JP2012032576A - Variable power optical system for endoscope and endoscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a variable power optical system for endoscope suitable for changing magnification into high magnification while keeping a constant observation distance at a telephoto end, while maintaining satisfactory optical performance at each variable power position from a wide angle end to the telephoto end.SOLUTION: In the variable power optical system which includes: a first lens group G1 having a first cemented lens CL1 formed by cementing a negative lens L1 turning its concave surface to an image side, a positive lens L2 turning its convex surface to the image side, a negative lens L3 and a positive lens L4, and has positive power; a second lens group G2 having a second cemented lens CL2 formed by cementing a negative lens L5 and a positive lens L6, and has negative power; and a third lens group G3 having a third cemented lens CL3 formed by cementing a positive lens L7, a negative lens L8 and a positive lens L9, and is an optical system for variably magnifying an optical image due to the movement of the second lens group, the focal lengths of the entire system and each group satisfy prescribed conditions.

Description

  The present invention relates to an endoscope variable magnification optical system and an endoscope incorporating the endoscope variable magnification optical system.

  In the medical field, endoscopes (fiberscopes or electronic scopes) are generally known as devices for diagnosing the inside of a body cavity of a patient and are put into practical use. Some endoscopes of this type are equipped with a variable magnification optical system having a variable magnification function in order to precisely observe a lesion. A specific configuration example of an endoscope equipped with a variable magnification optical system is described in Patent Document 1. The endoscope described in Patent Document 1 is set so that the focal length of each lens group satisfies a predetermined condition in order to ensure a sufficient depth of focus at each zoom position from the wide-angle end to the telephoto end. .

  By the way, if the distal end surface of the endoscope comes into contact with the tube wall, foreign matters such as mucus and residue may adhere to the objective lens surface and hinder good diagnosis. Further, since the light distribution lens surface is in contact with the tube wall, the tube wall is burdened by the heat of the illumination light. Therefore, as shown in Patent Document 1, a general variable magnification optical system mounted on an endoscope includes three groups including a movable group other than the first lens group (the lens group positioned closest to the object side). It has a configuration. In other words, this type of variable magnification optical system is designed such that the total length (the length from the lens surface closest to the object side to the image plane) is invariant regardless of the magnification. There is no extension like a camera lens barrel, and the distal end surface of the endoscope is immovable with respect to the body of the distal end of the endoscope, so inadvertent contact between the distal end surface of the endoscope and the tube wall is effective. can avoid.

  Negative-positive-negative or positive-negative-positive is assumed for the three-group configuration of the endoscope variable magnification optical system. If only the second lens group having a positive power in the negative-positive-negative three-group configuration is configured as a movable group, changes in various aberrations at each zoom position are large. Therefore, it is necessary to configure the third lens group as a movable group, and to suppress the aberration change at each zooming position by moving both groups. However, there is a problem that the more movable groups, the more complicated the movable mechanism. In the positive-negative-positive three-group configuration, correction of various aberrations by each positive lens group is relatively easy. For this reason, even if only the second lens group is configured as a movable group, a change in aberration at each zoom position can be suppressed.

JP 2007-233303 A

  However, in the positive-negative-positive three-group configuration, as disclosed in Patent Document 1, it is difficult to design a high magnification. Therefore, it is difficult to obtain a fine subject image during observation at the telephoto end. In order to obtain a fine subject image, it is necessary to physically bring the distal end surface of the endoscope close to the tube wall. However, when the distal end surface of the endoscope is brought close to the tube wall, there is a concern about the adhesion of the residue to the objective lens surface and the burden on the tube wall due to the heat of the illumination light. In the endoscope, the illumination optical system and the endoscope variable power optical system are not arranged coaxially. Since the illumination range and the photographing range are deviated, the light distribution unevenness is likely to occur as the distance between the endoscope distal end surface and the tube wall is shorter.

  That is, in luminal observation with an endoscope, it is desirable to keep the distal end surface of the endoscope as far as possible from the tube wall even during observation at the telephoto end. In order to obtain a fine subject image while separating the endoscope front end surface from the tube wall, it is necessary to increase the magnification at the telephoto end.

  The present invention has been made in view of the above circumstances, and its purpose is to maintain a constant observation distance at the telephoto end while maintaining good optical performance at each zoom position from the wide-angle end to the telephoto end. An object of the present invention is to provide an endoscopic variable magnification optical system and an endoscope suitable for increasing the magnification while maintaining it.

The variable power optical system for an endoscope according to an aspect of the present invention that solves the above problems is, in order from the object side, a first lens group having a positive power, a second lens group having a negative power, The optical system includes a third lens group having a positive power, and zooms the optical image by moving the second lens group. The first lens group includes, in order from the object side, at least a negative lens having a concave surface facing the image side, a positive lens having a convex surface facing the image side, and a first cemented lens in which the negative lens and the positive lens are cemented. The second lens group includes at least a second cemented lens in which a negative lens and a positive lens are cemented in order from the object side. The third lens group has at least a third cemented lens in which a positive lens, a negative lens, and a positive lens are cemented in order from the object side. In the variable magnification optical system for an endoscope according to the present invention, the focal length of the first lens group is defined as f ₁ (unit: mm), and the focal length of the second lens group is f ₂ (unit: mm). The focal length of the third lens group is defined as f ₃ (unit: mm), and the combined focal length of the first to third lens groups at the wide angle end is defined as f _W (unit: mm). The following conditional expressions (1) and (2)
−1.8 ≦ f ₂ / f _W ≦ −1.2 (1)
0.5 ≦ f ₁ / f ₃ ≦ 0.65 (2)
Meet.

