Objective lens for endoscope

An objective lens for an endoscope including a first lens group including one negative lens, a second lens group having a positive power, a third lens group having a negative power, and an aperture stop provided between the first lens group and the second lens group. The objective lens satisfies the relationships: EQU -1.4<f.sub.1 /f<-0.5, EQU 0.55<f.sub.2 /f<0.85; EQU and EQU f.sub.3 /f<-2.0, Where f designates a focal length of the entire objective lens; f.sub.1 designates a focal length of the first lens group; f.sub.2 designates a focal length of the second lens group; and f.sub.3 designates a focal length of the third lens group.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an objective lens for a medical or 
industrial endoscope. 
2. Description of Related Art In an objective lens for an endoscope, a 
negative lens having a high power is usually provided at a front end of 
the objective lens to obtain a wide angle of field. This inevitably causes 
a negative distortion. In particular, a considerable negative distortion 
occurs at first surface of frontmost negative lens. It has been proposed 
to decrease the radius of curvature of the first surface of the frontmost 
negative lens in order to reduce the negative distortion. However, the 
decreased radius of curvature of the first surface tends to cause water 
drops to remain on the first surface, and accordingly, the proposal is not 
practicable. 
It has been also proposed to incorporate an aspherical lens within the lens 
system to reduce the distortion. This however increases the manufacturing 
cost of the objective lens. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide an inexpensive 
objective lens for an endoscope, in which the above-noted distortion can 
be reduced. 
The basic concept of the present invention resides in a lens arrangement 
that requires no expensive aspherical lens or that the spherical lens 
elements, constituting the lens arrangement, must meet to reduce the 
distortion. 
To achieve the object mentioned above, according to an aspect of the 
present invention, there is provided an objective lens for an endoscope 
including a first lens group having of one negative lens, a second lens 
group having a positive power, a third lens group having a negative power, 
and an aperture-stop provided between the first lens group and the second 
lens group. 
Preferably, the objective lens satisfies the relationships: 
EQU -1.4&lt;f.sub.1 /f&lt;-0.5 (1) 
EQU 0.55&lt;f.sub.2 /f&lt;0.85 (2) 
EQU f.sub.3 /f&lt;-2.0 (3) 
wherein 
f designates a focal length of the entire objective lens, 
f.sub.1 designates a focal length of the first lens group, 
f.sub.2 designates a focal length of the second lens group, 
f.sub.3 designates a focal length of the third lens group. 
The second lens group can include one positive lens and a cemented lens 
pair including a positive and a negative lenses. 
The third lens group is preferably comprises of a negative meniscus lens 
having a convex surface that faces an object side. Preferably, the 
negative meniscus lens satisfies the relationships: 
EQU 0.7&lt;R.sub.L /f&lt;1.2; (4) 
EQU and 
EQU 1.2&lt;L.sub.A /f&lt;2.5 (5) 
wherein 
R.sub.L designates a radius of curvature of an image-side surface of the 
negative meniscus lens, and 
L.sub.A designates a distance between the image-side surface of the 
negative meniscus lens and the aperture stop. 
The present disclosure relates to subject matter contained in Japanese 
patent application No. HEI 6-7911 (filed on Jan. 27, 1994).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In general, a large amount of distortion occurs in a surface located far 
from an aperture stop. As mentioned above, if a negative lens is located 
at a front end of an objective lens, i.e., closest to an object to be 
observed, a large degree of negative distortion occurs on the first lens 
surface located farthest from the aperture stop. Consequently, it is 
necessary to produce a positive distortion on the lens surface other than 
the first lens surface to cancel or reduce the negative distortion. 
To this end, if a lens having a positive power is placed closer to the 
object to be observed than the aperture stop in order to produce a 
positive distortion that cancels or reduces the negative distortion, a 
technical effect to increase the angle of field of the negative lens can 
no be longer expected, and hence, its performance as an objective lens for 
an endoscope cannot be obtained. 
Under these circumstances, in the present invention, a negative lens is 
located as far from the aperture stop on the image side as possible to 
produce a positive distortion on a surface located closer to an image than 
the aperture stop, to thereby cancel the negative distortion. 
It is necessary for an objective lens system, as a whole, to have a 
positive power. To this end, a positive lens group is provided between the 
negative lens located far from the aperture stop on the image side first 
lens group including a negative lens. Namely, the objective lens according 
to the present invention comprises the first negative lens group, a second 
positive lens group and a third negative lens group. 
