Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-12-22
2001-11-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000, C359S686000, C359S689000, C359S691000
Reexamination Certificate
active
06324017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens system for a compact camera, and in particular, relates to a focusing operation thereof.
2. Description of the Related Art
In a zoom lens system for a compact camera, there is no need to provide a long back focal distance, unlike a zoom lens system for a single lens reflex (SLR) camera which requires a space for providing a mirror behind the photographing lens system. Accordingly, a compact camera generally employs a telephoto type lens system in which positive and negative lens groups are provided in this order from the object, while a SLR camera generally employs a retrofocus type lens system in which negative and positive lens groups are provided in this order from the object.
In a compact camera employing a telephoto type lens system, there has been a demand, in recent years, for a higher zoom ratio in a zoom lens system. In order to respond to this demand, a three-lens-group zoom lens system has been frequently employed. In such a three-lens-group zoom lens system, the three lens groups independently move along the optical axis upon zooming. On the other hand, focusing is performed by moving one of the lens groups, usually the first or second lens group, along the optical axis. In order to attain miniaturization of the three-lens-group zoom lens system, reducing the overall length, the lens diameters thereof, and the thickness of the lens groups are all important factors. However, if the number of lens elements is decreased in order to reduce the thicknesses of the lens groups, the number of lens elements constituting a focusing lens group is decreased accordingly. As a result, it becomes difficult to suitably correct aberrations from infinity to the closest photographing position with respect to any focal length points in an entire zooming range determined by the short focal length extremity and the long focal length extremity. This tendency becomes more remarkable when the zoom ratio is set higher. In order to correct aberrations suitably from infinity to the closest photographing position, the number of lens elements in the focusing lens group has to be increased, and therefore miniaturization of the three-lens-group zoom lens system cannot be achieved.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a miniaturized zoom lens system having a three-lens-group telephoto type lens system, with a small number of lens elements, which can suitably correct aberrations.
In the present invention, by integrally moving the first and second lens groups upon focusing, the number of lens elements constituting a lens group substantially performing an focusing operation is increased, whereby the correcting of aberrations from infinity to the closest photographing position becomes easier. This feature is unlike any well-known technology for focusing, i.e., only one lens group is arranged to perform focusing in a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the object, while zooming is performed by moving the first through third lens groups along the optical axis.
According to the present invention, there is provided a zoom lens system including a negative first lens group, a positive second lens group, and a negative third lens group, in this order from the object, and thereby upon zooming, the three lens groups independently move along the optical axis. On the other hand, upon focusing, the first and second lens groups, which move independently upon zooming, move integrally along the optical axis.
The zoom lens system preferably satisfies the following condition:
1<f
FT
/f
FW
<1.5 (1)
wherein
f
FT
designates the resultant focal length of the first lens group and the second lens group (a focusing lens group) at the long focal length extremity; and
f
FW
designates the resultant focal length of the first lens group and the second lens group (the focusing lens group) at the short focal length extremity.
The zoom lens system preferably satisfies the following condition:
20<K
F
<40 (2)
wherein
K
F
=(f
T
/f
FT
)
2
f
T
designates the focal length of the entire zoom lens system at the long focal length extremity; and
f
FT
designates the resultant focal length of the first lens group and the second lens group (a focusing lens group) at the long focal length extremity.
Preferably, the first lens group includes two lens elements, and the second lens group includes three lens elements, and thereby, with a small number of the lens elements, aberrations can be suitably corrected from infinity to the closest photographing position.
Further, according to the present invention, there is provided a focusing method for a zoom lens system including a negative first lens group, a positive second lens group and a negative third lens group, in this order from the object, wherein the method includes:
(i) independently moving the first, second and third lens groups along the optical axis upon zooming; and
(ii) integrally moving the first lens group and the second lens group along the optical axis upon focusing.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-10-368174 (filed on Dec. 24, 1998) which is expressly incorporated herein by reference in its entirety.
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patent: 4813773 (1989-03-01), Minefuji
patent: 4936661 (1990-06-01), Betensky
patent: 5061053 (1991-10-01), Hirakawa
patent: 5270866 (1993-12-01), Oizumi et al.
patent: 5455714 (1995-10-01), Kohno
patent: 5574599 (1996-11-01), Hoshi et al.
patent: 5666229 (1997-09-01), Ohtake
patent: 5815320 (1998-09-01), Hoshi et al.
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Lucas Michael A.
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