  If the upper limit of conditional expression (1) is exceeded, the Petzval sum is negatively large, making it difficult to correct field curvature. If the lower limit of conditional expression (1) is not reached, the power of the second lens group is weak, and the movement distance of the second lens group accompanying zooming is long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system. In addition, the change in coma accompanying the movement of the second lens group is large. For this reason, it is difficult to correct coma with a good balance over the entire zooming range from the wide-angle end to the telephoto end. For example, when coma is corrected with reference to the wide-angle end, the occurrence of coma at the telephoto end is large.

  If the upper limit of conditional expression (2) is exceeded, the power of the first lens group is too weak. In order to ensure the power of the first lens group, the effective light beam diameter of the first lens group must be set large. Moreover, in order to ensure the magnification of the entire system, it is necessary to weaken the power of the second lens group. However, as a compensation for weakening the power of the second lens group, the moving distance of the second lens group accompanying zooming becomes longer. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system. If the lower limit of conditional expression (2) is not reached, the power of the first lens group is strong, which is advantageous for setting the effective light beam diameter of the first lens group small. However, since the magnification of the lenses after the second lens group is large, the magnification change accompanying the change of the group interval due to the assembly error or individual component difference is large. Therefore, it is difficult to guarantee a viewing angle that satisfies the specifications. Further, in order to increase the power of the first lens group, it is necessary to increase the power of the negative lens in the first lens group. However, as a compensation for increasing the power of the negative lens, the amount of coma generated in the first lens group increases. Since the angle of incidence on the second lens group is particularly large at the wide-angle end, coma is greatly generated.

  When conditional expressions (1) and (2) are satisfied at the same time, a high zoom ratio can be obtained while satisfactorily correcting various aberrations over the entire zoom range from the wide-angle end to the telephoto end. The magnification can be increased while maintaining a constant observation distance at the telephoto end while making the wide-angle end a wide field of view. Therefore, a fine subject image can be obtained without bringing the distal end surface of the endoscope close to the tube wall even during observation at the telephoto end. Accordingly, it is possible to effectively avoid the burden on the tube wall due to the adhesion of residues on the objective lens surface and the heat of the illumination light. Furthermore, uneven light distribution within the observation field is suppressed. Further, satisfying the conditional expressions (1) and (2) simultaneously is advantageous for downsizing design of the variable magnification optical system for endoscopes and suppresses changes in the viewing angle due to assembly errors or individual component differences.

In order to further suppress the effective light beam diameter of the first lens group, the variable magnification optical system for an endoscope according to the present invention has the following conditional expression (3).
−0.85 ≦ f ₁ / f ₂ ≦ −0.65 (3)
It is good also as composition which satisfies.

  If the upper limit of conditional expression (3) is exceeded, the power of the negative lens in the first lens group is strong, so the amount of coma generated in the first lens group is large. Since the angle of incidence on the second lens group is particularly large at the wide-angle end, coma is greatly generated. Below the lower limit of conditional expression (3), the power of the first lens group is too weak. In order to ensure the power of the first lens group, the effective light beam diameter of the first lens group must be set large. When the effective light beam diameter of the first lens group is large, not only the enlargement / reduction optical system for the endoscope is enlarged, but also coma aberration is generated due to the movement of the second lens group. For example, when coma is corrected with reference to the wide-angle end, the occurrence of coma at the telephoto end is large.

The exit angle of the endoscope variable power optical system changes with zooming. Depending on the emission angle, the loss of the amount of light received on the imaging surface increases. The variable power optical system for endoscopes according to the present invention has the following conditional expression (4) in order to suppress a change in the emission angle accompanying the variable power.
−0.9 ≦ f ₂ / f ₃ ≦ −0.6 (4)
It is good also as composition which satisfies.

  If the upper limit of conditional expression (4) is exceeded, the power of the second lens group is too strong for the third lens group. Therefore, the closer the zoom position is to the telephoto end, the larger the emission angle of the entire system, and the amount of light received by the solid-state image sensor decreases. The decrease in the amount of light received during observation at the telephoto end is significant. In addition, since the incident angle to the third lens group is large, the occurrence of coma aberration is large. If the lower limit of conditional expression (4) is not reached, the power of the second lens group is weak, and the movement distance of the second lens group accompanying zooming becomes long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system.

The variable magnification optical system for an endoscope according to the present invention further satisfies the following conditional expression (5) in order to further suppress a viewing angle change caused by an assembly error or an individual component difference while reducing the outer dimension.
0.53 ≦ f ₁ / f ₃ ≦ 0.61 (5)
It is good also as composition which satisfies.

  If the upper limit of conditional expression (5) is exceeded, the power of the first lens group is too weak. In order to ensure the power of the first lens group, the effective light beam diameter of the first lens group must be set large. Moreover, in order to ensure the magnification of the entire system, it is necessary to weaken the power of the second lens group. However, as a compensation for weakening the power of the second lens group, the moving distance of the second lens group accompanying zooming becomes longer. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system. If the lower limit of conditional expression (5) is not reached, the magnification of the lenses after the second lens group is large, so that the magnification change accompanying the change in group spacing due to assembly errors or individual component differences is large. Therefore, it is difficult to guarantee a viewing angle that satisfies the specifications. Further, in order to increase the power of the first lens group, it is necessary to increase the power of the negative lens in the first lens group. However, as a compensation for increasing the power of the negative lens, the amount of coma generated in the first lens group increases. Since the angle of incidence on the second lens group is particularly large at the wide-angle end, coma is greatly generated.

