Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-10-18
2002-11-05
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000
Reexamination Certificate
active
06476977
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens and an optical device incorporating the zoom lens, and, more specifically, to a rear-focus zoom lens having a high zoom ratio with a relatively small number of lens elements, appropriate for use in a video camera, a film camera, a broadcast camera, and to an optical apparatus incorporating the zoom lens.
2. Description of the Related Art
Various rear-focus zoom lenses, which perform focusing by moving a lens unit behind a first lens unit closer to an object, have been proposed as zoom lenses used in film cameras or video cameras. Since the rear focus method moves a relatively compact and light-weight lens unit during focusing, the driving power required is small, and the focusing operation through the rear focus method is rapid enough to work compatibly with an auto-focusing system.
For instance, Japanese Patent Laid-Open No. 63-44614 discloses a so-called four-group rear-focus zoom lens which includes, in the order from the object end, a first lens unit having positive refractive power, a second lens unit for zooming having negative refractive power, a third lens unit having negative refractive power for correcting the shift of an image plane in response to zooming, and a fourth lens unit having positive refractive power. The focusing operation is performed by moving the third lens unit. The arrangement requires the space within which the third lens unit travels, and the overall length of the lens system increases.
Japanese Patent Laid-Open No. 63-278013 discloses a four-group rear-focus zoom lens which includes a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power, and a fourth lens unit having positive refractive power. The second lens unit effects zooming, and the fourth lens unit performs image plane correction and focusing. In the zooming method using the third lens unit having negative refractive power, diverging light rays from the second lens unit further diverge through the third lens unit. The lens diameter of the fourth lens unit increases, leading to a bulky design. Furthermore, a variation in aberration due to focusing becomes large.
Each of Japanese Patent Laid-Open No. 62-24213 and Japanese Patent Laid-Open No. 63-247316 discloses a four-group zoom lens which includes, in the order from the object end, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having positive refractive power, and a fourth lens unit having positive refractive power. The second lens unit is moved for zooming, and the fourth lens unit is moved to correct an image plane shift arising from zooming, while effecting focusing. Compact design is thus implemented in the overall lens system.
Japanese Patent Laid-Open No. 63-29718 discloses a four-group zoom lens which includes, in the order from the object end, a first lens unit having positive refractive power, a second lens unit, composed of three lens elements of a negative lens, a negative lens and a positive lens, and generally having negative refractive power and movable for zooming, a third negative lens having negative refractive power and having an aspherical surface, and a fourth lens unit, having positive refractive power and a relatively wide air gap therewithin. The fourth lens unit corrects an image plane shift arising from zooming, while moving for focusing.
Japanese Patent Laid-Open No. 5-72472 discloses a four-group zoom lens which employs aspherical surfaces and includes, in the order from the object end, a first lens unit having positive refractive power and remaining stationary during zooming and focusing, a second lens unit having negative refractive power and movable for zooming, a third lens unit remaining stationary and having a light converging effect with positive refractive power, and a fourth lens unit having positive refractive power and axially movable along an optical axis to keep the image plane position fixed against movements in response to zooming. In the zoom lens disclosed, the second lens unit is composed of a negative meniscus lens, a negative bi-concave lens, and a positive lens, the third lens unit is composed of a single lens element having at least one aspherical surface, and the fourth lens unit is composed of a lens having at least one aspherical surface.
In the above-referenced six disclosures, there is no mention of a zoom lens having the second lens unit composed of four lenses.
U.S. Pat. No. 4,299,454 discloses a zoom lens which includes, in the order from the object end, a first lens unit having positive refractive power, a second negative lens unit, and a rear positive lens unit. Two lenses including at least the second negative lens unit are movable to effect zooming. The second negative lens unit is composed of first and second negative lenses and a positive doublet from the object end. Since the third lens unit moves, the mechanism of the zoom lens become complicated. U.S. Reissue Pat. No. RE32,923 discloses a zoom lens which includes, in the order from the object end, a first positive lens unit, a second negative lens unit, a diaphragm, a third positive lens unit, and a fourth positive lens unit. The first and fourth lens units move in the same direction during zooming, and the diaphragm remains stationary during the zooming. The second lens unit employs a single cemented lens.
In the two U.S. Pat. Nos. 4,299,454 and RE32,923, there is no mention of the third lens unit which has aspherical surfaces on both sides thereof.
Each of Japanese Patent Laid-Open No. 7-270684 and Japanese Patent Laid-Open No. 7-318804 discloses a zoom lens which includes, in the order from the object end, a first lens unit remaining stationary and having positive refractive power, a second lens unit having negative refractive power, for zooming, a third lens unit remaining stationary and having positive refractive power, and a fourth lens unit having positive refractive power and axially movable in the optical axis for keeping the image plane fixed in position against zooming, and for focusing. The second lens unit is composed of four lens elements. In these disclosures, there is no mention of the third lens unit which has aspherical surfaces on the two sides thereof.
Japanese Patent Laid-Open No. 5-060974 discloses a short overall-lens-length zoom lens which includes, in the order from the object end, a first lens unit remaining stationary and having positive refractive power, a second lens unit, having negative refractive power, for zooming, a third lens unit remaining stationary and having positive refractive power, and a fourth lens unit having positive refractive power and axially movable along the optical axis to keep the image plane fixed in position against zooming and to perform focusing. There is neither mention of the second lens unit being composed of four single lens elements nor mention of the third lens unit having aspherical surfaces on the two sides thereof.
The implementation of the rear focus method in the zoom lens typically makes the design of the lens system compact, and permits focusing to be quickly performed, and close-up photographing is easily performed.
On the other hand, variations in aberration during focusing increase, and it is difficult to achieve high optical performance over an object range covering from infinity to a near object.
In particular, it is very difficult to maintain-high optical performance over the object range within an overall zoom range with a simplified construction of a zoom lens having a large zoom ratio and a high aperture ratio.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a zoom lens which has high optical performance for a high zoom ratio thereof, and features compact design with a short overall lens length and a simple construction with a reduced number of lens elements, and to provide an optical apparatus incorporating the zoom lens.
It is another object of
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Schwartz Jordan M.
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