificate
2001-06-28
2003-09-16
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Reverse telephoto
C359S793000
Reexamination Certificate
active
06621645
ABSTRACT:
INCORPORATION BY REFERENCE
The disclosures of the following priority application is herein incorporated by reference:
Japanese Patent Application No. 11-343694 filed Dec. 2, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wide-angle lens and, in particular, relates to a super wide-angle lens having a large relative-aperture and a large angle of view.
2. Description of Related Art
In conventional projection method, when the image height is denoted by y, the focal length of a lens is denoted by f, and an angle formed by an object measured from the optical axis is denoted by &thgr;, the expression y=f·tan &thgr; is satisfied. In this method, although a proposal of a super wide-angle lens having the inclusive angle (angle of view) of 2 &ohgr;=110° or more has been rare, and a proposal of a super wide-angle lens having the relative-aperture of F 3.5 or less has been very rare, lenses proposed by, for example, Japanese Patent Application No. 9-113798 and by Japanese Patent Application No. 9-113800 filed by the same assignee of the present application have been known.
Moreover, although a proposal of a super wide-angle lens having the inclusive angle of 2 &ohgr;=115° or more with the conventional projection method, which satisfies y=f·tan &thgr;, and having the relative-aperture of F 3.5 or less has been very rare, Japanese Patent Application No. 10-325923 filed by the same assignee of the present application, for example, has proposed a lens having the inclusive angle of 2 &ohgr;=115° or more and the relative-aperture of F 3.5 or less.
However, a super wide-angle lens having the inclusive angle of 2 &ohgr;=115° or more with the conventional projection method, which satisfies y=f·tan &thgr;, and having the relative-aperture of about F 2.8 has not been proposed, and not been commercialized. In this situation with the present inventor's exertion and diligence, an optical system is obtained that has the inclusive angle (angle of view) of 2 &ohgr;=118° or more with the conventional projection method, which is almost limit with the projection method and has not existed, and also has a large relative-aperture of F 2.8.
Problems for developing such an optical system are compactness to be able to use regularly, high optical performance, securing a peripheral quantity of light, and productivity of an aspherical surface. It is particularly important whether an aspherical lens to be used can be produced by using current mass production technique or not. In considering manufacturing method, instead of using a fine ground aspherical surface, which has not good productivity, to be able to use a glass molding method, which has high productivity, is directly connected to reducing costs, and is even advantageous to the user. In considering from this point of view, the aforementioned Japanese Patent Application No. 9-113798 or Japanese Patent Application No. 9-113800 proposed an optical system having a field of view of 2 &ohgr;=105.6° and the relative-aperture of F 2.87. However, aspherical lenses proposed in these optical systems have been difficult to be manufactured by using a fine ground method or a glass molding method and, as a result, productivity has been low. The angle of view has been only about 105°, so that it has been insufficient. If the angle of view is made wider without taking measures, then the aspherical lens will become even more difficult to be manufactured.
Japanese Patent Application No. 10-325923 proposed a large relative-aperture super wide-angle lens having the angle of view of 2 &ohgr;=118° and the relative-aperture of F 2.9. However, an aspherical lens located to an object side of the lens system was more difficult to be manufactured than the optical system described above, so that the productivity of the lens system was low. Moreover, the optical performance was insufficient, focusing method had a disadvantage, and the size was large.
SUMMARY OF THE INVENTION
The present invention is made in view of the aforementioned problem and has an object to provide a large relative-aperture super wide-angle lens having the inclusive angle (angle of view) of 2 &ohgr;=115° or more with the conventional projection method, which satisfies y=f·tan &thgr;, having the relative-aperture of about F 2.8, having compactness and high optical performance, and having small variation of aberration in relatively closed object distance.
In order to solve the problems described above, the present invention has, in order from the object, a divergent lens group having negative refractive power, and a convergent lens group having positive refractive power. The divergent lens system includes at least one negative aspherical lens element, and the aspherical lens satisfies the following conditional expressions (1) and (2):
0
<
(
d
φ
max
-
d
0
)
/
h
max
(
d
30
-
d
0
)
/
h
30
<
3
(
1
)
−15
<f
asp
/f
0
<−1.7 (2)
where d
0
denotes the thickness of the aspherical lens along the optical axis (center thickness), d
&phgr;max
denotes the thickness of the aspherical lens parallel to the optical axis at the height of the maximum effective aperture on the image side surfacer d
30
denotes the thickness of the aspherical lens parallel to the optical axis at the height of 30% of the maximum effective aperture when the maximum effective aperture on the image side surface is assumed to be is 100%, h
max
denotes the maximum effective radius of the aspherical lens on the image side surface, h
30
denotes the 30% of the maximum effective radius of the aspherical lens when the maximum effective aperture on the image side surface is assumed to be 100%, f
asp
denotes the paraxial focal length of the aspherical lens, and f
0
denotes the focal length of the super wide-angle lens.
In the present invention, there are at least two aspherical surfaces in the divergent lens group, and the most object side aspherical surface preferably has a third order term satisfied with conditional expression (3) when the aspherical surface is expressed by the following aspherical expression (A):
X(y)=(y
2
/r)/[1+(1−&kgr;·y
2
/r
2
)
1/2
]+C
2
·y
2
+C
3
·|y|
3
+C
4
·y
4
+C
6
·y
6
C
8
·y
8
+·y
8
+C
10
·y
10
+C
12
·y
12
+C
14
·y
14
+C
16
·y
16
(A)
1×10
−6
<|C3|<1×10
−2
(3)
where X(y) denotes the distance (sag amount) between the tangent plane to the vertex of the aspherical surface and the aspherical surface along the optical axis at the height y vertical from the optical axis, r denotes the reference radius of curvature, &kgr; denotes a conical coefficient, and Cn denotes the aspherical coefficient of n-th order.
In the present invention, the super wide-angle lens having the inclusive angle of the lens system of 2 &ohgr;=110° or more includes, in order from the object, a divergent lens group having negative refractive power, and a convergent lens group having positive refractive power. The divergent lens group includes, in order from the object, a negative meniscus lens having a convex surface facing to the object side, and a negative aspherical lens having at least one surface located to the image side with an aspherical shape that the radius of curvature becomes weak from the optical axis to the periphery. The aspherical lens satisfies the aforementioned conditional expression (1).
Moreover, the present invention has at least one focusing lens group for focusing to a near-distant object in the convergent lens group having positive refractive power, and the following conditional expression (4) is preferably satisfied:
3
<f
x
/f
0
<12 (4)
where f
x
denotes the focal length of the focusing lens group, or that having the shortest focal length when a plurality of focusing lens groups are there, and f
0
denotes the focal length of the whole system of the super wide-angle lens.
Epps Georgia
Miles & Stockbridge P.C.
Nikon Corporation
Spector David N.
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