In the endoscope variable magnification optical system according to the present invention, the focal length having the largest absolute value among the focal lengths of the single lens or the cemented lens constituting the third lens group is defined as f _X (unit: mm). If the following conditional expression (6)
| F _X / f ₃ | <5.0 (6)
It is good also as composition which satisfies.

  When the conditional expression (6) is satisfied, the amount of image plane tilt due to the eccentricity of the lenses constituting the third lens group can be suppressed without taking measures by strict tolerance management. Here, the image plane tilting ideally remains symmetrically with respect to the optical axis, but remains asymmetric with respect to the optical axis depending on the amount and direction of eccentricity when the imaging lens is assembled. It is a phenomenon. If the upper limit of conditional expression (6) is exceeded, the magnification of the lenses constituting the third lens group is high. For this reason, the image plane is greatly tilted when the third lens group is assembled eccentrically with respect to the optical axis, and image quality deterioration tends to occur around the observation field. In particular, the amount of image plane tilt during observation at the wide-angle end is large. When image plane tilting occurs during observation at the wide-angle end, the image plane tilting is prominent at each zoom position reaching the telephoto end.

The variable magnification optical system for an endoscope according to the present invention has the following when the angle of view at the maximum image height at the wide-angle end is defined as ω (unit: deg) in order to reduce oversight of a lesioned part or the like by an operator. Conditional expression (7)
ω ≧ 120 (7)
It is good also as composition which satisfies.

  The variable power optical system for an endoscope according to the present invention may be configured to have a diaphragm that moves integrally with the second lens group on the optical axis between the first and second lens groups. By moving the second lens group and the aperture together, the generation of astigmatism when the telephoto end is set can be effectively suppressed.

  An endoscope according to an embodiment of the present invention that solves the above-described problems is a device that includes the above-described endoscope variable power optical system mounted on a distal end.

  According to the present invention, it is suitable for an endoscope suitable for increasing the magnification while maintaining a constant observation distance at the telephoto end while maintaining good optical performance at each zoom position from the wide-angle end to the telephoto end. A variable magnification optical system and an endoscope are provided.

It is an external view which shows the external appearance of the electronic scope of embodiment of this invention. It is sectional drawing which shows arrangement | positioning of the variable magnification optical system for endoscopes of embodiment (Example 1) of this invention, and the optical component arrange | positioned in the back | latter stage. FIG. 6 is a diagram illustrating various aberrations of the variable magnification optical system for an endoscope according to Example 1 of the present invention. It is sectional drawing which shows arrangement | positioning of the variable magnification optical system for endoscopes of Example 2 of this invention, and the optical component arrange | positioned in the back | latter stage. FIG. 6 is a diagram showing various aberrations of the variable magnification optical system for an endoscope according to Example 2 of the present invention. It is sectional drawing which shows arrangement | positioning of the variable power optical system for endoscopes of Example 3 of this invention, and the optical component arrange | positioned in the back | latter stage. It is various aberrational figures of the variable power optical system for endoscopes of Example 3 of the present invention. It is sectional drawing which shows arrangement | positioning of the variable power optical system for endoscopes of Example 4 of this invention, and the optical component arrange | positioned in the back | latter stage. It is various aberrational figures of the variable power optical system for endoscopes of Example 4 of the present invention. It is sectional drawing which shows arrangement | positioning of the variable magnification optical system for endoscopes of Example 5 of this invention, and the optical component arrange | positioned in the back | latter stage. It is various aberrational figures of the variable power optical system for endoscopes of Example 5 of the present invention. It is sectional drawing which shows arrangement | positioning of the variable magnification optical system for endoscopes of Example 6 of this invention, and the optical component arrange | positioned in the back | latter stage. It is various aberrational figures of the variable power optical system for endoscopes of Example 6 of the present invention.

  Hereinafter, an endoscope variable magnification optical system and an electronic scope having an endoscope variable magnification optical system according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is an external view showing the external appearance of the electronic scope 1 of the present embodiment. As shown in FIG. 1, the electronic scope 1 has a flexible tube 11 covered with a flexible sheath (outer skin) 11a. Connected to the distal end of the flexible tube 11 is a distal end portion 12 that is sheathed by a rigid resin casing. The bending portion 14 at the connecting portion between the flexible tube 11 and the distal end portion 12 is remotely operated from the hand operating portion 13 connected to the proximal end of the flexible tube 11 (specifically, the rotation of the bending operation knob 13a). The operation is flexible. This bending mechanism is a well-known mechanism incorporated in a general electronic scope, and is configured to bend the bending portion 14 by pulling the operation wire in conjunction with the rotation operation of the bending operation knob 13a. By changing the direction of the distal end portion 12 according to the bending operation by the above operation, the imaging region by the electronic scope 1 moves.

  An endoscope variable magnification optical system 100 (blocks indicated by diagonal lines in FIG. 1) is incorporated in the resin casing of the distal end portion 12. The endoscope variable power optical system 100 forms an image of scattered light from a subject on a light receiving surface of a solid-state imaging device (not shown) in order to collect image data of the subject in the imaging region. As the solid-state imaging device, for example, a CCD (Charge Coupled Device) image sensor and a CMOS (Complementary Metal Oxide Semiconductor) image sensor are assumed.

