Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-05-02
2003-04-08
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S683000, C359S684000
Reexamination Certificate
active
06545818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens, and, more particularly, to a zoom lens which is suited for a television camera, a photographic camera, a digital camera, a video camera or the like, and in which an aspheric surface is appropriately employed in a part of a lens system thereof to obtain good optical performance over the entire variable magnification range while having, for example, a large relative aperture of 1.5 or thereabout in F-number at the wide-angle end, an ultrawide angle of view (2&ohgr;(angle of view in the wide-angle end)=78°-95°) and a high variable magnification ratio of 10-27 or thereabout. Further, the present invention relates to a zoom lens which is suited for a television camera, a video camera, a photographic camera, a video camera or the like, and, more particularly, to a zoom lens, in which the so-called inner focusing method, in which focusing is effected by moving a lens subunit that is a part of a first lens unit, is employed to obtain high optical performance over the entire object distance range while having a short minimum object distance.
2. Description of Related Art
Heretofore, for use with a television camera, a photographic camera, a digital 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.
For example, the zoom lens is required to have the various aberrations such as astigmatism, distortion and lateral chromatic aberration corrected well and to have high optical performance over the entire image plane. In addition, the zoom lens is required to have a large relative aperture, a wide angle of view and a high variable magnification ratio while being small in size and light in weight, and moreover to have a long back focal distance for enabling a color separation optical system and a variety of filters to be disposed in front of an image pickup means.
Among zoom lenses, the so-called four-unit zoom lens, which is composed of four lens units, i.e., in order from the object side, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for variation of magnification, a third lens unit of positive or negative refractive power for compensating for shift of an image plane caused by the variation of magnification, and a fourth lens unit of positive refractive power for image formation, is relatively easy to make have a high variable magnification ratio and a large relative aperture and is, therefore, widely used as a zoom lens for color television cameras for broadcasting.
Among the four-unit zoom lenses, a zoom lens having a large relative aperture and a high variable magnification ratio, such as having an F-number of 1.7 or thereabout, an angle of view at the wide-angle end 2&ohgr; of 86° or thereabout, and a variable magnification ratio of 8 or thereabout, has been proposed, for example, in Japanese Laid-Open Patent Application No. Hei 6-242378.
In order to obtain, in a zoom lens, a large relative aperture (F-number of 1.5-1.8), a high variable magnification ratio (variable magnification ratio of 10-27) and an ultra-wide angle of view (angle of view in the wide-angle end 2&ohgr; of 78°-95°) and, moreover, to have high optical performance over the entire variable magnification range, it is necessary to appropriately set the refractive power of each lens unit and the lens construction.
In general, in order to obtain high optical performance with little variation of aberrations over the entire variable magnification range, it becomes necessary to increase the freedom of design on aberration correction, for example, by increasing the number of lens elements of each lens unit.
Therefore, if it is attempted to attain a zoom lens having a large relative aperture, an ultra-wide angle of view and a high variable magnification ratio, a problem arises in that the number of lens elements would be inevitably increased to make the size of the whole lens system large. Thus, it would become impossible to meet the requirement for reduction in size and weight.
Further, with respect to the image forming performance, first, making reference to the ultra-wide angle of view of a zoom lens, the greatest problem is distortion. This is because distortion has influence according to the cube of an angle of view in a region of third-order aberration coefficients.
FIG. 45
is a schematic diagram showing the variation of distortion in every zoom position.
As shown in
FIG. 45
, distortion exhibits a considerably large under-tendency (minus tendency) when the zoom position is at a wide-angle end (focal length of fw). As zooming advances from the wide-angle end fw to a telephoto end (focal length of ft), distortion becomes gradually large in the direction of an over-tendency (plus tendency). Then, after zooming reaches a zoom position at which the value of distortion is “0”, the value of distortion in the over-tendency becomes maximum when the zoom position is in the vicinity of fm=fw×Z
¼
, where fw is a focal length at the wide-angle end and Z As a zoom ratio. After that, as zooming advances from the position of the focal length fm to the telephoto end ft, the value of distortion in the over-tendency becomes gradually small. Such an inclination of distortion becomes larger as an angle of view at the wide-angle end becomes larger. Therefore, in such an ultra-wide-angle zoom lens as to have an angle of view 2&ohgr; at the wide-angle end exceeding 78°, distortion in the under-tendency increases rapidly on the wide-angle side, so that it becomes very difficult to control distortion.
The next problem is that a point at which an image contrast becomes best in the center of an image plane, i.e., the so-called best image plane, varies due to the variation of magnification. This is mainly caused by the variation of spherical aberration due to the variation of magnification. Since the spherical aberration has influence according to the cube of an aperture in a region of third-order aberration coefficients, it presents the greatest problem for attaining a large relative aperture.
In general, the variation of spherical aberration due to the variation of magnification exhibits, as shown in
FIG. 46
, an under-tendency (minus tendency) with respect to a Gauss image plane when zooming advances from the wide-angle end at which the value of spherical aberration is “0” until the vicinity of the zoom position fm=fw×Z
¼
where Z is a zoom ratio and fw is a focal length at the wide-angle end. Then, when zooming passes the vicinity of the zoom position fm=fw×Z
¼
, the value of spherical aberration in the under-tendency becomes small. After zooming passes a zoom position at which the value of spherical aberration is “0”, spherical aberration comes to exhibit an over-tendency (plus tendency) in turn.
Then, in the vicinity of a zoom position fd=(Fno.w/Fno.t)×ft at which the so-called F drop begins, i.e., the zoom position where the F-number begins to become large (the lens-system begins to become dark) with the diameter of an on-axial light flux limited, spherical aberration exhibits the greatest over-tendency (plus tendency). When zooming passes the zoom position fd, the value of spherical aberration in the over-tendency becomes small. At the telephoto end, the value of spherical aberration bec
Fukami Kiyoshi
Usui Fumiaki
Yoshimi Takahiro
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