Optical: systems and elements – Lens – With variable magnification
Utility Patent
1999-02-04
2001-01-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
Utility Patent
active
06169635
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens system, and particularly to a zoom lens system applicable to cameras, such as digital cameras and video cameras, in which an image is formed on a solid state imaging element such as a CCD (Charge Coupled Device).
Conventionally, there has been known a two-group zoom lens system including a first lens group having negative power and a second lens group having positive power, which are arranged in this order from an object side. The first and second lens groups are movable along an optical axis for zooming.
In such a two-group zoom lens system, the second lens group moves to an object side when the focal length of the entire lens system is increased. That is, the second lens group is located at most image side position when the zoom lens system is positioned at wide extremity (i.e., when the zoom lens system has the smallest focal length), and when the focal length of the zoom lens system is to be increased, the second lens group is moved towards the object side. Further, an aperture stop of such a conventional two-group zoom lens system is provided within the second lens group or in the vicinity thereof, and moves together with the second lens group when the zooming operation is performed.
A zoom lens system for the CCD camera preferably has a telecentric property so that rays emitted from the lens system are perpendicularly incident on the CCD so that color difference is avoided. The telecentric system requires a certain amount of an exit pupil distance, which is defined as a distance from an exit pupil of the entire lens system to an image plane. Since the aperture stop moves together with the second lens group in the conventional two-group zoom lens system, the exit pupil distance decreases as the second lens group and the aperture stop move toward the image side.
However, the exit pupil distance of the conventional two-group zoom lens system is too short to have sufficient telecentric property within the entire zooming range. Lack of the telecentric property is significant especially when the conventional two-group zoom lens system is located at the wide extremity since the second lens group, together with the aperture stop, is moved to its most image side position at the wide extremity. Accordingly, the conventional two-group zoom lens system is not suitable for a CCD camera.
Further, in the conventional zoom lens system described above, since the aperture stop moves during the zooming operation, a structure of the lens barrel would be complicated, which increases the manufacturing cost.
It should be noted that, in the specification, terms “wide extremity” and “tele extremity” are used for expressing the zooming condition. When it is described that the zoom lens system is positioned to the “tele extremity”, all the elements included in the zoom lens are positioned such that the zoom lens system has the greatest focal length. Similarly, when it is described that the zoom lens system is positioned to the “wide extremity”, all the elements included in the zoom lens system are positioned such that the zoom lens system has the smallest focal length.
In the conventional two-group zoom lens system, when a zooming ratio exceeds two, a moving amount of the first lens group is relatively large. Therefore, the length of the entire lens system is relatively long. Further, variation of aberrations is relatively great, and accordingly, it is difficult to maintain a high optical performance in the entire zooming range.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide an improved zoom lens system that has a zooming ratio of approximately 2.5, and has a sufficient exit pupil distance so that a sufficient telecentric property can be obtained throughout the entire zooming range.
For the above object, according to the present invention, there is provided a zoom lens system that includes:
a negative first lens group that is movable along an optical axis for zooming;
a positive second lens group that is movable along the optical axis for zooming, the positive second lens group being disposed at an image side of the negative first lens group; and
an aperture stop that is disposed between the first and second lens groups, the aperture stop being fixed to keep the position thereof during zooming,
wherein the following conditions (1) and (2) are satisfied,
0.7<(
f
i
−f
w
)/(
f
t
−f
w
)<1.0 (1)
0.37
<d
i
/f
w
(2)
where
f
i
is a focal length of the entire system when the total length of the entire system becomes shortest,
f
w
is the shortest focal length of the entire system,
f
t
is the longest focal length of the entire system, and
d
i
is air gap, along the optical axis, between the first lens group and the aperture stop when the total length becomes shortest.
The zoom lens system of the present invention further satisfies at least one of the following conditions (3) and (4);
0.3
<f
w
/f
2<0.7 (3)
0.7<
D
w
/f
2
<1.2 (4)
where
f
2
is focal length of the second lens group,
D
w
is distance between the aperture stop and a first principal point of the second lens group at the wide extremity.
From a view point of moving loci of the lens groups, it is preferable that the first lens group is moved such that the distance between the first lens group and the aperture stop becomes the smallest at an intermediate position between the wide extremity and the tele extremity, and that the second lens group moves such that the distance between the second lens group and the aperture stop decreases gradually as the entire focal length increases.
The first lens group preferably includes at least two negative lenses and the following condition (5) is satisfied;
1.65
<N
n
(5)
where
N
n
is average refractive index of the negative lenses in the first lens group at d-line.
The zoom lens system of the present invention can be formed from two lens groups, while the present invention can be applied to a three-group zoom lens system that includes movable first and second lens group and a stationary third lens group disposed between the second lens group and an image plane.
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1
shows a structure of a zoom lens system, at a wide extremity, according to a first embodiment;
FIGS. 2A
,
2
B,
2
C and
2
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 1
is located at the wide extremity;
FIGS. 3A
,
3
B,
3
C and
3
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 1
is located at an intermediate position within a zooming range;
FIGS. 4A
,
4
B,
4
C and
4
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 1
is located at the tele extremity;
FIG. 5
shows a structure of a zoom lens system, at a wide extremity, according to a second embodiment;
FIGS. 6A
,
6
B,
6
C and
6
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 5
is located at the wide extremity;
FIGS. 7A
,
7
B,
7
C and
7
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 5
is located at an intermediate position within a zooming range;
FIGS. 8A
,
8
B,
8
C and
8
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respectively, when the zoom lens system shown in
FIG. 5
is located at the tele extremity;
FIG. 9
shows a structure of a zoom lens system, at a wide extremity, according to a third embodiment;
FIGS. 10A
,
10
B,
10
C and
10
D are graphs showing spherical aberration, lateral chromatic aberration, astigmatism and distortion, respec
Ito Takayuki
Ozaki Hiroyasu
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Thompson Tim
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