Zoom lens and imaging device incorporating the same

Details Zoom lens and imaging device incorporating the same Zoom lens and imaging device incorporating the same

1999-09-10

2001-02-20

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S683000

Reexamination Certificate

active

06191894

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a small-sized zoom lens which provides high variable power for use in imaging devices (e.g., video cameras or digital still cameras); and an imaging device incorporating such a zoom lens.
2. Description of the Related Art
Small-sized and high-variable-power zoom lenses for use in an imaging device (e.g., a video camera or a digital still camera) are plagued with five modes of aberration, i.e., spherical aberration, coma, astigmatism, field curvature, and distortion, as well as chromatic aberration. Conventionally, various methods have been adopted to compensate for such aberrations.
In recent years, there has been an increasing desire to miniaturize various image processing devices. This has led to a need to shorten the lens length of such zoom lenses.
Such miniaturization requires not only compensation for the aforementioned five modes of aberration and chromatic aberration, but also a configuration in which incident light on the image plane becomes parallel to the optical axis of the system as much as possible (that is, the image side of the lens needs to be “telecentric”). This has presented a major design constraint to those who wish to construct a zoom lens with a short lens length.
Now, the structure of a conventional zoom lens will be described in detail.
For example, Japanese Laid-Open Publication No. 5-297275 discloses a zoom lens of a four-array rear focus type (hereinafter referred to as “Conventional Example 1”), as shown in FIG.
13
. This zoom lens includes: a first lens array
101
which has positive refractive power and which is in a fixed position relative to an image plane
106
; a second lens array
102
which has negative refractive power and which provides variable power by moving along the optical axis direction; a third lens array
103
which has positive refractive power and which is in a fixed position relative to the image plane
106
; and a fourth lens array
104
which has positive refractive power and which is capable of moving along the optical axis direction so as to maintain the image plane
106
, which moves corresponding to the movement of the second lens array
102
and the object to be imaged (hereinafter referred to as an “imaging object”), at a predetermined distance from a reference plane. The lens arrays
101
to
104
are arranged in the above order so that the first lens array
101
lies adjacent to the object to be imaged. Reference numeral
105
denotes a flat plate which is equivalent to a low-pass filter, an infrared cut filter, and/or a cover glass of an imaging device.
In accordance with the structure of Comparative Example 1, the first lens array
101
, which is fixed relative to the image plane
106
, provides an image formation function. The second lens array
102
, which is capable of moving along the optical axis direction, provides a variable power function, i.e., ability to vary the focal length of the entire system. The third lens array
103
, which is fixed relative to the image plane
106
, provides a converging function. The fourth lens array
104
, which is capable of moving along the optical axis direction, provides a focusing function, i.e., the ability to minimize the variation in the image-forming position responsive to any movement of the second lens array
102
and any movement of the object to be imaged.
However, with the structure of Comparative Example 1, it has been very difficult to realize a high-variable-power zoom lens of a sufficiently small size.
On the other hand, Japanese Laid-Open Publication No. 9-269452 describes a zoom lens (hereinafter referred to as “Conventional Example 2”) which additionally includes a fifth lens array having negative refractive power in a four-array rear focus type zoom lens similar to that of Conventional Example 1, in an attempt to shorten the lens length of the zoom lens.
However, the zoom lenses of Comparative Examples 1 and 2 each have a problem in that, in order to achieve a high zooming ratio on the order of ×10 with such zoom lenses, the exit pupil will inevitably be located near the image plane. As a result, it is difficult to achieve telecentricity on the image side. These zoom lenses also have a problem in that their lens length cannot be sufficiently shortened, which makes it difficult to miniaturize such zoom lenses.
SUMMARY OF THE INVENTION
A zoom lens according to the present invention includes: a first lens array which has positive refractive power and which is in a fixed position relative to an image plane; a second lens array which has negative refractive power and which provides variable power by moving along an optical axis direction; a third lens array which has positive refractive power and which is in a fixed position relative to the image plane; a fourth lens array which has positive refractive power and which is capable of moving along the optical axis direction so as to maintain the image plane, which moves corresponding to movement of the second lens array and an object to be imaged, at a predetermined distance from a reference plane; a fifth lens array which has negative refractive power and which is in a fixed position relative to the image plane; and a sixth lens array which has positive refractive power and which is in a fixed position relative to the image plane, wherein the first to sixth lens arrays are arranged in a first through sixth order, respectively, so that the first lens array lies adjacent to the object to be imaged.
In one embodiment of the invention, the fifth lens array includes a single lens having negative refractive power, and the sixth lens array includes a single lens having positive refractive power.
In another embodiment of the invention, a wavelength selection filter is provided between the fifth lens array and the sixth lens array.
In still another embodiment of the invention, a focal length f
5
of the fifth lens array and a focal length f
6
of the sixth lens array satisfy the following relationship:
−3.0
<f
6
/f
5
<−1.0  (1)
In still another embodiment of the invention, a displacement D
z
of the second lens array for effecting power variation from a wide angle mode to a telescopic mode and a focal length f
W
of the second lens array in the wide angle mode satisfy the following relationship:
1.0
<D
z
/f
W
<2.2  (2)
In still another embodiment of the invention, a distance T
O
from a first lens surface to a proximal axis image plane on an imaging object side and a focal length f
T
of the zoom lens in the telescopic mode satisfy the following relationship:
0.7
<T
O
/f
T
<0.9  (3)
In still another embodiment of the invention, each of the third lens array, the fourth lens array, and the sixth lens array includes at least one aspheric surface.
In still another embodiment of the invention, the third lens array has a two-lens structure including a biconvex lens having positive refractive power and a lens having negative refractive power.
In another aspect of the embodiment of the invention, there is provided an imaging device incorporating any of the aforementioned zoom lenses.
In accordance with the above constitution, a telescopic or so-called telephoto type zoom lens can be constructed by combining the negative refractive power of the fifth lens array with the total positive refractive power provided by the first to fourth lens arrays. As a result, the lens length of the zoom lens can be effectively shortened. As will be appreciated, the “total positive refractive power provided by the first to fourth lens arrays” means an inverse of the focal length (having a positive value) of the total optical system spanning from the first to fourth lens arrays.
The addition of the sixth lens array having positive refractive power makes it possible to realize a telecentric zoom lens (i.e., the exit pupil can be located away from the image plane). This is accounted for by the function of the fifth lens array of increasing the focal length of the entire system and the function of th

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