JP2005091655A - Endoscope objective optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an endoscope objective optical system the entire length of which is short and the outside diameter of the lens of which is small, and which has excellent aberration performance as an enlarging endoscope objective optical system which varies the focal length of the entire system by moving a part of lens groups and realizes normal observation at the wide angle of visibility and enlarging observation at high magnification. <P>SOLUTION: The endoscope objective optical system is constituted of a 1st negative lens group, a 2nd positive lens group, a 3rd positive lens group and a 4th lens being a meniscus lens whose image side surface is convex in order from an object side. A focusing state is held while changing the focal length of the entire system by moving the 2nd and the 3rd lens groups in a state where an object distance is changed without changing the entire length from the 1st lens group to an image surface, and satisfies a following conditional expression (1). (1) -0.05<fw/f4<0.01 provided that f4; the focal length of the 4th lens and fw; the focal length at the short focal length end of the entire system. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  In the present invention, while maintaining the entire length from the first lens to the image plane, a part of the lens group is moved to change the focal length of the entire system, and normal observation at a wide viewing angle and enlargement at a high magnification are performed. The present invention relates to a magnifying endoscope objective optical system that enables observation.

2. Description of the Related Art As an endoscope objective optical system that enables normal observation and magnified observation, an endoscope observation optical system that changes a focal length by moving some lens groups is known. When there is only one movable group, it is common for the magnification of the moving group to be at the same magnification in order to increase the zoom ratio while maintaining the compactness of the entire system and the constant overall length, but the object-image plane The distance between objects (distance between object images) is the shortest at the point where the magnification of the moving group is equal, and the object distance increases when the lens group is moved from this shortest state to the enlargement side, so it can be used in the intermediate focal range. Not suitable for. Japanese Patent Application Laid-Open No. 2000-267002 includes an example in which only one group is moved and scaled to change the magnification, and the moving group does not sandwich the same magnification. Since the magnification is increased to be as short as about 0.8 mm (when the focal length at the wide angle end is 1 mm), there will be a portion where the illumination is not successful. In addition, the outer diameter of the first lens unit is increased in order to obtain a wide angle of view during normal observation. In Japanese Patent No. 2876252, zooming is performed by a negative lens unit having a large power, so that the number of lenses is large for aberration correction.

In Japanese Patent Laid-Open No. 11-295596, the lens group and the image sensor are moved.
Since an image pickup device such as this is accompanied by a signal processing board and a cable, a strong driving force is required to move the image pickup element. In particular, when driving with an actuator or motor, it is important to reduce the load on the moving group (body). Therefore, it is desirable that the image plane be fixed regardless of the magnification.

Japanese Patent Laid-Open No. 2001-166203 and Japanese Patent Laid-Open No. 2001-91832 are conventional examples of changing the magnification by moving a plurality of lens groups. The former is composed of three groups of negative-positive-positive as a whole.
Although the magnification is changed by moving the second and third lens groups, the power of the second group is small and the magnification ratio is small. In addition, the field curvature during magnification observation is large. The latter is a negative-positive-negative-positive four-group configuration, and zooming is performed by moving the third group and the second or fourth group. However, since the power of the third lens group is strong, the aberration is corrected. In addition, the number of lenses increases and the overall length becomes longer.
JP 2000-267002 A Japanese Patent No. 2876252 JP 11-295596 A Japanese Patent Laid-Open No. 2001-166203 JP 2001-91832 A

  The present invention relates to a magnifying endoscope objective optical system that allows normal observation at a wide viewing angle and magnified observation at a high magnification by changing the focal length of the entire system by moving some lens groups. An object of the present invention is to obtain an endoscope objective optical system having a small lens outer diameter and good aberration performance.

The endoscope objective optical system according to the present invention includes, in order from the object side, a first lens group having a negative power, a second lens group having a positive power, a third lens group having a positive power, and a convex surface on the image side. The entire system is obtained by moving the second lens group and the third lens group while changing the object distance without changing the total length from the first lens group to the image plane. The in-focus state is maintained while changing the focal length of and the following conditional expression (1) is satisfied.
(1) -0.05 <fw / f4 <0.01
However,
f4: focal length of the fourth lens,
fw: focal length at the short focal length end of the entire system,
It is.

