Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2001-12-21
2003-11-11
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S687000
Reexamination Certificate
active
06646815
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens, and in particular, to a zoom lens which is used for a digital still camera or for a video camera, and especially has a high resolving power suitable for a camera employing a high-density pixel type CCD, and has F number of about 2.8 at a wide-angle end and a ratio of variable magnification of about 2-3.
With the recent spread of personal computers and of handling of image data by the personal computers, digital still cameras for taking in image data have been demanded increasingly. Further, with a trend for densification of CCD, a demand for the zoom lens having higher image forming power has been increased. In addition, a compact one which is handy for carrying is demanded.
Zoom lenses with a magnification of about 3 which are suitable for a camera employing a solid-state image sensor such as CCD or the like have so far been disclosed in terms of technology, and examples thereof disclosed in TOKKAIHEI Nos. 10-39214, 11-52246 and 11-287953 and TOKKAI No. 2000-9997 are known.
However, in these conventional examples, there have been problems that distortion at a wide-angle end is extremely large, an angle of view at a wide-angle end is small and a total lens length is great.
SUMMARY OF THE INVENTION
The invention has been achieved in view of the problems stated above, and its object is to provide a zoom lens which has a ratio of variable magnification of about 2-3 as well as high image forming power and is suitable to be used for a digital still camera and a video camera both employing CCD of a high density pixel type.
The object mentioned above can be attained by either one of the following structures.
Structure (1)
A zoom lens having therein three lens groups including a first lens group having a negative refracting power, a second lens group having a positive refracting power and a third lens group having a positive refracting power in this order from the object side, in which the second lens group is composed of a positive lens, a cemented lens including a positive lens and a negative lens and one lens in this order from the object side, and when changing a magnification from the wide-angle side to the telephoto side, the first, second and third lens groups are moved on an optical axis so that a distance between the first lens group and the second lens group is reduced and a distance between the second lens group and the third lens group is increased for the magnification change, wherein the following conditional expression is satisfied;
0.4<
f
w
/f
2
<1.0 (1)
wherein, f
2
represents a focal length of the second lens group and f
w
represents a focal length of the total optical system at a wide-angle end.
Structure (2)
The zoom lens described in (1) wherein the first lens group has at least one aspheric surface and is composed of two pieces including a negative lens and a positive lens in this order from the object side, and the following conditional expression is satisfied;
−0.7<
f
w
/f
1
<−0.3 (2)
wherein, f
1
represents a focal length of the first lens group.
Structure (3)
The zoom lens described in (2) wherein the first lens group is composed of two pieces including a negative meniscus lens whose concave surface faces toward the image side and a positive meniscus lens whose convex surface faces toward the object side in this order from the object side.
Structure (4)
The zoom lens described in (1) wherein the first lens group is composed of three pieces including two negative lenses and one positive lens, and the following conditional expression is satisfied.
−0.7<
f
w
/f
1
<0.3 (2)
Structure (5)
The zoom lens described in (4) wherein the first lens group is composed of three pieces including a negative meniscus lens whose concave surface faces toward the image side, a negative lens and a positive lens in this order from the object side.
Structure (6)
The zoom lens described in (5) wherein the first lens group is composed of a negative meniscus lens whose concave surface faces toward the image side and a cemented lens including a negative lens and a positive meniscus lens whose convex surface faces toward the object side in this order from the object side.
Structure (7)
The zoom lens described in either one of (1)-(6) wherein the second lens group has an aspheric surface on at least one surface.
Structure (8)
The zoom lens described in (7) wherein at least one surface of the positive lens closest to the object in the second lens group has an aspheric surface.
Structure (9)
The zoom lens described in either one of (1)-(8) wherein the following conditional expression is satisfied;
|
f
w
/f
2i
|<0.5 (3)
wherein, f
2i
represents a focal length of the lens closest to the image in the second lens group.
Structure (10)
The zoom lens described in either one of (1)-(9) wherein the lens closest to the image in the second lens group is a meniscus lens whose convex surface faces toward the image side, and the following conditional expression is satisfied.
|
f
w
/f
2i
|<0.4 (4)
Structure (11)
The zoom lens described in either one of (1)-(10) wherein the lens closest to the image in the second lens group is a positive lens, and the following conditional expression is satisfied.
0.0<
f
w
/f
2i
<0.4 (5)
Structure (12)
The zoom lens described in either one of (1)-(10) wherein the lens closest to the image in the second lens group is a negative lens, and the following conditional expression is satisfied.
−0.3<
f
w
/f
2i
<0.0 (6)
Structure (13)
The zoom lens described in either one of (1)-(12) wherein the following conditional expression is satisfied.
0.5<
f
w
/f
2
<0.7 (7)
Structure (14)
The zoom lens described in either one of (1)-(13) wherein the third lens group is a single lens having a positive refracting power.
Structure (15)
The zoom lens described in (14) wherein the third lens group is a single lens having a positive refracting power of not less than 50 in terms of Abbe number.
Structure (16)
The zoom lens described in either one of (1)-(15) wherein the third lens group simply moves on an optical axis toward the object side when changing a magnification from the wide-angle side to the telephoto side, and satisfies the following conditional expression;
0.2<
f
w
/f
3
<0.7 (8)
wherein, f
3
represents a focal length of the third lens group.
Structure (17)
The zoom lens described in either one of (1)-(16) wherein the third lens group conducts focusing to cover from the object at the infinite point to the object at the short distance, and satisfies the following conditional expression.
0.2<
f
w
/f
3
<0.7 (8)
In the zoom lens of the invention, a first lens group having a negative refracting power, a second lens group having a positive refracting power and a third lens group having a positive refracting power are provided, and when zooming from the wide-angle end to the telephoto end, each lens group is moved so that a distance between the first lens group and the second lens group is reduced and a distance between the second lens group and the third lens group is increased. When the second lens group is composed of a positive lens, a cemented lens including a positive lens and a negative lens and one lens in this order from the object side, a magnification can be changed effectively and the total optical system can be made to be compact.
To be more effective, it is preferable that the first lens group is structured by a negative meniscus lens whose concave surface faces toward the image side and by a positive meniscus lens whose convex surface faces toward the object side in this order from the object side, and has an aspheric surface, and this structure makes it possible to reduce the number of lenses and thereby to correct satisfactorily the distortion that is easily caused at the wide-angle side. Or, it is preferable that the first lens group is structured by a negative meniscus lens whose
Choi William
Epps Georgia
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
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