  FIG. 2A is a cross-sectional view showing the arrangement of an endoscopic variable magnification optical system 100 according to Embodiment 1 (details will be described later) of the present invention and optical components arranged at the subsequent stage. Next, the variable magnification optical system 100 for an endoscope according to an embodiment of the present invention will be described in detail with reference to FIG.

  As shown in FIG. 2A, the endoscope variable magnification optical system 100 has, in order from the object (subject) side, a first lens group G1 having a positive power, an aperture S, and a negative power. A second lens group G2 and a third lens group G3 having a positive power are included. Each optical lens constituting each lens group G1 to G3 has a rotationally symmetric shape about the optical axis AX of the zooming optical system 100 for endoscope. A color correction filter F for a solid-state image sensor is disposed at the subsequent stage of the third lens group G3. The color correction filter F is bonded to a cover glass CG that protects the solid-state image sensor.

  The first lens group G1 is a lens group having a positive power and disposed closer to the object side than the stop S. The first lens group G1 includes, in order from the object side, a negative lens L1 having a concave surface directed to the image side, a positive lens L2 having a convex surface directed to the image side, and a cemented lens CL1 in which the negative lens L3 and the positive lens L4 are cemented. Have at least. The reason for having “at least” is that there may be a configuration example in which another optical element is additionally arranged within the scope of the technical idea of the present invention. In the description of the second lens group G2 and the third lens group G3, it is expressed as “having at least” for the same reason.

  The second lens group G2 is a lens group having negative power, and has at least a cemented lens CL2 in which a negative lens L5 and a positive lens L6 are cemented in order to suppress a change in chromatic aberration. The negative lens L5 is disposed on the object side, and the positive lens L6 is disposed on the image side. The second lens group G2 moves in the direction of the optical axis AX together with the stop S in order to change the optical image formed on the light receiving surface of the solid-state imaging device. By moving the second lens group G2 and the stop S together, the occurrence of astigmatism when the telephoto end is set can be effectively suppressed.

  The third lens group G3 is a lens group having positive power, and has at least a cemented lens CL3 in which a positive lens L7, a negative lens L8, and a positive lens L9 are cemented in order from the object side. The positive lens L7 is arranged mainly for the purpose of correcting spherical aberration, and the cemented lens CL3 is arranged mainly for the purpose of correcting lateral chromatic aberration.

  In the following, for convenience of explanation, the object-side surface and the image-side surface of each optical component are referred to as a first surface and a second surface, respectively. The diaphragm S is a plate-like member having a predetermined circular opening centered on the optical axis AX, or the lens surface closest to the diaphragm S of the second lens group G2 (in the configuration example of FIG. 2A). This is a light-shielding film coated on the first surface r9) of the negative lens L5 except for a predetermined circular area centered on the optical axis AX. The thickness of the diaphragm S is very thin compared to the thickness of each optical lens constituting the endoscope variable magnification optical system 100, and is ignored in calculating the optical performance of the endoscope variable magnification optical system 100. There is no problem. Therefore, in the present specification, the description will be made assuming that the thickness of the diaphragm S is 0.

The variable magnification optical system 100 for endoscope defines the focal length of the first lens group G1 as f ₁ (unit: mm), and the focal length of the second lens group G2 as f ₂ (unit: mm). And the focal length of the third lens group G3 is defined as f ₃ (unit: mm), and the focal length of the entire system at the wide-angle end (the combined focal point of the first lens group G1 to the third lens group G3). When the distance is defined as f _W (unit: mm), the following conditional expressions (1) and (2)
−1.8 ≦ f ₂ / f _W ≦ −1.2 (1)
0.5 ≦ f ₁ / f ₃ ≦ 0.65 (2)
It is configured to satisfy.

Condition (1) defines the ratio between the focal length f _W of the entire system at a focal length f ₂ and the wide-angle end of the second lens group G2. If the upper limit of conditional expression (1) is exceeded, the power of the second lens group G2 is strong, and the moving distance of the second lens group G2 due to zooming is short. Therefore, it is advantageous to design the total length of the endoscope variable magnification optical system 100 to be short. However, since the Petzval sum is negatively large, it is difficult to correct field curvature.

  If the lower limit of conditional expression (1) is not reached, the power of the second lens group G2 is weak, so that the moving distance of the second lens group G2 accompanying zooming is long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system 100. In addition, the change in coma accompanying the movement of the second lens group G2 is large. For this reason, it is difficult to correct coma with a good balance over the entire zooming range from the wide-angle end to the telephoto end. For example, when coma is corrected with reference to the wide-angle end, the occurrence of coma at the telephoto end is large.

Condition (2) defines the ratio between the focal length f ₁ and the focal length f ₃ of the third lens group G3 of the first lens group G1. If the upper limit of conditional expression (2) is exceeded, the power of the first lens group G1 is too weak. In order to ensure the power of the first lens group G1, the effective light beam diameter of the first lens group G1 must be set large. Moreover, in order to ensure the magnification of the entire system, it is necessary to weaken the power of the second lens group G2. However, as a compensation for weakening the power of the second lens group G2, the moving distance of the second lens group G2 accompanying zooming becomes long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system 100.

  If the lower limit of conditional expression (2) is not reached, the power of the first lens group G1 is strong, which is advantageous for setting the effective light beam diameter of the first lens group G1 small. However, since the magnification of the lenses after the second lens group G2 is large, the magnification change accompanying the change in the group interval due to the assembly error or individual component difference is large. In other words, it is difficult to guarantee the viewing angle satisfying the specifications because the viewing angle change due to assembly errors or individual component differences is large. Further, in order to increase the power of the first lens group G1, it is necessary to increase the power of the negative lens L1. However, as a price for increasing the power of the negative lens L1, the amount of coma generated in the first lens group G1 increases. Since the angle of incidence on the first lens group G1 is particularly large at the wide-angle end, coma is greatly generated.

