Photography – Having variable focal length of camera objective
Reexamination Certificate
1999-03-30
2001-10-23
Perkey, W. B. (Department: 2851)
Photography
Having variable focal length of camera objective
C359S689000
Reexamination Certificate
active
06308011
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to compact zoom lenses for use in photographic cameras, video cameras, electronic still cameras, etc.
2. Description of Related Art
Recently, as the home video camera has been reduced in size and weight, minimization of the bulk and size of the zoom lens, too, has made a remarkable advance, and great efforts are being devoted, in particular, to greatly reduce the total length of the zoom lens, to further reduce the diameter of the front lens members, and to seek a simpler configuration.
To attain these aims, optical system means has been developed to limit the range or zoom ratio to 2 to 3, and to provide a simple arrangement for the zoom lens of 2-unit or 3-unit configuration.
For example, Japanese Laid-Open Patent Applications No. Sho 55-35323 and No. Sho 56-158316 disclose a 3-unit zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, the second lens unit axially moving to vary the focal length, while simultaneously moving the first lens unit to compensate for the shift of the image plane.
Such a type of zoom lens in which the front lens unit is negative in refractive power, or the so-called “negative lead” type, is favorable for widening the field angle at the wide-angle end with relative ease. Therefore, it has found its use in many zoom lenses that cover a field of view of 60° or more.
For example, Japanese Patent Publications No. Sho 59-16248 and No. Hei 6-66008 propose zoom lenses having two lens units of negative and positive refractive powers with the separation therebetween varying to vary the focal length, or the so-called “short” zoom lenses.
Also, Japanese Patent Publication No. Hei 7-52256 proposes a zoom lens comprising, in order from an object side to an image side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, with the separation between the second and third lens units increasing as zooming occurs from the wide-angle end to the telephoto end.
Also, U.S. Pat. No. 5,434,710 discloses a zoom lens comprising, in order from an object side, a first lens unit of negative refractive power, a second lens unit of positive refractive power and a third lens unit of positive refractive power, with the separation between the second and third lens units decreasing as zooming occurs from the wide-angle end to the telephoto end.
Further, for the zoom lens of a video camera, the improvement of the compact form must be combined with a requirement of achieving a high resolution over the entire zooming range.
In general, in order to increase the resolution, the aberrations every lens unit produces may be minimized. To this purpose, a simple practice is to increase the number of constituent lenses in each lens unit to decrease the contributions of each lens unit to the aberrations. This method, however, increases the lens-system size, contradicting the goal of minimizing the size of the lens system.
In contrast, one of the prior known methods using an aspheric surface corrects spherical and zonal aberrations while minimizing the total number of lens elements. With the use of an aspheric surface, it becomes possible to expect an ever increasing reduction in the number of lens elements and the production of an aberration-correcting effect that cannot be obtained from the spherical surfaces.
Meanwhile, in order to achieve a lens system of high resolving power, it is important to correct chromatic aberrations as well as spherical and zonal aberrations. The above-described method that relies on the asphericity can, however, hardly correct chromatic aberrations.
Particularly for the 3-unit zoom lenses described above, the selection of the second lens unit as the main variator tends to increase the variation of chromatic aberrations with zooming. For this reason, in the prior art, the second lens unit has been achromatized by using a negative lens made from a material of high dispersion and a positive lens made from a material of low dispersion, each one or more in number.
Because of this arrangement, there are fewer degrees of freedom available to the second lens unit to correct aberrations other than chromatic aberrations. Therefore, the difficulty of correcting off-axial aberrations tends to increase at the wide-angle end of the zooming range.
A zoom lens with few members has previously been proposed in, for example, U.S. Pat. No. 4,999,007. In particular, the first and second embodiments disclosed in U.S. Pat. No. 4,999,007 show practical examples with a zoom ratio of no less than 3. However, the first lens unit is constructed with as small a number of members as 1 or 2. So, the aberrations the first lens unit produces, including chromatic aberrations, are not sufficiently corrected. Another problem arises from the fact that the aspherical first lens in the first embodiment is formed to a shape unfavorable for making it by molding. Concretely speaking, the axial and edge thicknesses differ so greatly from each other that the lens, although easy to make, is very hard to handle in detaching it from the mold. Also, in the second embodiment, although the drawbacks described above are slight, the field angle is narrow, as the wide-angle end is not designed to extend toward short enough focal lengths.
In Japanese Laid-Open Patent Application No. Hei 1-191820, too, a zoom lens with few members is proposed, in which embodiments of a zoom ratio of about 3 are disclosed. However, the first lens unit is constructed with one or two members and is not sufficiently corrected for aberrations, including chromatic ones. In addition, the zoom lens is designed not to make the maximum field angle wide enough.
Meanwhile, in Japanese Laid-Open Patent Application No. Hei 6-11650, a negative lens unit at the frontmost position is followed, on the image side thereof, by a positive lens unit. In such a zoom lens, the first lens unit is constructed with either positive and negative lenses, totaling two lenses, or negative and positive lenses, totaling two lenses, or a negative meniscus lens, a bi-concave lens and a positive meniscus lens, totaling three lenses, or a negative meniscus lens, a bi-convex lens and a bi-concave lens, totaling three lenses. The aberrations the first lens unit produces are not sufficiently corrected.
Japanese Laid-Open Patent Application No. Hei 3-240011 discloses a 3-unit zoom lens of a negative-positive-positive refractive power arrangement in this order from the object side to the image side. The first lens unit is constructed with a negative meniscus lens, a bi-concave lens and a positive meniscus lens, totaling three lenses. So, the aberrations the first lens unit produces are not sufficiently corrected.
Another Japanese Laid-Open Patent Application No. Hei 6-94996, too, discloses a 3-unit zoom lens of a negative-positive-positive refractive power arrangement in this order from the object side to the image side. The first and second lens units are both constructed in such a way as not to use any aspheric surfaces. So, the first lens unit produces distortion and, during zooming to the wide-angle region, lateral aberration. The second lens unit produces spherical aberration and astigmatism. These aberrations are not sufficiently corrected.
Yet another Japanese Laid-Open Patent Application No. Hei 8-152558 discloses a zoom lens including negative and positive lens units in this order from the object side to the image side. The second lens unit is constructed in such a way as not to use any aspheric surfaces. So the spherical aberration and astigmatism the second lens unit produces are not sufficiently corrected.
As described before, the negative lead type of zoom lens is favorable for reducing the size of the lens system and widening the field angle. To simultaneously fulfill the requirements of widening the field angle while still assuri
Kimura Ken-ichi
Wachi Fumihito
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Perkey W. B.
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