Zoom lens, and electronic imaging system using the same

Details Zoom lens, and electronic imaging system using the same Zoom lens, and electronic imaging system using the same

2002-11-15

2004-06-01

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S682000, C359S681000

Reexamination Certificate

active

06744564

ABSTRACT:

This application claims benefits of Japanese Application No. 2001-351623 filed in Japan on Nov. 16, 2001, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens and an electronic imaging system that incorporates the same, and more particularly to a zoom lens, the depth dimension of which is diminished by providing some contrivances to an optical system portion such as a zoom lens and an electronic imaging system using the same, such as a video or digital camera. According to the present invention, the zoom lens is also designed to be capable of rear focusing.
In recent years, digital cameras (electronic cameras) have received attention as cameras of the next generation, an alternative to silver-halide 35 mm-film (usually called Leica format) cameras. Currently available digital cameras are broken down into some categories in a wide range from the high-end type for commercial use to the portable low-end type. In view of the category of the portable low-end type in particular, the primary object of the present invention is to provide the technology for implementing video or digital cameras whose depth dimension is reduced while high image quality is ensured.
The gravest bottleneck in diminishing the depth dimension of cameras is the thickness of an optical system, especially a zoom lens system from the surface located nearest to its object side to an image pickup plane. To make use of a collapsible lens mount that allows the optical system to be taken out of a camera body for photo-taking and received therein for carrying now becomes mainstream. However, the thickness of an optical system received in a collapsible lens mount varies largely with the lens type or filters used. Especially in the case of a so-called +precedent type zoom lens wherein a lens group having positive refracting power is positioned nearest to its object side, the thickness of each lens element and dead space are too large to set such requirements as zoom ratios and F-numbers at high values; in other words, the optical system does not become thin as expected, even upon received in the lens mount (JP-A 11-258507). A − precedent type zoom lens, especially of two or three-group construction is advantageous in this regard. However, this type zoom lens, too, does not become slim upon received in a collapsible lens mount, even when the lens positioned nearest to the object side is formed of a positive lens (JP-A 11-52246), because the lens groups are each composed of an increased number of lens elements, and the thickness of lens elements is large. Among zoom lenses known so far in the art, those set forth typically in JP-A's 11-287953, 2000-267009 and 2000-275520 are suitable for use with electronic imaging systems with improved image-formation capabilities including zoom ratios, angles of view and F-numbers, and may possibly be reduced in thickness upon received in collapsible lens mounts.
To make the first lens group thin, it is preferable to make an entrance pupil position shallow; however, the magnification of the second lens group must be increased to this end. For this reason, some considerable load is applied on the second lens group. Thus, it is not only difficult to make the second lens group itself thin but it is also difficult to make correction for aberrations. In addition, the influence of production errors grows. Thickness and size reductions may be achieved by making the size of an image pickup device small. To ensure the same number of pixels, however, the pixel pitch must be diminished and insufficient sensitivity must be covered by the optical system. The same goes true for the influence of diffraction.
To obtain a camera body whose depth dimension is reduced, a rear focusing mode wherein the rear lens group is moved for focusing is effective in view of the layout of a driving system. It is then required to single out an optical system less susceptible to aberration fluctuations upon rear focusing.
SUMMARY OF THE INVENTION
In view of such problems as referred to above, the primary object of the invention is to thoroughly slim down a video or digital camera by singling out a zoom mode or zoom construction wherein a reduced number of lens elements are used to reduce the size of a zoom lens and simplify the layout thereof and stable yet high image-formation capabilities are kept over an infinite-to-nearby range, and optionally making lens elements thin thereby reducing the total thickness of each lens group and slimming down a zoom lens thoroughly by selection of filters.
According to the present invention, the aforesaid object is achievable by the provision of a zoom lens comprising, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, wherein for zooming from a wide-angle end to a telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves in a locus different from that of the second lens group with a varying spacing between adjacent lens groups, wherein:
the second lens group comprises two lens components, i.e., an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens component, and
the object side-lens component satisfies condition (1):
0.6
<R
2FR
/R
2FF
<1.05  (1)
where R
2FF
is the axial radius of curvature of the object side-surface of the object side-lens component in the second lens group, and R
2FR
is the axial radius of curvature of the image side-surface of the object side-lens component in the second lens group.
The advantages of, and the requirements for, the aforesaid zoom lens arrangement are now explained.
The zoom lens of the present invention comprises, in order from an object side thereof, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power. For zooming from the wide-angle end to the telephoto end of the zoom lens upon focused on an infinite object point, the second lens group moves toward the object side alone and the third lens group moves in a locus different from that of the second lens group with a varying spacing between the adjacent lens groups. The second lens group comprises two lens components, i.e., an object side-lens component and an image side-lens component, one of which is composed of a cemented lens component consisting of a positive lens element and a negative lens element and the other consists only of a positive single lens component (a two-component, three-lens arrangement).
Of two such lens components, the object side-lens component should preferably satisfy condition (1).
0.6
<R
2FR
/R
2FF
<1.05  (1)
Here R
2FF
is the axial radius of curvature of the object side-surface of the object side-lens component in the second lens group, and R
2FR
is the axial radius of curvature of the image side-surface of the object side-lens component in the second lens group.
In the present disclosure, the term “cemented lens” should be understood to comprise a plurality of lens elements wherein a lens element formed of a single medium is thought of as one unit, and the “lens component” should be understood to refer to a lens group with no air separation therein, i.e., a single lens or a cemented lens.
For reductions in the size of a two-group zoom lens of −+ construction commonly used as the zoom lens for long-standing silver-halide film cameras, it is preferable to increase the magnification of the positive rear group (the second lens group) at each focal length. To this end, it is already well known to locate an additional positive lens component as the third lens group on the image side of the

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