Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-05-18
2002-06-11
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06404562
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens, and in particular, to a small-sized 2-group zoom lens which has less restriction of back focus and is suitable for a camera of a lens shutter type, and to a camera.
As a type of a zoom lens, there has been a 2-group zoom lens. The 2-group zoom lens is composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, and is simple in structure and has a short total length. Due to this, the 2-group zoom lens is used widely for a camera such as a lens shutter type camera.
The 2-group zoom lens is classified into a negative lens preceding type and a positive lens preceding type, depending on whether the first lens is positive or negative. The positive lens preceding type is suitable for focal length range in a telephoto system because it is easy to make a telephoto ratio to be small.
However, it has been difficult to make the 2-group zoom lens to be a compact zoom lens wherein aspects of cost are taken into consideration, because of the reason that the 2-group zoom lens of a positive lens preceding type has many constituting lenses and has many glass lens structures.
The invention has been achieved in view of the problems stated above, and its object is to provide a zoom lens which is corrected in terms of various aberrations, and is compact in structure, low in cost, excellent in image-forming performance and is suitable for a lens shutter type camera.
Further, the invention makes it possible to correct chromatic aberration satisfactorily even when many plastic lenses are used and to compensate changes in focal length and back focus caused by temperature fluctuation.
In a conventional 2-group zoom lens employing a plastic lens, there has been a problem that the number of structural lenses is large when trying to obtain a large zoom ratio, resulting in complicated structure and an increase in cost. In particular, when many plastic lenses are used in a 2-group zoom lens, for the purpose of low cost, a large ratio of magnification tends not to be obtained, compared with an occasion wherein many glass lenses which are relatively expensive are used, resulting in poor balance between cost and function.
Another object of the invention is to provide a zoom lens which is of low cost, corrected satisfactorily in terms of various aberrations even when a zoom ratio is large, and is suitable for a lens shutter type camera.
SUMMARY OF THE INVENTION
The objects stated above can be attained by either one of the following structures.
Structure (1-1) A zoom lens to form an image of an object with variable magnification, comprises:
a first lens group having a positive refracting power, and
a second lens group positioned closer to the image than the first lens group and having a negative refracting power,
wherein, when the magnification is changed, a distance between the first lens group and the second lens group is changed,
wherein the first lens group comprises a 1-1 lens, a 1-2 lens and a 1-3 lens aligned in this order from the object, and
wherein the zoom lens comprises at least two plastic lenses.
Structure (1-2) In the zoom lens of Structure (1-1), the first lens group does not comprise a 1-4 lens and the second lens group comprises a 2-1 lens and a 2-2 lens aligned in this order from the object, and the 1-1 lens is a plastic lens, the 1-3 lens has a positive refracting power and the 2-2 lens has a negative refracting power.
Structure (1-3) In the zoom lens of Structure (1-2), the 1-2 lens has a negative refracting power and following formulas are satisfied:
1.50≦n
1-3
≦1.90
1.50≦n
2-2
≦1.90
where n
1-3
is a refractive index of the 1-3 lens, and n
2-2
is a refractive index of the 2-2 lens.
Structure (1-4) In the zoom lens of Structure (1-2), the 1-2 lens is a plastic lens, at least four surfaces of the lenses in the entire system of the zoom lens is an aspherical surface, and a following formula is satisfied:
1.50≦n
2-2
≦1.90
where n
2-2
is a refractive index of the 2-2 lens.
Structure (1-5) In the zoom lens of Structure (1-2), the 1-2 lens is a plastic lens and following formulas are satisfied:
1.50≦n
1-3
≦1.90
1.50≦n
2-2
≦1.90
where n
1-3
is a refractive index of the 1-3 lens, and n
2-2
is a refractive index of the 2-2 lens.
Structure (1-6) In the zoom lens of Structure (1-2), the 1-1 lens is a plastic lens having a positive refractive power, the 1-2 lens is a plastic lens, the 1-3 lens is a meniscus shaped glass lens having a positive refractive power wherein an image side surface of the meniscus shaped glass lens is concave, and the 2-2 lens is a plastic lens having a negative refractive power.
Structure (1-7) In the zoom lens of Structure (1-2), the 2-1 lens is a glass lens.
Structure (1-8) In the zoom lens of Structure (1-2), the 1-4 lens has at least one aspherical surface and following formulas are satisfied:
0.64≦f
W
/D≦1.0
0.50≦|f
RC
|/f
W
≦0.80
where f
W
is a focal length of the zoom lens at a wide angle end, D is a length of a diagonal line of an image plane, and f
RC
is a focal length of the second lens group.
Structure (1-9) In the zoom lens of Structure (1-2), the 1-1 lens is a positive refracting power and has at lease one aspherical surface, the 2-1 lens is a plastic lens and has at lease one aspherical surface, and a following formula is satisfied:
−5≦(&phgr;
P
+&phgr;
4
)·f
T
2
/F
T
≦5
where &phgr;
P
is a power of a plastic lens of the first lens group, &phgr;
4
is a power of the 2-1 lens, f
T
is a focal length of the zoom lens at a telephoto end and F
T
is a F-number of the zoom lens at a telephoto end.
Structure (1-10) In the zoom lens of Structure (1-1), the first lens group comprises a 1-4 lens which is a glass lens having a positive refracting power and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the first lens group comprises at least one plastic lens having a negative refracting power and at least one plastic lens having a positive refracting power.
Structure (1-11) In the zoom lens of Structure (1-1), the magnification ratio is 2.3 or more, the first lens group comprises a 1-4 lens and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the first lens group comprises at least one plastic lens having a negative refracting power and at least one plastic lens having a positive refracting power.
Structure (1-12) In the zoom lens of Structure (1-1), the first lens group is separated by the largest air space into a 1-a lens sub-group and a 1-b lens sub-group, and the second lens group comprises a 2-1 lens and a 2-2 lens, and wherein the 1-a lens sub-group comprises at least two lenses comprising at least one plastic lens having a negative power and the 1-b lens sub-group comprises at least two lenses comprising at least one plastic lens having a positive power
Structure (1-13) In the zoom lens of Structure (1-1), the first lens group comprises at least one glass lens and at least three plastic lenses and the second lens group comprises at most two lenses comprising at least one plastic lens.
Structure (1-14) In the zoom lens of Structure (1-1), the first lens group comprises at least two glass lenses and at least two plastic lenses, the second lens group comprises a 2-1 lens and a 2-2 lens and a following formula is satisfied:
−0.50≦|f
RC
|/f
2-1
≦0.20
where f
RC
is a focal length of the second lens group and f
2-1
is a focal length of the 2-1 lens.
Structure (1-15) In the zoom lens of Structure (1-1), all of the plastic lenses of the zoom lens have a saturated water absorption of 1.2% or less.
Structure (1-16) In the zoom lens of Structure (1-1), all of the plastic lenses of the zoom lens have a refractive index of 1.5 or more.
Structure (1-17) In the zoom lens of Structure (1-1), the 1-1 lens is a plastic lens having a positive refracting power.
Structure (1-18) A camera to photograph an image of an object with variable magnification, comp
Ota Kohei
Sato Masae
Yokota Minoru
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Konica Corporation
Schwartz Jordan M.
LandOfFree
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