Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-01-18
2002-10-01
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S690000, C359S687000, C359S688000
Reexamination Certificate
active
06459536
ABSTRACT:
This application claims benefit of Japanese Patent Application(s) No. 2000-9656 filed in Japan on Jan. 19, 2000, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
So far, a number of zoom lens systems, each comprising two lens groups or a positive and a negative lens group, wherein the space between them is varied for zooming, have been known as effective arrangements for achieving size and cost reductions and capable of zooming with a simple mechanism. Recently developed zoom lens system are increasingly required to have higher zoom ratios than ever before. Prior zoom lens arrangements to meet such requirements are disclosed in JP-A's 9-90220, 9-96761, etc.
These arrangements comprising a relatively small number of lenses have a zoom ratio of 2 to 3, and some of them have a zoom ratio of 3 or greater. To reduce the number of lenses used, the second lens group is composed of two lenses or a positive and a negative lens (one of which is an aspherical lens). By making correction for aberrations in the second lens group, performance is maintained all over the zooming zone. However, there is severe degradation of performance due to decentration in the second lens group, because various aberrations are corrected with two lenses. In addition, the power of the second lens group must be increased because the overall negative power of the diverging second lens group is compensated for by the negative lens in the second lens group. This is unfavorable for correction of aberrations.
Referring to JP-A's 5-119258, 4-22911, etc., a compact yet wide-angle zoom lens system is disclosed. The second lens group is composed of three lenses or a positive, a negative and a negative lens so that various aberrations therein can be corrected. The power of the second lens group is allocated to the three lenses so that the degradation of performance due to decentration can be reduced. However, all three lenses are formed of glass, and so the second lens group is higher in cost than that made up of two lenses. In addition, the back focus is short. This does not only add mechanical constrains to the zoom lens system but also offers several problems such as transfer onto film of dust deposits on the surface of a lens in the vicinity of an image plane, an increase in the diameter of the rear lens, etc. The zoom ratio is far short of 2.
Referring to JP-A 3-267909, etc., the second lens is composed of three lenses or a positive, a negative and a negative lens, with the positive lens being formed of a plastic lens. A plastic lens is excellent in mass productivity, and so has the merit of achieving lower costs as compared with a glass lens. However, a problem with the plastic lens is that its refractive index and shape are prone to large variations depending on ambient temperatures. Accordingly, meticulous care must be taken when the plastic lens is used for a camera's phototaking optical system. To this end, it is often attempted to make the power of the plastic lens weak. However, such care is not found in the example of JP-A 3-267909 because the power of the plastic lens is still strong.
Referring then to JP-A's 5-119258, 10-197793, etc., the second lens group is composed of three lenses or a positive, a negative and a negative lens, with the positive lens being formed of a plastic lens, as is the case with JP-A 3-267909. In consideration of the changes of the plastic lens depending on ambient temperatures, the power of the plastic lens is made weak. However, when the power becomes too weak, the effect on correction of aberrations becomes slender. In addition, the principal point positions of the second lens group are shifted to the object side under the power of the second negative lens, resulting in problems such as a decreased back focus. For this reason, how the power of the second negative lens located at a middle position in the second negative lens is determined is important for power profile. The examples show that the power of the second negative lens group is still strong, resulting in a decreased back focus. This does not only add mechanical constrains to the zoom lens system but also offers several problems such as transfer onto film of dust deposits on the surface of a lens in the vicinity of an image plane, an increase in the diameter of the rear lens, etc. When the power of the second negative lens is too weak, on the other hand, the effect on correction of aberrations becomes slender; in other words, the merit of +−− construction is lost.
SUMMARY OF THE INVENTION
In view of such problems associated with the prior art, an object of the present invention is to provide a compact, low-cost zoom lens system of +− construction, which comprises two lens groups, and an image pickup system using the same.
According to one aspect of the invention, this object is achieved by the provision of a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and a second lens group having negative refracting power, wherein:
said second lens group comprises, in order from an object side thereof, a positive lens component
2
-
1
, a negative lens component
2
-
2
and a negative lens component
2
-
3
, with said lens component
2
-
1
comprising a plastic lens element, and
said second lens group satisfies the following conditions (1) and (2):
1.05
≦f
21
/f
T
<5 (1)
3.8
<f
22
/f
G2
<8 (2)
where f
21
is the focal length of the lens component
2
-
1
in the second lens group, f
22
is the focal length of the lens component
2
-
2
in the second lens group, f
T
is the focal length of the zoom lens system at a telephoto end thereof, and f
G2
is the composite focal length of the second lens group.
According to another aspect of the invention, there is provided a zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having positive refracting power and a second lens group having negative refracting power, wherein:
said second lens group comprises, in order from an object side thereof, a positive lens component
2
-
1
, a negative lens component
2
-
2
and a negative lens component
2
-
3
, with said lens component
2
-
1
comprising a plastic lens element, and
said second lens group satisfies the following conditions (1), (2) and (4):
1.05
≦f
21
/f
T
<5 (1)
3.8
<f
22
/f
G2
<8 (2)
1.01
≦SG
21
<1.24 (4)
where f
21
is the focal length of the lens component
2
-
1
in the second lens group, f
22
is the focal length of the lens component
2
-
2
in said second lens group, f
T
is the focal length of the zoom lens system at a telephoto end thereof, f
G2
is the composite focal length of the second lens group, and SG
21
is the specific gravity of the lens component
2
-
1
in the second lens group.
Why the aforesaid arrangements are used in the invention, and how they work is now explained.
According to the present invention, the zoom lens system comprises a first lens group having positive refracting power and a second lens group having negative refracting power. The second lens group then comprises a positive lens
2
-
1
, a negative lens
2
-
2
and a negative lens
2
-
3
. The positive lens
2
-
1
is formed of a plastic lens. This arrangement is of the simplest two-group construction in zoom lens constructions, and is constructed of the telephoto type so as to achieve size reductions on the telephoto side. By providing the diverging second lens group of +−− construction, and especially allocating the high proportion of negative refracting power to two lenses, it is possible to reduce the influence of decentration produced within the second lens group and make correction for aberrations, especially off-axis coma. By constructing the positive lens
2
-
1
of a plastic lens, size and weight reductions can be achieved.
Referring here to why the plastic lens is used for the lens
2
-
1
rather than for the le
Epps Georgia
Hasan M.
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
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