Namely, in an objective lens for an endoscope according to the present 
invention, the first lens group is made of a negative lens to obtain a 
wide angle of field; 
the second lens group is made of a lens group having a positive power, so 
that the objective lens has a positive power; and, the third lens group is 
made of a lens having a negative power to correct a large amount of 
negative distortion caused by the first lens group. 
In this arrangement, the negative lens of the third lens group preferably 
comprises a meniscus lens having a convex surface that faces the object 
side. The convex surface of the meniscus lens provides a difference in the 
angle of refraction of an off-axis principal ray between a first and a 
second lens surface thereof, to thereby enhance the distortion correcting 
effect. Furthermore, the meniscus lens contributes to a reduction of a 
Petzval sum to restrict the curvature of the field. 
Furthermore, the inventors of the present invention have conceived the 
requirements defined by formulae (1) through (5), mentioned above, to 
obtain a better optical performance as an objective lens for an endoscope. 
Formula (1) specifies the power of the first lens group. The negative lens 
of the first lens group is adapted not only to increase the angle of field 
of the whole lens system, but also to correct aberrations, such as a 
spherical aberration, chromatic aberration, and curvature of the field, 
caused by the second lens group having a strong positive power. 
If value of the formula (1) is larger than the upper limit, the power is so 
strong that a large amount of negative distortion occurs. Conversely, if 
the value is smaller than the lower limit, the power is so weak that the 
angle of field is too small, although the distortion can be effectively 
restricted. Moreover, under-corrected aberrations caused in the second 
lens group cannot be corrected, thus resulting in a deteriorated image. 
Formula (2) specifies the power of the second lens group. In the whole lens 
system, it is only the second lens group that has a positive power and 
contributes to a formation of an object image. Namely, the second lens 
group has a power strong enough to cancel the negative power of the first 
lens group, so that the whole lens system has a positive power and causes 
under-corrected aberrations, as mentioned above. Formula (2) also 
specifies the requirement to make the whole lens system compact in 
connection with the formula (1). 
If value of the formula (2) exceeds the upper limit, the power is so weak 
that a length of the whole lens system increases, although the amount of 
aberration is reduced. Also, if the power of the second lens group is so 
weak that the value of formula (2) exceeds the upper limit, it is 
necessary to reduce the negative power of the first lens group accordingly 
in order to keep the power of the whole lens system constant. 
Consequently, the angle of field is too small to be practically accepted 
as an objective lens for an endoscope. 
Conversely, if the value of formula (2) is smaller than the lower limit, 
the power of the second lens group is so strong that there is a 
particularly large amount of spherical aberration and curvature of the 
field. This results in a decreased contrast of an object image and 
deterioration of uniform image quality over the entire image. To prevent 
this, it is theoretically possible to increase the negative power of the 
first lens group. However, in this solution, the distortion is increased, 
which is not preferable. 
Formula (3) specifies the power of the third lens group. The third lens 
group has a power weaker than the powers of the first and second lens 
groups, so as to have less influence on the image forming factors other 
than the distortion. If the value of formula (3) is above the upper limit, 
the power of the third lens group is too strong to correct astigmatism and 
transverse chromatic aberrations, which is not advisable in view of the 
image quality of the off-axis rays. 
Formula (4) specifies a radius of curvature of the lens surface of the 
negative meniscus lens of the third lens group that is located on the 
image side. The requirement defined in formula (4) is necessary to 
effectively correct distortion, in connection with the requirement defined 
in formula (3). If the radius of curvature of the surface of the third 
lens group that is located adjacent to the image side is determined to 
meet the requirement defined in formula (4), there is a difference in the 
angle of refraction of the off-axis rays between the first and second lens 
surfaces of the meniscus lens, so that the negative distortion caused by 
the first lens group can be cancelled by the positive distortion caused on 
the second lens surface. In other words, if the power of the third lens 
group is reduced, it is possible to provide a difference in height of the 
off-axis rays on the first and second lens surfaces to thereby provide a 
difference in the angle of refraction of the off-axis rays between the 
first and second lens surfaces, so long as the third lens group has an 
appropriate thickness and meets the requirement defined in formula (4) 
discussed below. Hence, the distortion can be effectively corrected. 
If the value of formula (4) exceeds the upper limit, the radius of 
curvature is so large that the difference in the angle of refraction is 
too small to produce an amount of positive distortion enough to cancel the 
negative distortion. If the value is smaller than the lower limit, the 
radius of curvature is reduced so that the large amount of positive 
distortion can be produced, but it is practically difficult to manufacture 
a meniscus lens having such a small radius of curvature. In addition to 
the difficulty, there is a possibility of a mechanical contact of the 
peripheral edge of the negative meniscus lens with an end surface of a 
bundle of fibers located at the image surface or a surface of an 
associated CCD, etc., which is not preferable. 
