Zoom lens and video camera comprising the same

Details Zoom lens and video camera comprising the same Zoom lens and video camera comprising the same

2001-05-30

2003-02-11

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S684000

Reexamination Certificate

active

06519094

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a zoom lens. More specifically, the present invention relates to an aspherical zoom lens that is suitably used for a video camera and has a zoom ratio of about 20 to 23 times, a brightness of an F number of 1.6, low cost and a long back focus.
BACKGROUND ART
In recent years, in the development of zoom lenses, in order to be competitive in the market, it strongly has been demanded that a small size zoom lens having a high resolution power while having a variable power action is realized at low cost. In other words, it is necessary to provide a zoom lens with a high magnification and high resolution power in which the number of lenses to be used is as small as possible. A high magnification zooming is proposed 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, thereby realizing a zoom ratio of about 12 times. Furthermore, JP 9(1997)-311272 A describes a zoom lens including ten lenses, five of which are plastic lenses, thereby realizing a zoom ratio of about 18 times.
However, in a zoom lens having a zoom ratio of about 20 times or more, if a plastic lens is employed, it is necessary to correct the change in the refractive index due to the temperature change of plastic materials, resulting in the increase of the full length of the zoom lens.
DISCLOSURE OF INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a compact and high function zoom lens with a brightness of an F number of 1.6 and with a high zoom ratio of about 20 times or more by providing a lens surface with an optimal power arrangement and an appropriate aspheric effect, and a video camera using the same.
In order to achieve the above-mentioned object, the first zoom lens of the present invention includes a first lens group having a positive refracting power that is fixed, a second lens group having a negative refracting power and varying power by moving along an optical axis, a third lens group having a positive refracting power that is fixed, and a fourth lens group having a positive refracting power and moving along the optical axis so that it keeps an image plane following up the movement of the second lens group and the object at a constant position with respect to the standard plane, the first, second, third and fourth lens groups being disposed from the side near the object to the side far away from the object in this order; wherein the first lens group includes a negative lens, a positive lens, and a positive lens having a convex surface facing the object side being disposed from the object side in this order; the second lens group includes a negative lens and a cemented lens of a negative lens and a positive lens in which the negative lens is located at the object side being disposed from the object side in this order, wherein at least one surface of the lenses is an aspherical surface; the third lens group includes a positive lens and a negative meniscus lens having a convex surface facing the object side being disposed from the object side in this order, wherein at least one surface of the lenses is an aspherical surface, and the fourth lens group includes a cemented lens of a negative lens and a positive lens in which the negative lens is located at the object side, wherein at least one surface of the lens has an aspherical surface; and the following relationships are satisfied:
9.0
<f
1
/
fw
<10.5
1.2
<|f
2
/
fw
|<1.6
4.5
<f
3
/
fw
<6.0
4.0
<f
4
/
fw
<5.5
where f
1
is a composed focal length of the first lens group, f
2
is a composed focal length of the second lens group, f
3
is a composed focal length of the third lens group, f
4
is a composed focal length of the fourth lens group, and fw is a composed focal length of the entire system at a wide-angle end. According to such a zoom lens, it is possible to form a compact zoom lens with an excellent aberration performance and a high magnification of 20 times or more. Furthermore, since an amount of movement at the time of zooming of the second lens group can be suppressed, it is possible to reduce the electric power consumption and to prevent the battery drive time from being shortened.
In the above-mentioned first zoom lens, it is preferable that an aspherical lens of the second lens group satisfies a relationship:
0.6
<r
21
/
r
29
<1.3,
where r
21
is a local radius of curvature in a diameter corresponding to 10% of the lens effective diameter and r29 is a local radius of curvature in a diameter corresponding to 90% of the lens effective diameter. According to such a zoom lens, satisfactory aberration performance to provide the high resolution can be obtained.
Furthermore, it is preferable that an aspherical lens of the third lens group satisfies a relationship:
0.3
<r
31
/
r
39
<1.9,
where r
31
is a local radius of curvature in a diameter corresponding to 10% of the lens effective diameter and r
39
is a local radius of curvature in a diameter corresponding to 90% of the lens effective diameter. According to such a zoom lens, satisfactory aberration performance to provide the high resolution can be obtained.
Furthermore, in the above-mentioned first zoom lens, it is preferable that an aspherical lens of the fourth lens group satisfies a relationship:
0.5
<r
41
/
r
49
<1.1,
where r
41
is a local radius of curvature in a diameter corresponding to 10% of the lens effective diameter and r
49
is a local radius of curvature in a diameter corresponding to 90% of the lens effective diameter. According to such a zoom lens, satisfactory aberration performance to provide the high resolution can be obtained.
Furthermore, it is preferable that the following relationship is satisfied:
0.8
<BF/fw
<1.7,
where fw is a composed focal length of the entire system at a wide-angle end and BF is an air distance between the zoom surface of lens and the image plane. According to such a zoom lens, it is possible to ensure a satisfactory back-focus enabling a low-pass filter such as a crystal filter, an IR cut filter, and the like to be inserted. Furthermore, since the back-focus does not become unnecessarily large, a compact zoom lens can be realized.
Furthermore, it is preferable that a radius of curvature of the surface of a lens disposed closest to the image plane side in the first lens group and a radius, of curvature of the surface of a lens disposed closest to the object side in the second lens group have the same value. According to such a zoom lens, it is possible to prevent the distance between the surface disposed closest to the image plane side of the first lens group and the surface disposed closest to the object side of the second lens group from being reduced, and therefore a lens barrel can be formed easily.
Furthermore, it is preferable that the negative lens of the cemented lens of the second lens group satisfies a relationship:
|{sag (
r
1
)−sag (
r
2
)−
d
8
}/
d
8
|<4.5
where sag (r
1
) denotes a sag amount between the center of the lens on the incident surface of the negative lens of the cemented lens and the position where the incident surface of the negative lens of the cemented lens is brought into contact with the outgoing surface of the negative lens disposed closest to the object side of the second lens group; sag (r
2
) denotes a sag amount between the center of the lens and the outer-most peripheral portion on the outgoing surface on the negative lens of the cemented lens; and d
8
denotes a thickness of a lens. According to such a zoom lens, a biconcave lens can be formed easily, suitably improving the yield.
Next, the second zoom lens of the present invention includes a first lens group having a positive refracting power that is fixed, a second lens group having a negative refracting power and varying power by moving along an optical axis, a third lens group having a positive refracting power that is fixed, and a fourth lens group having a posi

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