Wide-angle lens with long back focus

Details Wide-angle lens with long back focus Wide-angle lens with long back focus

1999-07-01

2001-01-30

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S680000, C359S682000

Reexamination Certificate

active

06181483

ABSTRACT:

RELATED APPLICATIONS
This application claims the priority of Japanese Patent Application No. 10-207381 filed on Jul. 23, 1998, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wide-angle lens having a taking angle of view of about 100 degrees used in a surveillance camera or the like; and, in particular, to a wide-angle lens having a sufficiently long back focus between the lens and its image surface.
2. Description of the Prior Art
In general, for viewing or capturing a wider area, lenses usable at a wide angle of about 100 degrees are demanded in TV cameras for commercial use which are employed as video cameras, surveillance cameras, and the like. For such a demand, there have been proposed wide-angle lenses disclosed in Japanese Unexamined Patent Publication Nos. 9-80303 and 9-127413.
In the wide-angle lenses disclosed in Japanese Unexamined Patent Publication Nos. 9-80303 and 9-127413, however, back focus is less than three times the focal length thereof. For disposing a three-color-separating optical system between the lens and the image surface, a longer back focus is demanded. Specifically, a back focus of about five times the focal length is required as a back focus sufficient for disposing the three-color-separating optical system.
Also, when disposing a three-color-separating optical system between a wide-angle lens and its image surface, the problem of color shading is needed to be taken into account. Luminous fluxes emitted from the wide-angle lens would be incident on a dichroic film of the three-color-separating optical system with their angles greater as they are directed nearer the corner portions of the screen. Color shading is a phenomenon in which, as the angle of light beam incident on the dichroic film varies, the optical path length within the thin film would vary, thus changing its spectral characteristics, whereby colors would appear differently between the upper and lower parts of the screen.
It is demanded for a wide-angle lens coexisting with a three-color-separating lens to suppress the occurrence of such color shading and favorably correct various kinds of aberration, while having a sufficient back focus.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a wide-angle lens having a long back focus which is about five times the focal length, while exhibiting a wide angle of 100 degrees in terms of taking angle of view and suppressing the occurrence of color shading.
It is another object of the present invention to favorably correct various kinds of aberration in the above-mentioned wide-angle lens having a long back focus.
The wide-angle lens having a long back focus in accordance with the present invention comprises, successively from an object side, a first lens group having a negative refracting power and a second lens group having a positive refracting power; wherein the first lens group comprises, successively from the object side, a first-a lens group having a negative refracting power and a first-b lens group having a positive refracting power; the wide-angle lens satisfying the following conditional expressions (1) and (2):
5.0
<Bf/f
  (1)
−0.05<tan &thgr;<0.05  (2)
where
f is the focal length of the whole system;
Bf is the back focus of the whole system; and
&thgr; is an angle formed between a principal ray directed to a screen corner portion and an optical axis.
Preferably, the wide-angle lens satisfies at least one of the following conditional expressions (3) and (4):
−0.50
<f/f
G1
<−0.10  (3)
−0.45
<f/f
G1a
<−0.23  (4)
where
f
G1
is the composite focal length of the first lens group; and
f
G1a
is the composite focal length of the first-a lens group.
Preferably, in the wide-angle lens having a long back focus in accordance with the present invention, the first lens group comprises, successively from the object side, a first lens made of a negative meniscus lens having a convex surface directed onto the object side, a second lens made of a negative lens, a third lens made of a biconvex lens, a fourth lens made of a negative lens, and a fifth lens made of a positive lens; and
the second lens group comprises, successively from the object side, a sixth lens made of a biconcave lens, a seventh lens made of a biconvex lens, eighth and ninth lenses constituting a cemented lens having a positive refracting power as a whole, and a tenth lens made of a biconvex lens;
the wide-angle lens satisfying the following conditional expressions (5) and (6):
0.18
<N
8
−N
9
<0.38  (5)
8.0
<R
13
/D
13
<30.0  (6)
where
N
i
is the refractive index of the i-th lens from the object side;
R
13
is the radius of curvature of the sixth lens on an image surface side; and
D
13
is the axial space between the sixth and seventh lenses.


REFERENCES:
patent: 5900987 (1999-05-01), Kreitzer
patent: 09080303 (1997-03-01), None
patent: 09127413 (1997-05-01), None

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