Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1998-08-31
2001-10-23
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06307685
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens, and specifically to a zoom lens having a variable magnification ratio from about 2.5 to 6, which is appropriate for an electronic still camera, video camera, or similar device, using a solid-state image sensor such as a CCD, or the like.
Recently, the development of software to process image data is remarkable with the technological advance or the spread of a personal computer, and a demand for an electronic still camera, used for image reading into a personal computer, increases.
As a zoom lens for a solid-state image sensor, conventionally, the zoom lens of a 3-group composition or 4-group composition is disclosed. As a zoom lens for a video camera, it has a tendency toward high variable magnification, and the zoom lens of 4-group composition is widely used. As a zoom lens for an electronic still camera, high resolution and high image quality are required, and the zoom lens having about 3 times-variable magnification ratio, which is a ratio of the focal length at the wide angle end and that at the telephoto end, is widely used, and 3-group composition is also disclosed for the zoom lens having about 3 times-variable magnification ratio.
Conventionally, the following are disclosed for the zoom lens having 3-group composition: Japanese Patent Publication Open to Public Inspection Nos. 79716/1989, 291515/1990, 89309/1991, 106512/1992, 307509/1992, 203875/1993, 271788/1996, etc.
However, in these conventional examples, there is a problem that the overall length of the lens is long in proportion to its variable magnification ratio in a zoom lens in which zooming is conducted by moving the second lens group and the third lens group, in a 3-group lens composed of positive, negative, and positive lens groups. Further, distortion at the wide angle end exceeds about −5%; the distortion is small, but the overall length of the lens is too large; or correction of the spherical aberration or astigmatism is insufficient.
Further, in the zoom lens whose aberration is sufficiently corrected, in order to have about 3 times-variable magnification ratio, cost is higher because it is necessary that the number of component lenses is 11-12, or many aspherical glass lenses whose production cost is relatively high, are used.
Further, in an example in which priority is given to cost reduction, all lenses are made of plastic, and variable magnification ratio is made large, therefore, there are problems in that the overall length of the lens is extremely increased, and it is necessary to compensate for the influence of temperature variation by any method except for lens, or because many aspherical surfaces are used, the sensitivity of performance deterioration, caused by eccentricity of the lens, is high, and assembling becomes difficult.
SUMMARY OF THE INVENTION
The present invention is accomplished in view of the foregoing problems, and an object of the present invention is to provide a zoom lens whose overall length is reduced while its variable magnification ratio is kept, without deteriorating the aberration, and further, to provide a digital still camera, video camera, etc., having the zoom lens. Another object of the present invention is to provide a compact zoom lens in which the number of component lenses is not more than 10, and various aberrations including distortion at the wide angle end, spherical aberration, or the like, is completely corrected, and which has a variable magnification ratio of 2.5 times or more.
Further object of the present invention is to provide a zoom lens in which, although it is compact and has the fewer number of component lenses, the performance deterioration due to lens eccentricity is smaller, and assembling operations are easy.
Still further object of the present invention is to provide a compact zoom lens in which plastic lenses are effectively used, and influence due to temperature variation is smaller, and which is inexpensive and has high performance.
The above objects can be attained by one of the following structures. That is, a zoom lens, which has a first lens group having a positive focal length, a second lens group having a negative focal length, and a third lens group having a positive focal length; in which in the order from an object side, the first lens group, the second lens group, and the third lens group are provided, and no other lens group is provided, and at the variable magnification, the second lens group and the third lens group are moved in the direction of optical axis; and which satisfies the following condition (0);
3.0
<Lt
/(
L{square root over (Z)}
)<6.0 (0)
where, Lt represents a distance from a lens surface located closest to the object side of the first lens group, to the image plane; L represents a diagonal length of the image plane; and Z represents a variable magnification ratio which is a ratio of the focal length at the wide angle end to that at the telephoto end of the zoom lens.
In this connection, when the upper limit of the condition (0) is exceeded, the overall length of the lens is too long, which is not desirable. When the condition (0) is lower than its lower limit, the refracting power of each lens group is increased, and correction of each aberration is difficult, which is not desirable.
Generally, a diagonal length of the image plane can be considered to be almost the same as that of a portion of an image sensor, or can be unconditionally determined by a field angle and the focal length. That is, a lens of the present invention is a zoom lens which has at least 3 lens groups of, in the order from an object side, the first lens group having a positive focal length; the second lens group having a negative focal length; and the third lens group having a positive focal length, and which conducts variable magnification by moving the second lens group and the third lens group, wherein the zoom lens satisfies the following condition,
0.60<(&bgr;
2T
/&bgr;
2W
)·(&bgr;
3W
/&bgr;
3T
)<4.0 (1)
where &bgr;
2T
, represents a lateral magnification of the second lens group at a telephoto end, &bgr;
2W
represents a lateral magnification of the second lens group at a wide angle end, &bgr;
3T
represents a lateral magnification of the third lens group at the telephoto end, and &bgr;
3W
represents a lateral magnification of the third lens group at the wide angle end, and wherein a variable magnification ratio representing a ratio of a focal length at the wide angle end to a focal length at the telephoto end, is not less than 2.5.
Further, a zoom lens which has at least 3 lens groups of, in the order from an object side, the first lens group having a positive focal length; the second lens group having a negative focal length; and the third lens group having a positive focal length, and which conducts variable magnification by moving the second lens group and the third lens group, wherein the zoom lens satisfies the following conditions,
0.60<(&bgr;
2T
/&bgr;
2W
)·(&bgr;
3W
/&bgr;
3T
)<1.2 (2)
30°<&ohgr;
W
(3)
where &bgr;
2T
represents a lateral magnification of the second lens group at a telephoto end, &bgr;
2W
represents a lateral magnification of the second lens group at a wide angle end, &bgr;
3T
represents a lateral magnification of the third lens group at the telephoto end, and &bgr;
3W
represents a lateral magnification of the third lens group at the wide angle end, and &ohgr;
W
represents a half field angle at the wide angle end.
Still further, the third lens group includes at least 3 lenses and at least one negative lens, and at least one surface of which is aspherical.
Further, a zoom lens which has at least 3 lens groups of, in the order from an object side, the first lens group having a positive focal length; the second lens group having a negative focal length; and the third lens group having a positive focal length, and which conducts variable magnification by moving the second lens group and the third lens group, wherein the zoom lens satisfies the following conditions
Mori Nobuyoshi
Sato Masae
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Schwartz Jordan M.
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