Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-12-17
2001-07-31
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S689000, C359S795000
Reexamination Certificate
active
06268966
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens system, and particularly to a two-group zoom lens system applicable to cameras, such as compact cameras that require shorter back focal distance than single lens reflex cameras.
Conventionally, there has been known a two-group zoom lens system for compact cameras including, in order from an object side, a positive first lens group and a negative second lens group. The first and second lens groups are movable along an optical axis for zooming.
In such a two-group zoom lens system, the larger the zoom ratio is, the greater aberrations are. Particularly, chromatic aberration becomes significantly larger.
Further, zoom lens systems having three or more lens groups have been known, while these zoom lens systems are complex in optical and mechanical constructions as compared with the two-group zoom lens system.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved zoom lens system that has a zooming ratio of approximately 3.0, and has sufficient optical performance with compact size.
For the above object, according to the present invention, there is provided a zoom lens system that includes a positive first lens group and a negative second lens group that are arranged from an object side. The lens groups move along an optical axis to change the distance therebetween forzooming. The first lens group includes a negative first lens unit and a positive second lens unit that are arranged from the object side and each of the first and second units of the first lens group includes at least one negative lens. The zoom lens system of the present invention satisfies the following conditions (1), (2), (3) and (4);
−3.1<f
t
/f
1a
<−2.5 (1)
5.8<f
t
/f
1b
<7.0 (2)
1.85<n
1an
(3)
1.85<n
1bn
(4)
where
fis the longest focal length of the entire system,
f
1a
is a focal length of the first lens unit;
f
1b
is a focal length of the second lens unit;
n
1an
is a refractive index of the negative lens in the first lens unit; and
n
1bn
is a refractive index of the negative lens in the second lens unit.
With this construction, the zoom lens system implements a zooming ratio of approximately 3.0 and sufficient optical performance with a compact two-group construction.
It should be noted that the longest gap between the lenses of the first lens group divides the first and second units of the first lens group.
Preferably, the first lens unit may include a positive lens at the most object side and the following condition (5) may be satisfied;
r
2
<0 (5)
where
r
2
is radius of curvature of an image side surface of the positive lens at the most object side in the first lens unit.
Further, the first lens unit may consist of the positive lens and a negative lens that are arranged from the object side, and the second lens unit may consist of a positive lens and a cemented lens of positive and negative lenses that are arranged from the object side. The order of the positive and negative lenses in the cemented lens is not limited to the above order, and another order can be used.
Still further, the second lens group may consist of a positive lens and a negative lens that are arranged from the object side. In such a case, the negative lens in the second lens group may be a meniscus lens whose concave surface is directed to the object side.
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Ishii Shinichiro
Ito Takayuki
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Lester Evelyn A
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