Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-11-27
2004-03-23
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06710933
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a zoom lens and a video camera using the same. More specifically, the present invention relates to a high-magnification spherical zoom lens that achieves a high magnification (zoom ratio: 23 times), high brightness (an F number of 1.6), low cost and a long back-focus, as well as to a video camera using the same.
BACKGROUND ART
Conventionally, to reduce the production cost of zoom lenses, plastic materials are used often as a lens material. Besides, in recent years, in the development of zoom lenses, in order to be competitive in the market, a zoom lens having a high resolution power while having a high varying power strongly has been demanded. In other words, it is necessary to provide a zoom lens with a high varying power and high resolution that can be produced at low cost.
A zoom lens that incorporates a plastic lens is disclosed in, for example, JP 8(1996)-106046 A, JP 9(1997)-311272 A. JP 8(1996)-106046 A discloses a zoom lens including ten lenses, four of which are plastic lenses, to provide a zoom ratio of 12 times. Furthermore, JP 9(1997)-311272 A discloses a zoom lens including ten lenses, five of which are plastic lenses, to provide a zoom ratio of 18 times.
However, in a zoom lens having a zoom ratio of 20 times or more, if a plastic lens is employed, the zoom lens incurs a great change in refractive indices of plastic materials due to a temperature change. Thus, since it is difficult to apply plastic lenses in a high-magnification zoom lens, most of lenses composing a zoom lens are glass lenses in the current state.
DISCLOSURE OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a zoom lens that achieves high brightness at an F number of 1.6, a high magnification at a zoom ratio of 23 times, as well as high performance and low cost by applying an optimal power arrangement and an optimal arrangement of plastic lenses, and also to provide a video camera employing the foregoing zoom lens.
To achieve the foregoing object, a zoom lens according to a first aspect of the present invention includes: a first lens group having positive refracting power and being fixed with respect to the image plane; a second lens group having negative refracting power and varying power by moving along an optical axis; a third lens group having positive refracting power and being fixed with respect to the image plane; and a fourth lens group having positive refracting power and moving along the optical axis so that the image plane varied by a movement of the second lens group and a movement of an object is kept at a predetermined position from a reference plane. The first, second, third, and fourth lens groups are arranged in this order from an object side to an image plane side. In the zoom lens, the first lens group includes a negative lens, a positive lens, and a positive meniscus lens arranged from the object side in this order, in which the positive meniscus lens has a convex surface on the object side. The second lens group includes a negative lens, a double-concave lens, and a positive lens arranged from the object side in this order, and includes at least one aspherical surface, in which the double-concave lens and the positive lens are cemented with each other. The third lens group includes a positive lens and a negative plastic lens arranged from the object side in this order, and includes at least one aspherical surface. The fourth lens group includes a negative plastic lens and a positive plastic lens that are arranged from the object side in this order and cemented with each other, and includes at least one aspherical surface. In this zoom lens, the following expression (36) is satisfied:
5<|(
fp
1
+
fp
2
+fp
3
)/
fw|<
12 (36)
where fp
1
represents a focal length of the negative plastic lens of the third lens group, fp
2
represents a focal length of the negative plastic lens of the fourth lens group, fp
3
represents a focal length of the positive plastic lens of the fourth lens group, and fw represents a combined focal length of the entire system at a wide position.
With the above configuration of the zoom lens of the first aspect, it is possible to provide a zoom lens with a high magnification at a zoom ratio of 20 times or more, while balancing various aberrations thereof well. Besides, it is possible to cancel changes in respective refractive indices of plastic lens materials caused by temperature changes, thereby reducing deviations of the position of the image plane.
Furthermore, in the zoom lens according to the first aspect of the present invention, the following expression (37) preferably is satisfied:
7<|(
fp
1
+fp
2
+fp
3
)/
fw|<
10.5 (37)
Using this preferable example, it is possible to cancel changes in the respective refractive indices of the plastic lens materials caused by temperature changes, thereby substantially eliminating deviations of the position of the image plane. In this case, furthermore, the following expressions (38) to (41) preferably are satisfied:
9<
f
1
/
fw<
11 (38)
1
<|f
2
/fw|<
2 (39)
4.5
<f
3
/fw<
6 (40)
4.5<
f
4
/
fw<
6.5 (41)
where f
1
represents a combined focal length of the first lens group, f
2
represents a combined focal length of the second lens group, f
3
represents a combined focal length of the third lens group, and f
4
represents a combined focal length of the fourth lens group.
Using this preferable example, it is possible to make the zobm lens compact, while adjusting the various aberration performances excellently. In this. case, furthermore, the following expression (42) preferably is satisfied:
d
12
×fw<
1.2 (42)
where d
12
represents a distance between the positive lens and the negative plastic lens of the third lens group.
Using this preferable example, a chromatic aberration can be corrected excellently in a zooming range from the wide position to a tele position.
Furthermore, in the zoom lens according to the first aspect of the present invention, the following expression (43) preferably is satisfied:
(sag(
r
1
)+sag(
r
2
)+
d
8
)/
d
8
<4.5 (43)
where sag (r
1
) represents a sag amount between a center of an incident surface of the double-concave lens of the second lens group and a position where the incident surface of the double-concave lens is brought into contact with an outgoing surface of the negative lens disposed on the object side in the second lens group, sag (r
2
) represents a sag amount between a center and an outer-most peripheral portion of the outgoing surface of the double-concave lens, and d
8
denotes a thickness of the double-concave lens.
Using this preferable example, the double-concave lens can be formed readily, whereby the yield thereof can be improved.
Furthermore, in the zoom lens according to the first aspect of the present invention, it is preferable that a radius of curvature of a lens surface closest to the image plane of the first lens group and a radius of curvature of a lens surface closest to the object of the second lens group are equal to each other. Using this preferable example, it is possible to prevent a distance between the surface closest to the image plane of the first lens group and the surface closest to the object of the second lens group from decreasing with increasing proximity to a lens periphery. This facilitates the production of a lens barrel.
Furthermore, in the zoom lens according to the first aspect of the present invention, the following expression (44) preferably is satisfied:
0.6
<BF/fw<
1.1 (44)
where BF represents an air distance between an image-plane-side surface of the lens closest to the image plane and the image plane.
Using this preferable example, it is possible to ensure a back-focus necessary for allowing an infrared cut-off filter or a low-pass filter such as a crystal filter to be inserted. Besides, the back-focus is pre
Boku Kazutake
Ono Shuusuke
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Schwartz Jordan M.
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