  When conditional expressions (1) and (2) are satisfied at the same time, a high zoom ratio can be obtained while satisfactorily correcting various aberrations over the entire zoom range from the wide-angle end to the telephoto end. The magnification can be increased while maintaining a constant observation distance at the telephoto end while making the wide-angle end a wide field of view. Therefore, a fine subject image can be obtained without bringing the distal end surface of the distal end portion 12 close to the tube wall even during observation at the telephoto end. Accordingly, it is possible to effectively avoid the burden on the tube wall due to the adhesion of residues on the objective lens surface and the heat of the illumination light. Furthermore, uneven light distribution within the observation field is suppressed. Further, satisfying the conditional expressions (1) and (2) simultaneously is advantageous for downsizing design of the variable magnification optical system 100 for an endoscope, and suppresses a change in viewing angle due to an assembly error or individual component differences. .

In order to further suppress the effective light beam diameter of the first lens group G1, the variable power optical system for endoscope 100 satisfies the following conditional expression (3).
−0.85 ≦ f ₁ / f ₂ ≦ −0.65 (3)
It is good also as composition which satisfies.

Condition (3) defines the ratio between the focal length f ₁ and the focal length f ₂ of the second lens group G2 of the first lens group G1. If the upper limit of conditional expression (3) is exceeded, the power of the first lens group G1 is strong, which is advantageous for setting the effective light beam diameter of the first lens group G1 small. However, since the power of the negative lens L1 is strong, the amount of coma generated in the first lens group G1 is large. Since the angle of incidence on the second lens group G2 is particularly large at the wide-angle end, coma is greatly generated.

  If the lower limit of conditional expression (3) is not reached, the power of the first lens group G1 is too weak. In order to ensure the power of the first lens group G1, the effective light beam diameter of the first lens group G1 must be set large. When the effective light beam diameter of the first lens group G1 is large, not only the enlargement / reduction optical system 100 for an endoscope is enlarged, but also coma aberration is generated due to the movement of the second lens group G2. For example, when coma is corrected with reference to the wide-angle end, the occurrence of coma at the telephoto end is large.

The light receiving surface of the solid-state imaging device has an incident angle dependency. Therefore, depending on the incident angle to the light receiving surface (the exit angle of the endoscope variable magnification optical system 100), the amount of light may be greatly lost around the effective pixel region. Therefore, in order to suppress the change in the emission angle that accompanies zooming, the zooming optical system 100 for endoscopes satisfies the following conditional expression (4).
−0.9 ≦ f ₂ / f ₃ ≦ −0.6 (4)
It is good also as composition which satisfies.

Condition (4) defines the focal length f ₂ of the second lens group G2 and the ratio between the focal length f ₃ of the third lens group G3. If the upper limit of conditional expression (4) is exceeded, the power of the second lens group G2 is too strong for the third lens group G3. Therefore, the closer the zoom position is to the telephoto end, the larger the emission angle of the entire system, and the amount of light received by the solid-state image sensor decreases. The decrease in the amount of light received during observation at the telephoto end is significant. Further, since the incident angle to the third lens group G3 is large, the occurrence of coma aberration is large.

  If the lower limit of conditional expression (4) is surpassed, the power of the second lens group G2 is weaker than that of the third lens group G3, so the exit angle of the entire system during observation at the wide angle end is small. However, since the power of the second lens group G2 is too weak, the moving distance of the second lens group G2 due to zooming becomes long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system 100.

In order to further suppress the viewing angle change caused by the assembly error or the individual component difference while reducing the external dimension, the variable power optical system for endoscope 100 satisfies the following conditional expression (5).
0.53 ≦ f ₁ / f ₃ ≦ 0.61 (5)
It is good also as composition which satisfies.

Condition (5) defines the ratio between the focal length f ₁ and the focal length f ₃ of the third lens group G3 of the first lens group G1. If the upper limit of conditional expression (5) is exceeded, the power of the first lens group G1 is too weak. In order to ensure the power of the first lens group G1, the effective light beam diameter of the first lens group G1 must be set large. Moreover, in order to ensure the magnification of the entire system, it is necessary to weaken the power of the second lens group G2. However, as a compensation for weakening the power of the second lens group G2, the moving distance of the second lens group G2 accompanying zooming becomes long. Therefore, it is difficult to design to reduce the overall length of the endoscope variable magnification optical system 100.

  If the lower limit of conditional expression (5) is not reached, the power of the first lens group G1 is strong, which is advantageous for setting the effective light beam diameter of the first lens group G1 small. However, since the magnification of the lenses after the second lens group G2 is large, the magnification change accompanying the change in the group interval due to the assembly error or individual component difference is large. Therefore, it is difficult to guarantee a viewing angle that satisfies the specifications. Further, in order to increase the power of the first lens group G1, it is necessary to increase the power of the negative lens L1. However, as a price for increasing the power of the negative lens L1, the amount of coma generated in the first lens group G1 increases. Since the angle of incidence on the second lens group G2 is particularly large at the wide-angle end, coma is greatly generated.

