Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
1998-12-01
2001-05-22
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S689000, C359S686000
Reexamination Certificate
active
06236522
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a photographic optical system and, more particularly, to a photographic optical system used in a camera for photography.
Photographic optical systems used in cameras for photography in recent years, particularly zoom optical systems are demanded to provide a high zoom ratio and to be compact in size. As the zoom ratio becomes higher, it becomes likelier that variations in aberrations, particularly variations in chromatic aberration, will limit the improvement in performance by aberration correction. Attempts have heretofore been made to improve chromatic aberration, for example, by using ED lenses.
ED lenses are low-refractive index and low-dispersive glass and have anomalous partial dispersion. Therefore, they allow removal of the secondary spectrum, which has heretofore been impossible to remove by the conventional optical glass alone.
Many examples are known as the prior art that uses ED lenses, including Japanese Patent Application Unexamined Publication (KOKAI) Nos. 8-160298, 4-95912, 3-260611 and 3-158816.
The above-mentioned prior art attains a compact zoom optical system by effectively using ED lenses in a lens arrangement comprising a minimal number of lens elements. Furthermore, the final lens unit is formed from a single negative lens having a concave surface directed toward the object side, thereby allowing the lens unit to come closer to the forward end of the lens system when it is collapsed. Accordingly, the arrangement is also favorable for achieving a reduction in the size of the optical system.
However, all these conventional zoom optical systems have a zoom ratio of 2 or less. Accordingly, the specifications of these zoom optical systems cannot be said to be attractive to users who demand zoom optical systems of high zoom ratio in recent years. Meanwhile, zoom optical systems designed to attain a high zoom ratio have an unfavorably large number of constituent lens elements, which is contrary to the purpose of attaining a compact optical system and likely to cause the cost to rise.
ED lenses are soft and readily scratchable in comparison to ordinary optical glasses and have a relatively large specific gravity (3.5 g/cm
3
or more). Therefore, when ED lenses are used, it is necessary to consider that if a large number of ED lenses are used, the cost rises, and the optical system becomes heavy.
SUMMARY OF THE INVENTION
In view of the above-described problems with the prior art, an object of the present invention is to provide a photographic optical system which is greatly improved in chromatic aberration and capable of providing high image quality with a compact, lightweight and low-cost arrangement having a minimal number of constituent elements.
Another object of the present invention is to provide a photographic lens system arranged as a zoom optical system, in particular, which is designed so that variations in chromatic aberration due to zooming are minimized and the focal length at the telephoto end, in particular, is increased to a considerable extent despite a lens arrangement which is similar to the conventional lens arrangement and has the number of constituent lens elements that is similar to that of the conventional lens arrangement.
A first photographic optical system according to the present invention, which is provided to attain the above-described objects, has a negative lens placed closest to the image side of the photographic optical system, and a positive lens, a negative lens and a positive lens, which are placed in order from the object side of the photographic optical system on the object side of the negative lens closest to the image side. The negative lens closest to the image side satisfies the following conditions:
75.0≦&ngr;
d
(1)
0.8≦SG≦2.2 (2)
where &ngr;
d
and SG are the Abbe's number and lens specific gravity (g/cm
3
) of the negative lens closest to the image side.
A second photographic optical system according to the present invention is a zoom optical system including a single lens of negative refracting power that is placed closest to the image side of the optical system. The single lens of negative refracting power satisfies the following conditions:
75.0≦&ngr;
d
(3)
0.8≦SG≦2.2 (4)
where &ngr;
d
and SG are the Abbe's number and lens specific gravity (g/cm
3
) of the single lens of negative refracting power that is closest to the image side.
A third photographic optical system according to the present invention is a zoom optical system including at least a negative lens unit placed closest to the image side of the optical system, and two positive lens units placed on the object side of the negative lens unit closest to the image side. The negative lens unit is formed from a single lens. The single lens satisfies the following condition:
75.0≦&ngr;
d
(5)
where &ngr;
d
is the Abbe's number of the single lens that forms the negative lens unit.
A fourth photographic optical system according to the present invention includes a single lens of negative refracting power that is placed closest to the image side of the optical system. The single lens of negative refracting power that is closest to the image side satisfies the following condition:
n
d
≦1.40 (6)
where n
d
is the refractive index of the single lens of negative refracting power that is closest to the image side.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 4757336 (1988-07-01), Nakayama et al.
patent: 4952038 (1990-08-01), Ito
patent: 5087988 (1992-02-01), Nakayama
patent: 5305148 (1994-04-01), Ikemori et al.
patent: 5325235 (1994-06-01), Takashima et al.
patent: 5485313 (1996-01-01), Betensky
patent: 5808808 (1998-09-01), Yamanashi
patent: 8-160298 (1996-06-01), None
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
Sugarman Scott J.
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