The endoscope objective optical system according to the present invention preferably satisfies the following conditional expression (2).
(2) -2.2 <f1 / fw <-1.5
However,
f1: focal length of the first lens group,
It is.

Moreover, it is preferable that the following conditional expression (3) is satisfied.
(3) 0.80 <d _2w / d _2t <1.32
However,
d _2w ; distance from the rear principal point of the second lens group to the front principal point of the third lens group at the short focal length end;
d _2t ; distance from the rear principal point of the second lens group to the front principal point of the third lens group at the maximum magnification position;
It is.

The endoscope objective optical system of the present invention preferably satisfies the following conditional expressions (4) and (5).
(4) 1.9 <f2 / fw <2.3
(5) 6 <f3 / fw <7
However,
f2: focal length of the second lens group,
f3: focal length of the third lens group,
It is.

Moreover, it is preferable that the following conditional expression (6) is satisfied.
_{(6) 0.4 <(f 23t} × fw) / (f 23w × ft) <0.65
However,
f _23t ; composite focal length at the maximum magnification position of the second lens group and the third lens group,
f _23w ; composite focal length at the short focal length end of the second lens group and the third lens group,
ft: focal length at the maximum magnification position of the entire system,
It is.

  According to the present invention, in a magnifying endoscope objective optical system that moves a part of lens groups to make the focal length of the entire system variable, and enables normal observation at a wide viewing angle and magnified observation at a high magnification. It is possible to obtain an endoscope objective optical system that has a small overall length and a lens outer diameter and good aberration performance.

FIG. 17 shows an aspect in which the endoscope objective optical system according to the present invention is applied to an electronic endoscope. The endoscope insertion unit 100 includes, in order from the object side, a fixed first lens group 10 having a negative power, a diaphragm S, a movable second lens group 20 having a positive power, and a positive power. , A fixed fourth lens group 40 of a meniscus lens having a convex surface on the image side, a cover glass (filters) CG, and an image sensor 50 are located. The fourth lens group 40 can take either positive or negative power. The diaphragm S is mounted on the second lens group 20 and moves together with the second lens group 20.

  In the above-described endoscope objective optical system, in order to shift from the normal observation state at the wide angle to the magnification observation state, the object distance is not changed without changing the total length from the first lens group 10 to the image sensor 50 (image plane). The second lens group 20 and the third lens group 30 are moved to the object side independently while changing the focal length of the entire system, while reducing the minimum magnification. Change from the end to the maximum magnification end. In other words, when the second lens group 20 and the third lens group 30 are based on the object distance OS at the short focal length end S, the second lens group 20 and the third lens group 30 are independently set on the object side. And the in-focus object distance OL is shortened toward the maximum magnification observation position L. The distance from the first lens group 10 to the image sensor 50 (image plane) does not change. Depending on the movement trajectory of the second and third lens groups, even if the object distance and magnification change monotonously when switching from the normal observation state at the wide angle to the magnification observation state, the change in the focal length of the entire system is monotonous. In some cases, the focal length becomes maximum on the way, and the focal length may be shorter in the maximum magnification state.

In this way, by moving the two lens groups, the second lens group 20 and the third lens group 30, the image plane is fixed with respect to the first lens group 10 when switching between normal observation and magnified observation at a wide angle. Furthermore, the change of the in-focus object distance can be made monotonous. The fact that this change is not monotonic means that the object moves relatively close to and away from the first lens during the transition from normal observation at wide angle to magnified observation, so that the object distance changes monotonously. This is important when observing in the middle region between the short focal length end and the maximum magnification position. When the object distance change is not monotonous, an operation that continuously moves from normal observation to magnified observation requires an unnatural operation of approaching the object to the middle of the scope but moving away from the middle. Poor nature (or skill required for operation).