Formula (5) specifies a distance between an aperture stop and the lens 
surface of the negative meniscus lens that is located on the image side. 
As mentioned above, positive distortion is caused by the lens surface of 
the negative meniscus lens on the image side so as to cancel the negative 
distortion of the first lens group. To enhance the cancellation effect, 
the lens surface should be spaced from the aperture stop as much as 
possible. However, if the distance is so large that the value of formula 
(5) exceeds the upper limit, the length of the whole lens system 
increases, which would not allow for a compact lens. If the value of 
formula (5) is below the lower limit, the lens surface of the negative 
meniscus lens on the image side is too close to the aperture stop to 
correct the negative distortion. 
To reduce the whole lens length and the number of the lens elements, to 
thereby curtail the manufacturing cost, the second lens group preferably 
comprises one positive sub-lens (IIa sub-lens) and a cemented sub-lens 
(IIb sub-lens), consisting of a pair of positive and negative lenses. 
To obtain a better optical performance, the lens system preferably 
satisfies the following relationships: 
EQU -0.75&lt;R.sub.A /f&lt;-0.55; (6) 
EQU 0.50&lt;.vertline.R.sub.B .vertline./f&lt;1.30; (7) 
EQU and 
EQU 1.67&lt;N.sub.2, (8) 
wherein 
R.sub.A designates a radius of curvature of the surface of the positive 
sub-lens of the second lens group, that is located on the image side, 
R.sub.B designates a radius of curvature of a cemented surface of the 
cemented sub-lens of the second lens group, and 
N.sub.2 designates an average value of the refractive index of the positive 
sub-lens and the positive lens in the cemented sub-lens. 
Formula (6) specifies the radius of curvature of the surface of the 
positive sub-lens belonging to the second lens group, that is located on 
the image side. If the lens system meets the requirements defined in 
formulae (2) and (6), the positive sub-lens of the second lens group is 
shaped such that the radius of curvature of the lens surface thereof 
located on the image side is small. Consequently, an appropriate amount of 
spherical aberration can be caused, so that the focal position on the axis 
is slightly shifted toward an object to be observed from a Gaussian image 
plane. Consequently, the bundle of off-axis rays which tend to occur due 
to an under-corrected abberation can be well balanced. 
If the value of formula (6) exceeds the upper limit, the absolute value of 
the radius of curvature of the lens surface is so small, that is, the 
surface power is so strong that the amount of the under spherical 
aberration is too large to balance the axial rays and the off-axis rays. 
Moreover, there is an increased astigmatism which is not acceptable, 
depending on the purpose of usage. If the value of formula (6) is below 
the lower limit, the surface power is too small to obtain the necessary 
amount of under spherical aberration to balance the axial rays and the 
off-axis rays. 
Formula (7) specifies a radius of curvature of a connecting surface 
(cemented surface) of the cemented sub-lens belonging to the second lens 
group. The connecting surface influences the balance of the axial 
chromatic aberration and the transverse chromatic aberration. If the value 
of formula (7) is above (or below) the upper limit (or lower limit), it is 
difficult to appropriately balance the axial chromatic aberration and the 
transverse chromatic aberration. Consequently, no optical performance of 
an objective lens for an endoscope can be obtained, depending on the 
purpose of usage thereof. 
Formula (8) specifies a mean value of the refractive index of the positive 
lenses belonging to the second lens group. If the positive lenses of the 
second lens group are made of glasses having a mean refractive index which 
meets the requirement defined in formula (8), not only can the Petzval sum 
be restricted, but also the curvature of the field can be reduced. In 
connection with the requirement defined in formula (8), the requirement 
defined in formula (6) ensures that a good image quality can be obtained 
over the entire image surface. If the value of formula (8) is above the 
upper limit, it is difficult to obtain a good image quality over the 
entire image surface. 
As mentioned above, according to the present invention, an objective lens 
having less distortion can be realized by spherical lenses in combination, 
without using an aspherical lens. However, it is possible to incorporate 
an aspherical lens in a basic arrangement of the present invention to 
enhance the distortion correcting effect or aberration correcting effect. 
Several examples of numerical data will be discussed below. In the 
following seven embodiments, the objective lens system comprises a first 
negative lens group 11, a second positive lens group 12, a third lens 
group 13 made of a negative meniscus lens, and two plane-parallel plates 
14 (including filter and glass cover, etc.), in this order from the object 
side. The second positive lens group 12 comprises of one positive sub-lens 
12-1 and a cemented sub-lens consisting of a pair of positive and negative 
lenses 12-2 and 12-3. Aperture stop S is provided between the first lens 
group 11 and the second lens group 12. The thickness (or optical constant) 
of the plane-parallel plates 14 does not have a substantial influence on 
the optical efficiency of the objective lens system. 
