Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2000-03-06
2001-12-04
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S656000, C359S657000, C359S658000, C359S659000, C359S660000, C359S661000, C359S642000
Reexamination Certificate
active
06327101
ABSTRACT:
RELATED APPLICATIONS
This application claims the priorities of Japanese Patent Application No. 11-066651 filed on Mar. 12, 1999 and Japanese Patent Application No. 2000-057634 filed on Mar. 2, 2000, which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an objective lens disposed at the tip portion of an endoscope, and particularly relates to an objective lens for an endoscope having a focusing mechanism.
2. Description of the Prior Art
Conventionally, a general objective lens for an endoscope comprises a first lens group with positive refractive power and a second lens group with negative refractive power, and has about an 8-100 mm viewing range on the object side.
In an endoscope employing such an objective lens, the viewing subject is located in the depth of field of the objective lens, and picture images are transmitted by a solid imaging element such as a CCD or an image guide fiber.
Moreover, in order to observe viewing subjects such as the affected parts as a whole, it is preferable to locate an objective lens apart from the viewing subjects, and to locate the objective lens close to the viewing subjects so as to observe the viewing subjects in detail. In particular, in recent medical treatments demanding highly precise observation and more accurate diagnosis, it is desirable to develop an objective lens for an endoscope which can provide preferable picture images over a wide viewing range.
However, in the above-described conventional objective lens for an endoscope, the depth of field is fixed because of a fixed diaphragm, and viewing subjects cannot be clearly observed beyond the depth of field.
There is also an objective lens for an endoscope having a focusing mechanism. However, since the objective lens consists of a first lens group having negative refractive power and a second lens group having positive refractive power, it is difficult to preferably correct various aberrations such as coma when a concave lens is too powerful in the first lens group with negative refractive power, which thus requires further improvements.
SUMMARY OF THE INVENTION
In view of such circumstances, it is an object of the present invention to provide a simple objective lens for an endoscope which can provide preferable picture images over a wide perspective viewing range.
An objective lens for a variable viewing depth endoscope in accordance with the present invention comprises, successively from the object side, a first lens group having positive refractive power and a second lens group which has positive refractive power and which is movable along the optical axis; wherein near point focusing is made possible with the second lens group moved to the object side, and far point focusing is made possible with the second lens group moved to the image side; and wherein the composite focal length f
N
of a whole system at the near point view is set to be shorter than a composite focal length f
F
of the whole system at the far point view.
Also, the objective lens for a variable viewing depth endoscope in accordance with the present invention satisfies the following conditional expression (1):
0.5
<f
1
/f
2
<0.7 (1)
wherein f
1
is a focal length of the first lens group, and f
2
is a focal length of the second lens group.
Moreover, the objective lens for a variable viewing depth endoscope in accordance with the present invention has a brightness diaphragm disposed in the first lens group.
Furthermore, in the objective lens for a variable viewing depth endoscope in accordance with the present invention, the first lens group, successively from the object side, comprises a concave lens having a surface with strong curvature directed onto the image side, a convex lens having a surface with strong curvature directed onto the object side, a brightness diaphragm and a convex lens having a surface with strong curvature directed onto the image side.
Here, both of the image-side surfaces of the convex lens having a surface with strong curvature directed onto the object side and the object-side surface of the convex lens having a surface with strong curvature directed onto the image side may be made flat.
In the objective lens for a variable viewing depth endoscope in accordance with the present invention, moreover, the mobility of the second lens group is set to be located at a location nearly identical to a near point of depth of field with the second lens group moved closest to the image side and the far point of depth of field with the second lens group moved closest to the object side.
REFERENCES:
patent: 4854685 (1989-08-01), Corbasson
patent: 5940220 (1999-08-01), Suenaga
patent: 6160668 (2000-12-01), Rogers
Epps Georgia
Fuji Photo Optical Co., Ltd.
Lucas Michael A.
Snider Ronald R.
Snider & Associates
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