  One of the features of this embodiment is that a meniscus fourth lens having a weak power and a convex surface on the image side is disposed at the rear (image surface side) of the moving group (second lens group and third lens group). In the point. The power of the fourth lens group can be positive or negative. In order to obtain a wide viewing angle during normal observation while reducing the lens outer diameter, it is preferable to increase the negative power of the first lens group. However, if the negative power of the first lens group is simply increased, the magnification of the first lens group is reduced. Therefore, in order to obtain a high magnification at the time of enlargement, it is necessary to increase the focal length of the moving group having a positive power. Arise. If the focal length of the moving group is increased, the moving amount increases and the overall length increases. Therefore, in the present embodiment, the fourth lens having a magnification of 1 or more (m4> 1, m4; lateral magnification of the fourth lens) is disposed at the rear of the second lens group and the third lens group. The power of the second lens group and the third lens group can be increased, and the overall length can be shortened while improving the aberration performance.

  Conditional expression (1) is a condition for the focal length of the fourth lens.

  If the upper limit of conditional expression (1) is exceeded, the magnification of the fourth lens becomes small, and it becomes difficult to increase the magnification of the entire system during enlargement while shortening the overall length and reducing the outer diameter. If the lower limit of conditional expression (1) is surpassed, the negative power of the fourth lens increases, and the telecentricity deteriorates. When the telecentricity deteriorates, the amount of peripheral light decreases due to shading that occurs on the CCD. In addition, coma aberration increases and imaging performance deteriorates. Further, in a fiber endoscope using an image guide fiber instead of the image pickup element 50, the peripheral light amount is similarly reduced by deviating from the range of NA (generally as small as about 0.3).

  The fourth lens has a small lens power, and can be reduced in overall length by reducing the telecentricity due to the positive refractive power of the final surface by forming a meniscus shape with a convex surface facing the image side.

Further, the observation distance of the endoscope is mainly in the range of about 2 to 15 mm from the first lens in magnified observation and normal observation, and the lateral magnification of the fourth lens at that time satisfies the conditional expression (7). desirable.
(7) 1.0 <m4 <1.2
However,
m4: lateral magnification of the fourth lens,
It is.

  If the lower limit of conditional expression (7) is not reached, the effect of reducing the overall length and diameter will be lost. If the magnification of the fourth lens is increased beyond the upper limit of conditional expression (7) (negative power is increased), the influence of the aberration generated in the fourth lens increases, resulting in a deterioration in telecentricity and aberration performance. Will increase.

  In order to shorten the overall length and reduce the diameter, it is desirable that the first lens group is composed of a single negative lens. When the first lens group is composed of a plurality of lenses, the distance between the stop and the first lens incident surface is increased, so that the entire length is increased and the first incident surface is increased in diameter. At this time, it is preferable that the first lens group satisfies the conditional expression (2) regarding the focal length.

  If the lower limit of conditional expression (2) is not reached, the outer diameter of the lens increases when attempting to obtain a wide angle of view through normal observation. In addition, the field curvature increases during enlargement. If the upper limit of conditional expression (2) is exceeded, the magnification of the first lens group decreases, and in order to increase the magnification of the entire system at the time of magnification, the focal length of the positive lens group becomes longer and the entire system becomes larger. To do. In addition, astigmatism and coma generated in the first lens group become large, and correction with other lens groups becomes difficult.

  In order to suppress a change in aberration performance during zooming (particularly performance degradation during zooming), it is preferable to satisfy conditional expression (3).

  If the lower limit of conditional expression (3) is not reached, the change in lateral chromatic aberration during normal observation and enlargement will become large, making it difficult to achieve a balance, and the resolution of either will decrease. If the upper limit of conditional expression (3) is exceeded, the combined focal length of the second lens group and the third lens group becomes short (magnification becomes small) at the time of enlargement, so in order to increase the magnification of the entire system at the time of enlargement, 2. The amount of movement of the third lens group is increased, or the magnification of the first lens group is decreased and the entire system is lengthened. In addition, astigmatism and coma generated in the first lens group become large, and correction with other lens groups becomes difficult.