First Embodiment 
FIG. 1 shows a lens arrangement of an objective lens, according to a first 
embodiment of the present invention. 
Numerical data of the objective lens shown in FIG. 1 is shown in Table 1 
below. Diagrams of various aberrations thereof are shown in FIG. 2. 
In FIG. 2, "SA" designates a spherical aberration, "SC" designates a sine 
condition, "d-line", "g-line" and "C-line" designates chromatic aberration 
represented by the spherical aberration and the transverse chromatic 
aberration, at the respective wavelengths, "S" designates a sagittal ray, 
and "M" designates a meridional ray. 
In the Tables and the drawings, "F.sub.NO " designates the f-number, "f" 
designates a focal length, "M" designates a lateral magnification, 
".OMEGA." designates a half angle of view, "Y" designates an image height, 
"r.sub.i " designates a radius of curvature of each lens surface, "d.sub.i 
" designates a lens thicknesses or distance between the lenses, "N" 
designates a refractive index of the d-line, and "d" designates an Abbe 
number of the d-line, respectively. 
TABLE 1 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.69 
M = -0.143 
.omega. = 54.7.degree. 
Y = 1.7 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 0.72 1.88300 
40.8 
2 1.004 0.05 -- -- 
STOP .infin. 0.02 -- -- 
3 5.325 1.43 1.78650 
50.0 
4 -1.061 0.06 -- -- 
5 19.642 1.21 1.65160 
58.5 
6 -1.073 0.30 1.84666 
23.9 
7 -2.355 0.05 -- -- 
8 3.462 0.30 1.61293 
37.0 
9 1.657 0.87 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Second Embodiment 
FIG. 3 shows a lens arrangement of an objective lens according to a second 
embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 3 is shown in Table 2 
below. Diagrams of various aberrations thereof are shown in FIG. 4. 
TABLE 2 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.67 
M = -0.135 
.omega. 54.2.degree. 
Y = 1.7 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.89 1.88300 
40.8 
2 1.141 0.06 -- -- 
STOP .infin. 0.02 -- -- 
3 6.587 1.28 1.73520 
41.1 
4 -1.092 0.05 -- -- 
5 7.305 0.30 1.80518 
25.4 
6 1.384 0.95 1.74100 
52.7 
7 -3.099 0.05 -- -- 
8 2.715 0.30 1.74077 
27.8 
9 1.478 0.82 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Third Embodiment 
FIG. 5 shows a lens arrangement of an objective lens according to a third 
embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 5 is shown in Table 3 
below. Diagrams of various aberrations thereof are shown in FIG. 6. 
TABLE 3 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.69 
M = -0.140 
.omega. = 54.0.degree. 
Y = 1.7 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.35 1.88300 
40.8 
2 1.407 0.07 -- -- 
STOP .infin. 0.02 -- -- 
3 .infin. 1.27 1.77250 
49.6 
4 -1.158 0.05 -- -- 
5 7.916 0.30 1.80518 
25.4 
6 1.720 0.91 1.72916 
54.7 
7 -2.782 0.05 -- -- 
8 2.841 0.30 1.75550 
25.1 
9 1.497 0.83 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Fourth Embodiment 
FIG. 7 shows a lens arrangement of an objective lens according to a fourth 
embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 7 is shown in Table 4 
below. Diagrams of various aberrations thereof are shown in FIG. 8. 
TABLE 4 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.68 
M = -0.139 
.omega. = 53.8.degree. 
Y = 1.7 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.47 1.88300 
40.8 
2 1.399 0.07 -- -- 
STOP .infin. 0.02 -- -- 
3 .infin. 1.26 1.77250 
49.6 
4 -1.141 0.05 -- -- 
5 8.039 0.30 1.80518 
25.4 
6 1.682 0.91 1.72916 
54.7 
7 -2.799 0.05 -- -- 
8 2.908 0.30 1.75520 
27.5 
9 1.504 0.82 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Fifth Embodiment 
FIG. 9 shows a lens arrangement of an objective lens according to a fifth 
embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 9 is shown in Table 5 
below. Diagrams of various aberrations thereof are shown in FIG. 10. 
TABLE 5 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.63 
M = -0.134 
.omega. = 53.3.degree. 
