Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2003-04-09
2004-08-03
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S686000
Reexamination Certificate
active
06771432
ABSTRACT:
This application claims benefits of Japanese Application No. 2002-106378 filed in Japan on Apr. 9, 2002, the contents of which are incorporated herein by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens and an electronic imaging system using the same, and more particularly to an electronic imaging system such as a video camera or a digital camera, the depth dimension of which is diminished by providing some contrivances to an optical system portion such as a zoom lens.
In recent years, digital cameras (electronic cameras) have received attention as the coming generation of cameras, 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, and which are easy to handle.
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.
Recent technologies for slimming down cameras rely primarily on a collapsible lens mount that allows the optical system to be taken out of a camera body for phototaking and received therein for carrying. Typical examples of an optical system that can effectively be slimmed down while relying on the collapsible lens mount are disclosed in JP-A's 11-194274, 11-287953 and 2000-9997. Each publication discloses an optical system comprising, in order from its object side, a first lens group having negative refracting power and a second lens group having positive refracting power, wherein both lens groups move during zooming.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, there is provided a zoom lens, comprising, in order from its object side, a first lens group that remains fixed during zooming, a second lens group that has negative refracting power and moves during zooming, a third lens group that has positive refracting power and moves during zooming, and a fourth lens group that has positive refracting power and moves during zooming and focusing, characterized in that the first lens group comprises, in order from its object side, a negative meniscus lens component convex on its object side, a reflecting optical element for bending an optical path and a positive lens.
According to another aspect of the present invention, there is provided a zoom lens, comprising, in order from its object side, a first lens group that remains fixed during zooming, a second lens group that has negative refracting power and moves during zooming, a third lens group that has positive refracting power and moves during zooming, and a fourth lens group that has positive refracting power and moves during zooming and focusing, characterized in that the first lens group comprises a reflecting optical element for bending an optical path, and upon focusing on an infinite object point, the fourth lens group moves in a locus opposite to that of movement of the third lens group during zooming.
The advantages of, and the requirements, for the above arrangements used herein are now explained.
While relying upon the arrangement comprising, in order from its object side, the first lens group that remains fixed during zooming, the second lens group that has negative refracting power and moves during zooming, the third lens group that has positive refracting power and moves during zooming and the fourth lens group that has positive refracting power and moves during both zooming and focusing, the zoom lens of the present invention enables an associated camera to be immediately put into the ready state unlike a collapsible lens mount camera. To be favorable for water-proofing and dust-proofing purposes, the first lens group is designed to remain during zooming, and for considerably reducing the depth dimension of the camera, at least one reflecting optical element for bending an optical path is located in the first lens group nearest to the object side of the lens system.
However, the location of the optical path-bending reflecting optical element in the first lens group would give rise to the following two demerits.
A. The depth of an entrance pupil increases, leading unavoidably to an increase in the size of each lens element forming the first lens group that, by definition, has a large diameter.
B. The magnification of a combined system comprising the second or the third lens group that, by definition, has a zooming function and the subsequent lens group or groups is close to zero, and so the zoom ratio becomes low relative to the amount of zooming movement.
First of all, the condition necessary for bending is explained. Referring to a zoom type such as one intended herein, the location of the optical path-bending reflecting optical element in the first lens group necessarily makes the position of the entrance pupil likely to become deep, as in the case of JP-A 10-62687 or 11-258507, resulting in an increase in the size of each optical element that forms the first lens group. It is thus preferable that the first lens group comprises, in order from its object side, a negative meniscus lens component convex on its object side, a reflecting optical element for bending an optical path and a positive lens and satisfies the following conditions (1), (2), (3) and (4).
1.4
<−f
11
/{square root over ( )}(
f
W
·f
T
)<2.4 (1)
1.2
<f
12
/{square root over ( )}(
f
W
·f
T
)<2.2 (2)
0.8
<d/L
<2.0 (3)
1.55
<n
PRI
(4)
Here f
11
is the focal length of the negative meniscus lens component in the first lens group, f
12
is the focal length of the positive lens element in the first lens group, f
W
and f
T
are the focal lengths of the zoom lens at the wide-angle end and the telephoto end of the zoom lens, respectively, d is an air-based length from the image side-surface of the negative meniscus lens component to the object side-surface of the positive lens element in the first lens group, as measured on the optical axis of the zoom lens, L is the diagonal length of the (substantially rectangular) effective image pickup area of an electronic image pickup device, and N
PRI
is the d-line refractive index of the medium of a prism used as the optical path-bending reflecting optical element in the first lens group.
In order to locate the entrance pupil at a shallow position thereby enabling the optical path to be physically bent, it is preferable to increase the powers of the lens elements on both sides of the first lens group, as defined by conditions (1) and (2). As the upper limits of 2.4 and 2.2 to both conditions are exceeded, the entrance pupil remains at a deep position. Hence, when it is intended to ensure some angle of view, the diameter or size of each optical element forming the first lens group becomes too large to physically bend the optical path. As the lower limits of 1.4 and 1.2 are not reached, the magnification that the lens groups subsequent to the first lens group and designed to move for zooming can have becomes close to zero, offering problems such as an increase in the amount of zooming movement or a zoom ratio drop and, at the same time, rendering correction of off-axis aberrations such as distortion and chromatic aberrations difficult.
Condition (3) is provided to determine the length necessary for the location of the optical path-bending reflecting optical element, as measured along the optical axis of the zoom lens. Although the value of this condition should preferably be as small as possible, it is understood that as the lower limit of 0.8 thereto is not reached, a light beam contributing to the formation of an image at the periphery of a screen does no
Epps Georgia
Hasan M.
Kenyon & Kenyon
Olympus Corporation
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