Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-11-29
2001-04-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S687000
Reexamination Certificate
active
06212017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a soft-focus zoom lens system having a soft-focusing effect over a range from a shorter focal length toward a medium-longer focal length, the zoom lens system which can be utilized in an interchangeable photographing lens for a single lens reflex camera.
2. Description of the Related Art
A soft-focus lens system has conventionally been used for taking portraits, and has therefore been employed in a medium-telephoto photographing lens having a relatively narrower angle-of-view. On the other hand, a soft-focus lens system has recently been employed even in a photographing lens having a wider angle-of-view in order to take photographs of distant views such as landscapes and the like. However, a zoom lens system having a soft-focusing effect over a range from a shorter focal length toward a medium-longer focal length has not been provided.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a soft-focus zoom lens system through which a soft-focusing effect can be obtained in photographing of both distant views, such as landscapes, and portraits.
In order to achieve the above mentioned object, there is provided a soft-focus zoom lens system including a positive front lens group, a negative intermediate lens group, and a positive rear lens group, in this order from the object. Zooming is performed by respectively moving the front lens group, the intermediate lens group, and the rear lens group, along the optical axis. The zoom lens system satisfies the following conditions:
SAU/
fw
<−0.10 (1)
0.30<D
23w
/fw
(2)
0.25<D
12t
/ft
(3)
wherein
SAU designates the amount of spherical aberration at an open aperture, with respect to a focal length at which the amount of spherical aberration, upon zooming, becomes the smallest;
fw designates the focal length of the entire lens system at the short focal length extremity;
D
23w
designates the axial distance, at the short focal length extremity, between the most image-side surface of the intermediate lens group and the most object-side surface of the rear lens group;
D
12t
designates the axial distance, at the long focal length extremity, between the most image-side surface of the front lens group and the most object-side surface of the intermediate lens group; and
ft designates the focal length of the entire zoom lens system at the long focal length extremity.
In the soft-focus zoom lens system according to the present invention, in order to generate spherical aberration and reduce the occurrences of other off-axis aberrations, the rear lens group preferably includes at least one positive lens element, and a diaphragm is positioned between the intermediate lens group and the rear lens group. Further, the zoom lens system preferably satisfies the following condition:
0.5
<r
R-1
/fw
<1.6 (4)
wherein
r
R-1
designates the radius of curvature of the object-side surface of the most object-side positive lens element in the rear lens group.
The soft-focus zoom lens system according to the present invention preferably has a wider half angle-of-view at the short focal length extremity to the extend that the lens system satisfies the following condition:
25°<Ww (5)
wherein
Ww designates the half angle-of-view at the short focal length extremity.
The present disclosure relates to subject matter contained in Japanese Patent Application No.Hei-10-340076 (filed on Nov. 30, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 4826301 (1989-05-01), Ikemori
patent: 5822132 (1998-10-01), Hirakawa
patent: 5841590 (1998-11-01), Sato
patent: 5991093 (1999-11-01), Murata et al.
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Schwartz Jordan M.
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