  In a variable magnification optical system for an endoscope having a small size and a wide viewing angle, the image plane is greatly tilted when the lens is assembled decentered with respect to the optical axis, and image quality is likely to deteriorate around the observation field. In order to suppress the image plane tilt amount, for example, a measure in manufacturing management in which strict tolerance management is performed to manufacture an optical lens or a component around the optical lens (for example, a lens holding frame) with a small manufacturing error can be considered. However, in this case, adverse effects such as a decrease in yield and an increase in manufacturing unit cost occur. Further, in a small optical lens, there is a problem that it is technically difficult to grasp and manage an error generation amount. Therefore, measures based on strict tolerance management cannot be easily adopted.

Therefore, the variable magnification optical system 100 for endoscopes has the following when the focal length having the largest absolute value among the focal lengths of the lenses constituting the third lens group G3 is defined as f _X (unit: mm). Conditional expression (6)
The following conditional expression (6)
| F _X / f ₃ | <5.0 (6)
It is good also as composition which satisfies.

  The third lens group G3 is higher in the occurrence sensitivity of image plane tilt with respect to the decentration of the lens than in other lens groups. When the conditional expression (6) is satisfied, it is possible to suppress the amount of image plane tilt due to the eccentricity of the lenses constituting the third lens group G3 without taking measures by strict tolerance management.

  If the upper limit of conditional expression (6) is exceeded, the magnification of the lenses constituting the third lens group G3 is high. Therefore, when the third lens group G3 is assembled with being decentered with respect to the optical axis AX, the image plane is greatly tilted, and image quality deterioration is likely to occur around the observation field. In particular, the amount of image plane tilt during observation at the wide-angle end is large. When image plane tilting occurs during observation at the wide-angle end, the image plane tilting is prominent at each zoom position reaching the telephoto end.

When the angle of view ω (unit: deg) at the maximum image height at the wide-angle end is set narrow, various aberrations can be easily corrected. Therefore, it is easy to design to maintain good optical performance at each zoom position. However, since the endoscope assumed in the present embodiment is used for lower digestive organ observation, there is a concern that a lesioned part or the like may be overlooked at a narrow viewing angle. Therefore, the variable magnification optical system 100 for an endoscope has the following conditional expression (7):
ω ≧ 120 (7)
It is good also as composition which satisfies.

  Next, six specific numerical examples of the endoscope variable magnification optical system 100 described so far will be described. The variable magnification optical system 100 for endoscopes of each numerical example 1 to 6 is disposed at the distal end portion 12 of the electronic scope 1 shown in FIG.

  As described above, the configuration of the variable magnification optical system 100 for endoscope according to the first embodiment of the present invention is as shown in FIG. FIG. 2A is a cross-sectional view showing the lens arrangement when the zoom position is at the wide-angle end. A cross-sectional view showing the lens arrangement when the zoom position is at the telephoto end is shown in FIG.

  Table 1 shows specific numerical configurations (design values) of the variable magnification optical system 100 for an endoscope according to the first embodiment (and optical components arranged at the subsequent stage). The surface number NO shown in Table 1 corresponds to the surface code rn (n is a natural number) in FIG. 2 except for the surface number 8 corresponding to the stop S. In Table 1, R (unit: mm) is the radius of curvature of each surface of the optical member, D (unit: mm) is the optical member thickness or optical member interval on the optical axis AX, and N (d) is the d line ( The refractive index at a wavelength of 588 nm), and νd, the Abbe number of the d line. Table 2 shows the specifications of the variable magnification optical system 100 for an endoscope (effective F number, focal length (unit: mm) of the entire system, optical magnification, half angle of view (unit: deg), and image height (unit: mm). , Group spacing D7 (unit: mm) and group spacing D11 (unit: mm) are shown for the wide-angle end and the telephoto end, respectively. The group interval D7 is a group interval between the first lens group G1 and the second lens group G2. The group interval D11 is a group interval between the first lens group G1 and the third lens group G3. The group interval D7 and the group interval D11 change according to the zoom position.

  Graphs A to D in FIG. 3A are various aberration diagrams when the zooming position is at the wide-angle end in the zooming optical system 100 for an endoscope according to the first embodiment. Graphs A to D in FIG. 3B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the first embodiment. Graphs A in FIGS. 3A and 3B show spherical aberration and axial chromatic aberration at d-line, g-line, and C-line. Graphs B in FIGS. 3A and 3B show lateral chromatic aberration at d-line, g-line, and C-line. In the graphs A and B, the solid line indicates the aberration at the d line, the dotted line indicates the aberration at the g line, and the alternate long and short dash line indicates the aberration at the C line. Graph C in FIGS. 3A and 3B shows astigmatism. In the graph C, a solid line indicates a sagittal component, and a dotted line indicates a meridional component. A graph D in FIGS. 3A and 3B shows distortion. In the graphs A to C, the vertical axis represents the image height, and the horizontal axis represents the aberration amount. The vertical axis of the graph D represents the image height, and the horizontal axis represents the distortion. In addition, the description about each table | surface or each drawing of the present Example 1 is applied also to each table | surface or each drawing shown by each subsequent numerical example.

  As shown in Tables 1 and 2, the variable magnification optical system 100 for endoscope according to the first embodiment has a high magnification at the telephoto end while being small and having a wide viewing angle. Further, as shown in FIGS. 3A and 3B, various aberrations are satisfactorily corrected at both the wide-angle end and the telephoto end. In the intermediate region between the wide-angle end and the telephoto end, various aberrations change within the range shown in FIGS. 3 (a) and 3 (b). In other words, the variable magnification optical system 100 for endoscope according to the first embodiment is small and has a wide viewing angle, but has good optical performance at each variable magnification position from the wide angle end to the telephoto end and is constant at the telephoto end. The magnification is high while maintaining the observation distance.

