Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-10-16
2003-01-28
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S684000
Reexamination Certificate
active
06512637
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens and a photographing apparatus, and, more particularly, to a zoom lens in which the variation of an angle of view thereof due to the movement of a focusing lens is suppressed, and a photographing apparatus having such a zoom lens.
2. Description of Related Art
Heretofore, for use with a television camera, a photographic camera, a video camera or the like, there has been a demand for a zoom lens having high optical performance while having a large relative aperture and a high variable magnification ratio.
In addition to such a demand, in the case of a color television camera for broadcasting, in particular, importance is attached to operability and mobility. In response to such a requirement, the usage of a CCD (charge-coupled device) of ⅔ inch or ½ inch has become the mainstream for an image pickup device in the color television camera.
Since the CCD has an almost uniform resolution over the entire image pickup range, a zoom lens to be associated with the CCD is also required to have an almost uniform resolution from the center of an image plane to the periphery thereof. Further, with advances in high-resolution products in recent years, the high density construction of the CCD has progressed, so that the demand for high performance of zoom lenses has become stronger increasingly.
In addition, there is a great demand for a wide angle of view for zoom lenses. In recent years, some zoom lenses have a horizontal angle of view exceeding 80 degrees. In these wide-angle zoom lenses, such an arrangement has become the mainstream that the first lens unit is composed of three lens subunits of negative, positive and positive refractive powers, respectively, so as to correct the various aberrations caused by the wide angle of view, and the second lens subunit of positive refractive power is moved toward the image side during focusing from an infinitely distant object to a minimum-distance object.
In the meantime, there is such a problem that the angle of view is changed due to focusing. As one means for solving the problem, there has been proposed a system in which position signals indicative of the positions for focusing and zooming are detected and, during focusing, the zooming operation is performed in such a way as to cancel the change of the angle of view due to the focusing. However, in the proposed system, the correction for the change of the angle of view cannot be performed both at the wide-angle end and the telephoto end, so that the change of an angle of view is left uncorrected in part. At the telephoto side, where the depth of field is shallow, the change of the angle of view due to focusing is inconspicuous even if left uncorrected. However, at the wide-angel side, the change of the angle of view appears as if the zooming operation occurs. On the other hand, as has been proposed as a focusing type adapted for a wide-angle zoom lens in, for example, Japanese Patent Publication No. Sho 59-4686 (corresponding to U.S. Pat. No. 4,099,845), the above-mentioned zoom lens of the type in which the first lens unit is composed of three lens subunits of negative, positive and positive refractive powers, respectively, is known as a type in which the change of the angle of view due to focusing is small, as compared with zoom lenses of other focusing types.
Further, in Japanese Laid-Open Patent Application No. Hei 10-062686 (corresponding to U.S. Pat. No. 5,966,246), as a large-aperture, wide-angle and high-variable-magnification zoom lens having the F-number of 1.8 or thereabout and a variable magnification ratio of 10 or thereabout, there has been proposed a four-unit zoom lens which comprises four lens units, i.e., in order from the object side, a first lens unit of positive refractive power arranged to be stationary during the variation of magnification, a second lens unit of negative refractive power arranged to move during the variation of magnification, a third lens unit of positive refractive power for compensating for the shift of an image plane due to the variation of magnification, and a fourth lens unit of positive refractive power for image formation, wherein the first lens unit is composed of three lens subunits of negative, positive and positive refractive powers, respectively.
In order to make a zoom lens have a large aperture ratio (F-number of 1.5 to 1.8), a wide angle of view and a high variable magnification ratio (variable magnification ratio of 10 to 30) and have high optical performance over the entire variable magnification ratio, and in order to lessen the change of the angle of view due to focusing in the zoom lens, it is necessary to appropriately set the refractive power of each lens unit, the lens construction, the sharing of correction of aberration, the sharing of achromatism, etc.
For example, if it is intended to attain a zoom lens having high optical performance with little variation of aberration over the entire variable magnification range and the entire focusing range, having little change of an angle of view due to focusing at the wide-angle end, and having a large aperture ratio and a high variable magnification ratio, a problem arises in that the number of constituent lens elements of the zoom lens increases inevitably and, therefore, the size of the entire lens system increases.
There have been proposed a number of zoom lenses in which the first lens unit is composed of three lens subunits of negative, positive and positive refractive powers, respectively, for the purpose of realizing a wide angle of view.
For example, in Japanese Patent Publication No. Sho 59-4686, there has been proposed a focusing type adapted for a wide-angle zoom lens in which the first lens unit is composed of three lens subunits of negative, positive and positive refractive powers, respectively. However, since the positive lens subunit movable for focusing is allowed to have a refractive power relatively stronger than that of the negative lens subunit disposed before that positive lens subunit, the zoom lens is not appropriate to the attainment of high magnification, high performance and high quality. Further, although the lens construction in which the change of an angle of view due to focusing is small has been proposed, no mention has been made about the relationship between the lateral magnification of the first lens unit and the lateral magnification of the second and subsequent lens units, and the zoom lens can not be said to have a lens construction sufficient for controlling the degree of the change of the angle of view at the wide-angle end.
In addition, in a zoom lens disclosed in Japanese Laid-Open Patent Application No. Hei 10-062686, the positive lens subunit movable for focusing has a refractive power weaker than that of the negative lens unit disposed before that positive lens unit. Therefore, as has been mentioned in Japanese Patent Publication No. Sho 59-4686, the zoom lens has a lens construction which has low efficiency for focusing and which is unsuitable for the reduction in size of the entire lens system.
In order to make a zoom lens have high quality while attaining a reduction in size and weight of the entire lens system, it is necessary to appropriately set the refractive power of each lens unit and the lens construction. In order to attain a wide angle of view and a high variable magnification ratio in a zoom lens, how to balance, with respect to the entire lens system, the refractive power of each of the front lens unit, the magnification varying lens unit and the lens unit for compensating for the shift of an image plane due to the variation of magnification is an important factor. Further, in order to suppress the change of the angle of view due to focusing, it is also necessary to appropriately set the refractive powers of lens subunits included in the front lens unit and the relationship between the lateral magnification of the front lens unit and the lateral magnification of the lens units subsequent to the front lens unit.
BRIEF SU
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Sugarman Scott J.
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