Variable-focus lens system

Details Variable-focus lens system Variable-focus lens system

2001-12-21

2004-01-27

Sugarman, Scott J. (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S682000

Reexamination Certificate

active

06683730

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a variable-focus lens system in which a focal length is varied, and in particular, relates to a variable-focus lens system which is usable in wavelength ranges, particularly in a visible light wavelength range (about 400~700 nm) through to a near-infrared light wavelength range (about 700~1000 nm).
2. Description of the Prior Art
A surveillance camera preferably has a photographing lens system which can perform photographing operation in a visible light wavelength range at daytime, and can perform photographing operation in a near-infrared light wavelength range at nighttime. Some of such photographing lens systems have been put into practical use. However, it is still difficult to suitably correct aberrations, particularly chromatic aberration, in a visible wavelength light range over to a near-infrared light wavelength range, without making the lens arrangement complicated.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a variable-focus lens system which can suitably correct aberrations in a visible light wavelength range and a near-infrared light wavelength range.
In order to achieve the above-mentioned object, there is provided a variable-focus lens system including a negative powered (hereinafter, negative) front lens group, and a positive powered (hereinafter, positive) rear lens group, in this order from the object. The focal length of the variable-focus lens system is varied by changing the distance between the front lens group and the rear lens group, and the lens system satisfies the following conditions:
−3.0
<fx/Fw<−
2.5  (1)
3.0
<fy/Fw<
3.5  (2)
wherein
fx designates the focal length of the front lens group (fx<0);
fy designates the focal length of the rear lens group (fy>0); and
Fw designates the focal length of the entire variable-focus lens system at the short focal length extremity.
It is preferable that the rear lens group is used within a range to satisfy the following condition:
−0.9
<m<−
0.3  (3)
wherein
m designates the image-forming magnification of the rear lens group.
More concretely, the front lens group
10
can include two sub-lens groups constituted by three lens elements L1 through L3, and the rear lens group
20
includes five sub-lens groups constituted by six lens elements. Furthermore, the rear lens group
20
is preferably constituted by a positive lens element L4, a positive lens element L5, a negative lens element L6, a positive lens element L7, and positive cemented lens elements constituted by a negative lens element L8 and a positive lens element L9, in this order from the front lens group
10
. In addition, the positive lens element L7 can satisfy the following conditions:
1.73
<n
7<1.83  (4)
30<&ngr;7<40  (5)
wherein
n7 designates the refractive index of the lens element L7; and
&ngr;7 designates the Abbe number of the lens element L7.
Still further, one of the lens elements L8 and L9 which constitute the cemented lens elements can be composed of a convex lens element satisfying conditions (6) and (7), and the other of the lens elements L8 and L9 can be composed of a concave lens element satisfying 13 conditions (8) and (9):
70<&ngr;-convex  (6)
14.0<f-convex/Fw<25.0  (7)
30 >&ngr;-concave  (8)
−7.3<f-concave/
Fw<−
4.5  (9)
wherein
&ngr;-convex designates the Abbe number of the convex lens element;
f-convex designates the focal length of the convex lens elements;
&ngr;-concave designates the Abbe number of the concave lens element; and
f-concave designates the focal length of the concave lens elements.
The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-3691 (filed on Jan. 11, 2001) which is expressly incorporated herein in its entirety.


REFERENCES:
patent: 4838666 (1989-06-01), Shiraishi
patent: 0620467 (1994-10-01), None
patent: 2090998 (1982-07-01), None

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