Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-05-24
2001-11-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S691000, C359S745000, C359S794000
Reexamination Certificate
active
06313955
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rear-focusing telephoto lens system which is used in a single lens reflex (SLR) camera, a video camera and the like.
2. Description of the Related Art
In a telephoto lens system having a smaller F-number of 2.8 or less and an angle-of-view of about 26°, the number of lens elements tends to be larger due to a smaller F-number, the overall length thereof becomes longer due to being a telephoto lens system, and further the diameter of lens elements tends to be larger due to an angle-of-view of 26° which is relatively wider for a telephoto lens system. Consequently, the weight of the lens system also tends to be heavier.
On the other hand, an auto-focusing (AF) camera is required to have a lighter (less heavier) lens group to be moved upon focusing (hereinafter, a focusing lens group) in order to attain quick movement of the focusing lens group and to reduce a load imposed on a power source. For the purpose of satisfying these requirements, a lens system, such as rear-focusing and inner-focusing type lens systems, in which a part of lens groups is arranged to be moved for focusing, is employed. In this regard, a rear-focusing type lens system is suitable for a lens system having a relatively larger angle-of-view, while an inner-focusing type lens system is not.
Various rear-focusing telephoto lens systems are taught, for example, in the U.S. Pat. No. 4,426,137,the Japanese Unexamined Patent Publication (JPUEPP) No. Sho-58-199311, JPUEPP Sho-60-166912, JPUEPP Sho-60-201313, and U.S. Pat. No. 4,812,027. However, in these lens systems, the rear lens group in a Gauss type lens system is made movable as a focusing lens group. Upon the rear lens group being moved, a height of incidence on a large-curvature concave surface of the object-side surface of the rear lens group varies, and an angle of incidence varies accordingly, so that fluctuation of spherical aberration becomes larger.
As an improved type of the above described lens systems, in U.S. Pat. No. 5,172,274, the applicant of the present invention has disclosed a telephoto lens system. This telephoto lens system has acceptable optical performance; however, an aperture stop is provided in the rear lens group (focusing lens group), because the distance between the front and rear lens groups is shorter upon photographing an object at a closer photographing distance. An arrangement in which the aperture stop with a variable aperture moves upon focusing makes the structure of a lens barrel complicated. Furthermore, since a rear lens frame is divided by the aperture stop, it becomes difficult to maintain precision on decenteration.
Still further, more improved types of the above described lens system are disclosed in U.S. Pat. No. 5,640,277 and U.S. Pat. No. 5,627,686. However, a large number of lens elements are required, and the structure of a lens barrel is complicated, since the aperture stop is arranged to move together with the focusing lens group.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a rear-focusing telephoto lens system, with a F-number of about 2.8 and with an angle-of-view of about 26°, which can simplify the structure of a lens barrel, and is suitable for an automatic focusing operation.
In order to achieve the above-mentioned object, there is provided a rear-focusing telephoto lens system including a positive front lens group, and a positive rear lens group, in this order from the object. The front lens group includes at least two positive lens elements, each of which has a large-curvature convex surface facing toward the object, and a negative lens element having a large-curvature concave surface facing toward the image. The rear lens group includes at least one lens element which has a convex surface facing toward the object, and is positioned at the most object-side in the rear lens group. Upon focusing, only the rear lens group moves along the optical axis, and the lens system satisfies the following condition:
0.5<f/fF<0.8 (1)
wherein
f designates the focal length of the entire lens system; and
fF designates the focal length of the front lens group.
An aperture stop is preferably positioned between the front and rear lens groups so that the aperture stop is made immoveable upon focusing, and the lens system preferably satisfies the following condition:
0.23<d/f<0.50 (2)
wherein
d designates the distance between the front and rear lens groups when the lens system is focused on an object at an infinite photographing distance.
The rear lens group includes a positive lens element having a convex surface facing toward the object, a negative lens element, a negative lens element, and a positive lens element, in this order from the object, and the lens system preferably satisfies the following condition:
−0.8<f/fGr3<−0.2 (3)
wherein
fGr3 designates the focal length of the third lens element, i.e., the second negative lens element, from the object.
On the other hand, the rear lens group can be constituted by three lens elements, i.e., a positive lens element having a convex surface facing toward the object, a negative lens element, and a positive lens element, in this order from the object.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-11-144821 (filed on May 25, 1999) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 4426137 (1984-01-01), Mori
patent: 4575198 (1986-03-01), Tanaka
patent: 4812027 (1989-03-01), Yanagisawa
patent: 5172274 (1992-12-01), Hirakawa
patent: 5627686 (1997-05-01), Ohshita
patent: 5640277 (1997-06-01), Ohshita
patent: 5793536 (1998-08-01), Sato
patent: 58199311 (1983-11-01), None
patent: 60166912 (1985-08-01), None
patent: 60201313 (1985-10-01), None
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Lucas Michael A.
LandOfFree
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