  FIGS. 4A and 4B are cross-sectional views illustrating the arrangement of optical components including the endoscope variable magnification optical system 100 according to the second embodiment. FIG. 4A shows the lens arrangement when the zoom position is at the wide angle end. FIG. 4B shows the lens arrangement when the zoom position is at the telephoto end. The variable magnification optical system 100 for endoscope according to the second embodiment includes a positive lens L10 between the cemented lens CL3 and the color correction filter F as shown in FIGS. 4 (a) and 4 (b). Yes. That is, in the second embodiment, the third lens group G3 has a three-lens configuration in order from the object side, that is, a positive lens L7, a cemented lens CL3, and a positive lens L10. The first lens group G1 and the second lens group G2 have the same configuration as that of the first embodiment.

  Graphs A to D in FIG. 5A are various aberration diagrams when the zooming position is at the wide angle end in the zooming optical system 100 for an endoscope according to the second embodiment. Graphs A to D in FIG. 5B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the second embodiment. Table 3 shows specific numerical configurations of optical components including the endoscope variable magnification optical system 100 of the second embodiment, and Table 4 shows specifications of the endoscope variable magnification optical system 100 of the second embodiment. Are shown respectively. As shown in Tables 3 and 4 and FIGS. 5A and 5B, the variable magnification optical system 100 for endoscope according to the second embodiment is small and has a wide viewing angle, but also from the wide-angle end to the telephoto end. The optical performance is good at each zooming position up to and the magnification is high while maintaining a constant observation distance at the telephoto end.

  6A and 6B are cross-sectional views showing the arrangement of optical components including the endoscope variable magnification optical system 100 according to the third embodiment. FIG. 6A shows the lens arrangement when the zoom position is at the wide angle end. FIG. 6B shows the lens arrangement when the zoom position is at the telephoto end. The endoscope variable power optical system 100 of the third embodiment has the same number of sheets as that of the first embodiment, as shown in FIGS.

  Graphs A to D in FIG. 7A are various aberration diagrams when the zooming position is at the wide-angle end in the zooming optical system 100 for an endoscope according to the third embodiment. Graphs A to D in FIG. 7B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the third embodiment. Table 5 shows the specific numerical configuration of each optical component including the endoscope variable magnification optical system 100 of the third embodiment, and Table 6 shows the specifications of the endoscope variable magnification optical system 100 of the third embodiment. Are shown respectively. As shown in Tables 5 and 6 and FIGS. 7A and 7B, the variable magnification optical system 100 for endoscope according to the third embodiment is small and has a wide viewing angle, but also from the wide-angle end to the telephoto end. The optical performance is good at each zooming position up to and the magnification is high while maintaining a constant observation distance at the telephoto end.

  FIGS. 8A and 8B are sectional views showing the arrangement of optical components including the endoscope variable magnification optical system 100 according to the fourth embodiment. FIG. 8A shows the lens arrangement when the zoom position is at the wide angle end. FIG. 8B shows the lens arrangement when the zoom position is at the telephoto end. The endoscope variable magnification optical system 100 of the fourth embodiment has the same number of sheets as that of the second embodiment, as shown in FIGS.

  Graphs A to D in FIG. 9A are various aberration diagrams when the zooming position is at the wide-angle end in the zooming optical system 100 for an endoscope according to the fourth embodiment. Graphs A to D in FIG. 9B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the fourth embodiment. Table 7 shows specific numerical configurations of optical components including the endoscope variable magnification optical system 100 of the fourth embodiment, and Table 8 shows specifications of the endoscope variable magnification optical system 100 of the fourth embodiment. Are shown respectively. As shown in Tables 7 and 8 and FIGS. 9A and 9B, the variable magnification optical system 100 for endoscope according to the fourth embodiment is small and has a wide viewing angle, but from the wide-angle end to the telephoto end. The optical performance is good at each zooming position up to and the magnification is high while maintaining a constant observation distance at the telephoto end.

  FIGS. 10A and 10B are cross-sectional views showing the arrangement of optical components including the endoscope variable magnification optical system 100 according to the fifth embodiment. FIG. 10A shows the lens arrangement when the zoom position is at the wide angle end. FIG. 10B shows the lens arrangement when the zoom position is at the telephoto end. The variable magnification optical system 100 for an endoscope of the fifth embodiment has the same number of sheets as that of the second embodiment, as shown in FIGS. 10 (a) and 10 (b).

  Graphs A to D in FIG. 11A are various aberration diagrams when the zooming position is at the wide-angle end in the zooming optical system 100 for an endoscope according to the fifth embodiment. Graphs A to D in FIG. 11B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the fifth embodiment. Table 9 shows specific numerical configurations of optical components including the endoscope variable magnification optical system 100 according to the fifth embodiment, and Table 10 shows specifications of the endoscope variable magnification optical system 100 according to the fifth embodiment. Are shown respectively. As shown in Tables 9 and 10 and FIGS. 11A and 11B, the variable magnification optical system 100 for an endoscope according to the fifth embodiment is small and has a wide viewing angle but also from the wide-angle end to the telephoto end. The optical performance is good at each zooming position up to and the magnification is high while maintaining a constant observation distance at the telephoto end.