  Conditional expression (4) defines the focal length of the second lens group.

  If the lower limit of conditional expression (4) is not satisfied, sufficient magnification cannot be obtained during magnified observation. Furthermore, the spherical aberration increases and the resolution decreases. If the upper limit of conditional expression (4) is deviated, the amount of movement of the second lens unit increases to increase the zoom ratio, and the overall length becomes longer.

  Conditional expression (5) defines the focal length of the third lens group.

  If the lower limit of conditional expression (5) is not satisfied, aberration performance, particularly astigmatism at the time of enlargement, will increase and resolution will decrease. If the upper limit of conditional expression (5) is not satisfied, the focal length of the second lens group will be shortened to maintain a wide angle during normal observation, or the negative power of the first lens group will become weak and the diameter will increase.

  Conditional expression (6) defines the range of change in the combined focal length of the moving group.

  If the lower limit of conditional expression (6) is not reached, the power variation of the moving group at the time of zooming is large, so that the fluctuation of aberration becomes large, and it becomes difficult to balance the aberration in each focal region. If the upper limit of conditional expression (6) is not met, the distance between the second and third lens groups becomes large, and the total length becomes long.

Next, specific examples will be described. In the various aberration diagrams and tables, the d-line, g-line, and c-line in the chromatic aberration (axial chromatic aberration) diagram and magnification chromatic aberration diagram represented by spherical aberration are aberrations for each wavelength, S is sagittal, and M is meridional. , M is the lateral magnification of the entire system, f is the focal length of the entire system, ODIS is the object distance (air equivalent distance from the object to the lens surface closest to the object), fB Is the back focus (air equivalent distance from the most image side surface of the cover glass CG to the imaging surface of the image sensor 50), FE is the effective F number, W is the half field angle (°), Y is the image height, and r is the curvature. Radius, d is the lens thickness or lens interval, N _d is the refractive index of the d line, and ν _d is the Abbe number.