Y = 1.7 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.52 1.88300 
40.8 
2 1.066 0.10 -- -- 
STOP .infin. 0.02 -- -- 
3 7.378 1.06 1.77250 
49.6 
4 -1.050 0.05 -- -- 
5 16.536 0.30 1.80518 
25.4 
6 1.567 0.83 1.72916 
54.7 
7 -2.531 0.05 -- -- 
8 2.348 0.37 1.75520 
27.5 
9 1.333 0.78 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Sixth Embodiment 
FIG. 11 shows a lens arrangement of an objective lens according to a sixth 
embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 11 is shown in Table 6 
below. Diagrams of various aberrations thereof are shown in FIG. 12. 
TABLE 6 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.77 
M = -0.148 
.omega. = 49.9.degree. 
Y = 1.6 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.55 1.88300 
40.8 
2 1.892 0.05 -- -- 
STOP .infin. 0.02 -- -- 
3 .infin. 1.33 1.77250 
49.6 
4 -1.122 0.05 -- -- 
5 20.684 0.30 1.80518 
25.4 
6 1.418 0.65 1.72916 
54.7 
7 -29.304 0.05 -- -- 
8 1.647 0.48 1.75520 
27.5 
9 1.419 0.70 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
Seventh Embodiment 
FIG. 13 shows a lens arrangement of an objective lens according to a 
seventh embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 13 is shown in Table 7 
below. Diagrams of various aberrations thereof are shown in FIG. 14. 
TABLE 7 
______________________________________ 
F.sub.NO = 1:5.6 
f = 1.75 
M = -0.145 
.omega. = 49.7.degree. 
Y = 1.6 
surface No. r.sub.i d.sub.i N .nu. 
______________________________________ 
1 .infin. 1.50 1.88300 
40.8 
2 1.331 0.05 -- -- 
STOP .infin. 0.02 -- -- 
3 7.143 1.31 1.77250 
49.6 
4 -1.085 0.13 -- -- 
5 -43.866 0.32 1.80518 
25.4 
6 1.591 0.59 1.72916 
54.7 
7 -6.830 0.05 -- -- 
8 1.651 0.49 1.75520 
27.5 
9 1.366 0.80 -- -- 
10 .infin. 0.50 1.52400 
69.0 
11 .infin. 1.00 1.53996 
59.5 
12 .infin. -- -- -- 
______________________________________ 
The values of formulae (1) through (5) in each embodiment are shown in 
Table 8 below. 
TABLE 8 
______________________________________ 
formula(1) formula(2) 
formula(3) 
______________________________________ 
Embodiment 1 
-0.671 0.717 -3.267 
Embodiment 2 
-0.775 0.692 -2.928 
Embodiment 3 
-0.947 0.700 -2.750 
Embodiment 4 
-0.941 0.696 -2.698 
Embodiment 5 
-0.740 0.674 -2.969 
Embodiment 6 
-1.209 0.812 -90.3 
Embodiment 7 
-0.864 0.751 -22.9 
______________________________________ 
formula(4) formula(5) 
______________________________________ 
Embodiment 1 
0.978 1.978 
Embodiment 2 
0.886 1.763 
Embodiment 3 
0.888 1.715 
Embodiment 4 
0.893 1.704 
Embodiment 5 
0.817 1.631 
Embodiment 6 
0.801 1.614 
Embodiment 7 
0.783 1.656 
______________________________________ 
As can be seen from Table 8 above, all seven of the embodiments satisfy the 
requirements defined by formulae (1) through (5). Moreover, an objective 
lens for an endoscope according to the present invention has a small 
transverse chromatic aberration and can effectively correct other 
aberrations. 
The values of formulae (6) through (8) in each embodiment are shown in 
Table 9 below. 
TABLE 9 
______________________________________ 
formula(6) formula(7) 
formula(8) 
______________________________________ 
Embodiment 1 
-0.628 0.635 1.719 
Embodiment 2 
-0.654 0.829 1.738 
Embodiment 3 
-0.685 1.018 1.751 
Embodiment 4 
-0.679 1.001 1.751 
Embodiment 5 
-0.644 0.961 1.751 
Embodiment 6 
-0.633 0.800 1.751 
Embodiment 7 
-0.622 0.912 1.780 
______________________________________ 
As can be seen from Table 9 above, all seven embodiments satisfy the 
requirements defined by formulae (6) through (8). 
As may be understood from the above discussion, according to the present 
invention, the distortion of an objective lens for an endoscope can be 
effectively restricted or eliminated by the spherical lenses in 
combination, without using an aspherical lens. Moreover, according to the 
present invention, an inexpensive objective lens for an endoscope can be 
obtained.