  FIGS. 12A and 12B are cross-sectional views showing the arrangement of optical components including the endoscope variable magnification optical system 100 according to the sixth embodiment. FIG. 12A shows the lens arrangement when the zoom position is at the wide angle end. FIG. 12B shows the lens arrangement when the zoom position is at the telephoto end. As shown in FIGS. 12A and 12B, the variable magnification optical system 100 for endoscope according to the sixth embodiment includes a positive lens L10 between the positive lens L7 and the cemented lens CL3. . That is, in Example 6, the third lens group G3 has a three-lens configuration in order from the object side: a positive lens L7, a positive lens L10, and a cemented lens CL3. The first lens group G1 and the second lens group G2 have the same configuration as that of the first embodiment.

  Graphs A to D in FIG. 13A are various aberration diagrams when the zooming position is at the wide-angle end in the zooming optical system 100 for an endoscope according to the sixth embodiment. Graphs A to D in FIG. 13B are various aberration diagrams when the zooming position is at the telephoto end in the zooming optical system 100 for an endoscope according to the sixth embodiment. Table 11 shows specific numerical configurations of optical components including the endoscope variable magnification optical system 100 according to the sixth embodiment, and Table 12 shows specifications of the endoscope variable magnification optical system 100 according to the sixth embodiment. Are shown respectively. As shown in Tables 11 and 12 and FIG. 13, the zooming optical system 100 for an endoscope of Example 6 is small and has a wide viewing angle, but at each zooming position from the wide-angle end to the telephoto end. The optical performance is good and the magnification is high while maintaining a constant observation distance at the telephoto end.

(Comparison verification)
Table 13 is a list of values calculated when the conditional expressions (1) to (7) are applied in the nine examples in which the first to third comparative examples are added to the first to sixth examples. It is. Comparative Examples 1, 2, and 3 are Examples 1, 2, and 4 of Patent Document 1, respectively.

  As shown in Table 13, the variable magnification optical system 100 for endoscopes of Examples 1 to 6 is presented in the description of each Example by satisfying conditional expressions (1) and (2) at the same time. As shown in the figure or table, the optical performance is good at each zoom position from the wide-angle end to the telephoto end, and the magnification is high while maintaining a constant observation distance at the telephoto end, while being small and having a wide viewing angle. On the other hand, the variable power optical system for endoscopes of Comparative Examples 1 to 3 does not satisfy at least one of conditional expressions (1) and (2) as shown in Table 13. Therefore, in Comparative Examples 1 to 3, at least one of the external dimensions, the viewing angle, the magnification, and the optical performance at each zoom position does not satisfy a level sufficient for performing good lumen observation.

  As shown in Table 13, the endoscope variable magnification optical system 100 of Examples 1 to 6 also satisfies the conditional expressions (3), (4), and (7). Examples 1 and 3 further satisfy conditional expressions (5) and (6), and Examples 2 and 6 further satisfy conditional expression (5). Therefore, in the first to sixth embodiments, further effects are achieved by satisfying each conditional expression.

  The above is the description of the embodiment of the present invention. The present invention is not limited to the above-described configuration, and various modifications can be made within the scope of the technical idea of the present invention.

1 Electronic scope 100 Variable magnification optical system for endoscope

Claims (8)

In order from the object side, a first lens group having a positive power, a second lens group having a negative power, and a third lens group having a positive power, and by moving the second lens group In a variable magnification optical system for an endoscope that changes an optical image,
The first lens group includes, in order from the object side, at least a negative lens having a concave surface facing the image side, a positive lens having a convex surface facing the image side, and a first cemented lens in which the negative lens and the positive lens are cemented. And
The second lens group has at least a second cemented lens in which a negative lens and a positive lens are cemented in order from the object side,
The third lens group has at least a third cemented lens in which a positive lens, a negative lens and a positive lens are cemented in order from the object side,
The focal length of the first lens group is defined as f ₁ (unit: mm), the focal length of the second lens group is defined as f ₂ (unit: mm), and the focal point of the third lens group. When the distance is defined as f ₃ (unit: mm) and the combined focal length of the first to third lens groups at the wide angle end is defined as f _W (unit: mm), the following conditional expression ( 1), (2)
−1.8 ≦ f ₂ / f _W ≦ −1.2 (1)
0.5 ≦ f ₁ / f ₃ ≦ 0.65 (2)
A variable power optical system for an endoscope characterized by satisfying the above. The following conditional expression (3)
−0.85 ≦ f ₁ / f ₂ ≦ −0.65 (3)
The variable power optical system for an endoscope according to claim 1, wherein: The following conditional expression (4)
−0.9 ≦ f ₂ / f ₃ ≦ −0.6 (4)
The variable power optical system for an endoscope according to claim 1 or 2, wherein: The following conditional expression (5)
0.53 ≦ f ₁ / f ₃ ≦ 0.61 (5)
The variable power optical system for an endoscope according to any one of claims 1 to 3, wherein: When the focal length having the largest absolute value among the focal lengths of the single lens or the cemented lens constituting the third lens group is defined as f _X (unit: mm), the following conditional expression (6)
| F _X / f ₃ | <5.0 (6)
The variable power optical system for an endoscope according to any one of claims 1 to 4, wherein: When the angle of view at the maximum image height at the wide angle end is defined as ω (unit: deg), the following conditional expression (7)
ω ≧ 120 (7)
The variable power optical system for an endoscope according to any one of claims 1 to 5, wherein:   7. The diaphragm according to claim 1, further comprising a diaphragm that moves integrally with the second lens group on the optical axis between the first lens group and the second lens group. 8. A variable magnification optical system for an endoscope according to 1.   An endoscope comprising the endoscope variable magnification optical system according to any one of claims 1 to 7 mounted at a tip thereof.

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