A rotationally symmetric aspherical surface is defined by the following equation.
x = cy ² / [1+ [1- (1 + K) c ² y ² ] ^1/2 ] + A4y ⁴ + A6y ⁶ + A8y ⁸ + A10y ¹⁰ + A12y ¹² ...
(Where x is an aspherical shape, c is a curvature, y is a height from the optical axis, K is a conical coefficient, A4, A6, A8, A10... Are aspherical coefficients of each order)
[Example 1]
1 to 4 show Embodiment 1 of the endoscope objective optical system of the present invention. FIGS. 1 and 3 are lens configuration diagrams at the short focal length end (minimum magnification position) and the maximum magnification position, respectively. FIGS. 2 and 4 are graphs showing various aberrations in the lens configuration of FIGS. is there. Table 1 shows the numerical data. The negative first lens group 10 is composed of a single lens, the positive second lens group 20 is composed of a cemented lens of a positive lens and a negative lens, and the positive third lens group 30 is composed of a cemented lens of a negative lens and a positive lens. The positive fourth lens 40 is a meniscus single lens convex on the image side. The stop S is at a position 0.119 in front (object side) of the second lens group 20 (three surfaces).
(Table 1)
FE = 1: 6.6-9.4
f = 1.01-2.12
ODIS = -7.729--2.973
m = -0.114--0.741
W = 65.6-22.0
fB = 0.05
Surface NO. R d N _d ν _d
1 ∞ 0.357 1.88300 40.8
2 1.860 2.703-0.595
3 -13.525 0.595 1.88 300 40.8
4 -3.298 0.357 1.84666 23.8
5 -1.546 0.635
6 ∞ 0.357 1.92286 18.9
7 1.246 0.752 1.77250 49.6
8 -2.936 0.663-3.058
9 -3.245 0.357 1.84666 23.8
10 -3.340 0.286
11 ∞ 0.595 1.52400
12 ∞ 0.357 1.53000
13 ∞-
[Example 2]
5 to 8 show Embodiment 2 of the endoscope objective optical system of the present invention. 5 and 7
FIG. 6 is a lens configuration diagram at the short focal length end (minimum magnification position) and the maximum magnification position, respectively, and FIGS. 6 and 8 are aberration diagrams in the lens configurations of FIGS. 5 and 7, respectively. Table 2 shows the numerical data. The basic lens configuration is the same as that of Example 1 except that the second lens group 20 is composed of a positive single lens and the fourth lens group is composed of a negative single lens. The stop S is at a position 0.116 in front (object side) of the second lens group 20 (three surfaces).
(Table 2)
FE = 1: 6.6-9.3
f = 1.00-2.07
ODIS = -7.574--2.913
m = -0.115--0.741
W = 60.3-20.4
fB = 0.05
Surface NO. R d N _d ν _d
1 ∞ 0.350 1.88300 40.8
2 1.697 2.867-0.699
3 -6.966 0.946 1.88300 40.8
4 -1.504 1.030
5 10.940 0.350 1.92286 18.9
6 1.352 0.978 1.77250 49.6
7 -4.438 0.396-2.937
8 -2.840 0.350 1.84666 23.8
9 -3.293 0.234
10 ∞ 0.583 1.52400
11 ∞ 0.350 1.53000
12 ∞-
[Example 3]
9 to 12 show a third embodiment of the endoscope objective optical system according to the present invention. 9 and 11 are lens configuration diagrams at the short focal length end (minimum magnification position) and the maximum magnification position, respectively, and FIGS. 10 and 12 are aberration diagrams in the lens configurations of FIGS. 9 and 11, respectively. is there. Table 3 shows the numerical data. The basic lens configuration is the same as in the second embodiment. The stop S is at a position 0.140 in front (object side) of the second lens group 20 (three surfaces).
(Table 3)
FE = 1: 5.2-6.1
f = 0.99-1.67
ODIS = -12.382--3.095
m = -0.075--0.494
W = 70.0-31.9
fB = 0.05
Surface NO. R d N _d ν _d
1 ∞ 0.371 1.88300 40.8
2 1.377 2.037-0.759
3 -103.642 1.542 1.77250 49.6
4 -1.505 0.604
5 7.456 0.371 1.92286 18.9
6 1.354 0.771 1.77250 49.6
7 -5.304 0.366-1.694
8 -1.993 0.371 1.84666 23.8
9 -2.349 0.630
10 ∞ 0.619 1.52400
11 ∞ 0.371 1.53000
12 ∞-
[Example 4]
FIGS. 13 to 16 show Embodiment 4 of the endoscope objective optical system of the present invention. 13 and 15 are lens configuration diagrams at the short focal length end (minimum magnification position) and the maximum magnification position, respectively, and FIGS. 14 and 16 are aberration diagrams in the lens configurations of FIGS. 13 and 15, respectively. . Table 4 shows the numerical data. The basic lens configuration is the same as that of Example 1 except that the second lens group 20 is a positive single lens. The stop S is at a position 0.123 in front (object side) of the second lens group 20 (three surfaces).
(Table 4)
FE = 1: 5.2-6.2
f = 1.00-1.73
ODIS = -12.354--3.089
m = -0.075--0.494
W = 70.3-31.9
fB = 0.05
Surface NO. R d N _d ν _d
1 ∞ 0.371 1.88300 40.8
2 1.644 2.160-0.741
3 * -2.768 1.272 1.77250 49.6
4 -1.183 0.924
5 15.428 0.371 1.92286 18.9
6 1.579 0.751 1.77250 49.6
7 -4.144 0.365-1.539
8 -2.264 0.371 1.84666 23.8
9 -2.409 0.618
10 ∞ 0.618 1.52400
11 ∞ 0.371 1.53000
12 ∞-
* Is a rotationally symmetric aspherical surface.
Aspheric data (Aspheric coefficient not shown is 0.00);
NO A4 A6 A8
3 -0.54874 × 10 ^-1 -0.44 115 × 10 0.18410 × 10 ²
Table 5 shows values for the conditional expressions in the respective examples.
(Table 5)

  As is apparent from Table 5, Examples 1 to 4 satisfy the conditional expressions (1) to (7), and various aberrations are relatively well corrected.

