Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-11-06
2004-08-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S689000, C359S740000
Reexamination Certificate
active
06771431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens and an optical equipment employing the zoom lens. More specifically, the present invention relates to a zoom lens suited for a video camera and a still camera for films, a digital camera and the like.
2. Related Background Art
Recently, as the functions of an optical equipment (a camera), such as a video camera or a digital camera, employing a solid state image pickup device have advanced, a high-performance, small-sized zoom lens is desired as an optical system employed therein.
A camera of this type is required to use, as an optical system, a lens system having a relatively long back focus since various types of optical members such as a low-pass filter and a color correction filter, are arranged between the last lens and the image pickup device. Further, a color camera which employs an image pickup device for color images is desired to employ an optical system having a good image-side telecentric characteristic to avoid color shading.
Various types of short zoom, wide angle, two-unit zoom lenses each consisting of two lens units, i.e., a first lens unit having a negative optical power (reciprocal number of focal length) and a second lens unit having a positive optical power and which zooms in and out by changing the interval between the two lens units, have been conventionally proposed. A short zoom optical system of this type performs zooming by moving the second lens unit having the positive optical power and focusing in association with the zooming by moving the first lens unit having the negative optical power.
To make a zoom lens compact while keeping a higher zoom ratio, three-unit zoom lenses have been proposed in, for example, Japanese Patent Publication Nos. 7-3507 (corresponding to U.S. Pat. No. 4,810,072) and 6-40170 (corresponding to U.S. Pat. No. 4,647,160). A zoom lens of this type has a third lens unit having a negative or positive optical power, arranged on the image side of the second lens system to thereby compensate for various aberrations which are generated as the magnification of the lens is higher. However, the three-unit zoom lenses are mainly designed for 35-mm film photographs, so that it is difficult to say that the three-unit zoom lens satisfies a requirement of a relatively long back focus and that of a good telecentric characteristic, which are required for a camera employing a solid state image pickup device.
Three-unit zoom lenses which satisfy a back focus requirement and a telecentric characteristic requirement are proposed in, for example, Japanese Patent Application Laid-Open Nos. 63-135913 and 7-261083. In addition, Japanese Patent Application Laid-Open (JP-A) No. 3-288113 (corresponding to U.S. Pat. No. 5,270,863) discloses a three-unit zoom lens in which a first unit having a negative optical power is fixed and a second unit having a positive optical power and a third unit having a positive optical power are moved to perform zooming. According to each of the conventional zoom lenses, however, the number of the constituent elements of each lens unit tends to be relatively large and the entire lens length tends to be large.
According to the three-unit zoom lens disclosed in JP-A-7-261083, a positive lens is arranged to be closest to the object side in a first unit having a negative optical power. Due to this, particularly if the angle of the lens widens, the outer diameter of the lens tends to increase. Further, since the first unit having the negative optical power is moved to thereby focus on a short-distance object, the mechanical structure of the zoom lens tends to be complicated due to the movement of the first unit for focusing and the movement thereof for zooming.
Moreover, U.S. Pat. No. 4,999,007 discloses a three-unit zoom lens consisting of three lens units having negative, positive, and positive optical powers respectively, each lens unit consisting of a single lens. The entire length of this three-unit zoom lens is relatively large on a wide angle end and the first unit is far away from a stop on the wide angle end. Due to this, the incidence height of an off-axis beam is large, whereby the diameter of the lens constituting the first lens unit increases and the entire lens system tends to be large in size. Further, since each of the first and second lens units consists of one lens, it is difficult to compensate for aberrations in each unit. A change in chromatic aberration of magnification during zooming tends to occur particularly in the first unit which has a large change in the height of the off-axis beam from the optical axis. Since the first unit consists of a concave lens, it is insufficient to compensate for the change in the unit and a change in chromatic aberration of magnification tends to increase in the entire lens system.
U.S. Pat. No. 4,824,223 discloses an optical system for a projector which consists of three units having negative, positive and positive optical powers, respectively. According to this optical system, since the first unit consists of one negative lens, aberrations cannot be always sufficiently compensated in the lens unit and a zoom ratio is about 1.7.
The assignee of the present application proposed a three-unit zoom lens consisting of three lens units having negative, positive and positive optical powers, respectively in JP-A-2000-111798 (corresponding to U.S. Pat. No. 6,308,011). This three-unit zoom lens satisfies both the requirement of a back focus having a length necessary to insert a filter or the like into the rear portion of the lens unit and that of telecentric characteristic necessary for a solid state image pickup device, has a zoom ratio of not less than 2, has an entire lens length as short as possible, and thereby attains a compact zoom lens.
JP-A-11-84243 (corresponding to U.S. Pat. No. 6,191,896) discloses a three-unit zoom lens consisting of three lens units having negative, positive and positive optical powers wherein a stop is disposed in the second lens unit. In zooming from a wide angle end to a telephoto end, the third lens unit is largely moved toward an object side while the interval between the third lens unit and the second lens unit is not greatly changed. If the third lens unit is held in a lens barrel independently of the second lens unit, the third lens is required to have a shaft length corresponding to the movement quantity thereof, making it difficult to make the lens barrel small in size in the optical axis direction.
Further, if the third lens unit is held in the lens barrel of the second lens unit, the shaft length of the third lens unit can be reduced. However, since the driving means of the third lens unit is disposed in the second lens unit which moves during zooming, the mechanical structure of the system tends to be complicated. Besides, since it is also necessary to arrange a stop unit in the lens barrel of the second lens unit, the lens barrel of the second lens unit tends to be large in size.
Likewise, a zoom lens which performs zooming without greatly changing the interval between the second lens unit and the third lens unit is proposed in U.S. Pat. No. 5,270,863.
Furthermore, a three-unit zoom lens consisting of three lens units having negative, positive and positive optical powers, respectively wherein a stop is arranged on an image side of the lens closest to an object side in the second lens unit is disclosed in U.S. Pat. Nos. 5,035,492, 4,844,599 and 5,262,897 and the like. However, each zoom lens disclosed therein has not less than three lenses closer to an image side than the stop in the second lens unit and it is disadvantageously difficult to make the second lens unit compact.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a zoom lens which is compact and has excellent optical performance and a camera which employs the zoom lens.
To attain this object, a zoom lens according to one aspect of the present invention comprises, in order from an object side to an image side:
a first lens unit having a negative optical
Canon Kabushiki Kaisha
Epps Georgia
Morgan & Finnegan , LLP
Raizen Deborah
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