It is a lens block diagram in the short focal distance end of Example 1 of the endoscope objective optical system by this invention. FIG. 2 is a diagram illustrating various aberrations of the lens configuration in FIG. 1. It is a lens block diagram in the maximum magnification position of Example 1 of the endoscope objective optical system by this invention. FIG. 4 is a diagram illustrating various aberrations of the lens configuration in FIG. 3. It is a lens block diagram in the short focal distance end of Example 2 of the endoscope objective optical system by this invention. FIG. 6 is a diagram illustrating various aberrations of the lens configuration in FIG. 5. It is a lens block diagram in the maximum magnification position of Example 2 of the endoscope objective optical system by this invention. FIG. 8 is a diagram illustrating various aberrations of the lens configuration in FIG. 7. It is a lens block diagram in the short focal distance end of Example 3 of the endoscope objective optical system by this invention. FIG. 10 is a diagram illustrating various aberrations of the lens configuration in FIG. 9. It is a lens block diagram in the maximum magnification position of Example 3 of the endoscope objective optical system by this invention. FIG. 12 is a diagram illustrating various aberrations of the lens configuration in FIG. 11. It is a lens block diagram in the short focal distance end of Example 4 of the endoscope objective optical system by this invention. FIG. 14 is a diagram illustrating various aberrations of the lens configuration in FIG. 13. It is a lens block diagram in the maximum magnification position of Example 4 of the endoscope objective optical system by this invention. FIG. 16 is a diagram illustrating various aberrations of the lens configuration in FIG. 15. It is the mounting schematic diagram and the simple movement figure in the endoscope front-end | tip part of the endoscope objective optical system by this invention.

Claims (6)

In order from the object side, a first lens group having negative power, a second lens group having positive power, a third lens group having positive power, and a fourth lens of a meniscus lens having a convex surface on the image side, By moving the second lens group and the third lens group while changing the object distance without changing the total length from the first lens group to the image plane, the in-focus state can be changed while changing the focal length of the entire system. An endoscope objective optical system that holds and satisfies the following conditional expression (1): (1) -0.05 <fw / f4 <0.01
However,
f4: focal length of the fourth lens,
fw: focal length at the short focal length end of the entire system. The endoscope objective optical system according to claim 1, wherein the endoscope objective optical system satisfies the following conditional expression (2).
(2) -2.2 <f1 / fw <-1.5
However,
f1: focal length of the first lens unit. The endoscope objective optical system according to claim 1 or 2, wherein the endoscope objective optical system satisfies the following conditional expression (3).
(3) 0.80 <d _2w / d _2t <1.32
However,
d _2w ; distance from the rear principal point of the second lens group to the front principal point of the third lens group at the short focal length end;
d _2t : Distance from the rear principal point of the second lens group to the front principal point of the third lens group at the maximum magnification position. The endoscope objective optical system according to any one of claims 1 to 3, wherein the endoscope objective optical system satisfies the following conditional expressions (4) and (5).
(4) 1.9 <f2 / fw <2.3
(5) 6 <f3 / fw <7
However,
f2: focal length of the second lens group,
f3: focal length of the third lens group. The endoscope objective optical system according to any one of claims 1 to 4, wherein the endoscope objective optical system satisfies the following conditional expression (6).
_{(6) 0.4 <(f 23t} × fw) / (f 23w × ft) <0.65
However,
f _23t ; composite focal length at the maximum magnification position of the second lens group and the third lens group,
f _23w ; composite focal length at the short focal length end of the second lens group and the third lens group,
ft: focal length at the maximum magnification position of the entire system. An endoscope provided with the endoscope objective optical system according to any one of claims 1 to 5.