Objective optical system for endoscope

The present application is a continuation of PCT/JP2015/064582, filed on May 21, 2015 which is based upon and claims the benefit of priority from Japanese Patent Application No. 2014-127318 filed on Jun. 20, 2014; the entire contents of which are incorporated herein by reference.

The present invention relates to an objective optical system such as an objective optical system that can be used in an endoscope apparatus to be used in fields such as a medical field and an industrial field.

An endoscope is an apparatus that has been used widely in a medical field and an industrial field. In the medical field, images of various parts in a body cavity are achieved by an endoscope inserted inside the body cavity. Diagnosis of apart observed, is made by using these images. In such manner, endoscopes have been used for the observation of various parts in the body cavity and diagnosis.

In the observation by an endoscope, it is desirable that focused images in a wide range from a near point to a far point are captured. In this case, generally, an observation depth is determined by an F-number (hereinafter, referred to as ‘Fno’). Consequently, the observation depth has limitations.

Therefore, in recent years, an image forming optical system which enables to carry out observation with wider observation depth has been proposed. In such image forming optical system, zoom and focus are used. As an image forming optical system having a focus function (hereinafter, referred to as “focusing optical system”), an objective optical system in Japanese Patent No. 4934233 Publication is available.

The present invention provides the following means. An objective optical system for endoscope comprising in order from an object side:

a first negative lens which is planoconcave;

a second meniscus lens having a convex surface directed toward an image side;

a third positive meniscus lens having a convex surface directed toward the object side;

a positive lens unit; and

a cemented lens in which, a positive lens and a negative lens are cemented, wherein

focusing is carried out by moving the third positive meniscus lens along an optical axis, and

the second meniscus lens is a negative lens, and

the objective optical system for endoscope satisfies the following conditional expression (1), and satisfies at least one of the following conditional expressions (4-1) and (4-2)

2.3≦ fp/Ih≦ 5  (1)

−1.5≦ r 2/ r 3≦−0.28  (4-1)

−0.2≦ r 2/ r 4≦−0.08  (4-2)

where,

fp denotes a focal length of the positive lens unit, and

Ih denotes the maximum image height of the objective optical system for endoscope,

r 2 denotes an image-side radius of curvature of the first negative lens,

r 3 denotes an object-side radius of curvature of the second meniscus lens, and

r 4 denotes an image-side radius of curvature of the second meniscus lens.

FIG. 1A and FIG. 1B are diagrams showing a cross-sectional arrangement of an objective optical system for endoscope according to an embodiment of the present invention, where, FIG. 1A is a cross-sectional view in a normal observation state, and FIG. 15 is a cross-sectional view in a close observation state.

FIG. 2A and FIG. 2B are diagrams showing a cross-sectional arrangement of an objective optical system for endoscope according to an example 1 of the present invention, where, FIG. 2A is a cross-sectional view in a normal observation state, and FIG. 25 is a cross-sectional view in a close observation state;

FIG. 3A , FIG. 3B , FIG. 3C , FIG. 3D , FIG. 3E , FIG. 35 , FIG. 30 , and FIG. 3H are aberration diagrams showing a spherical aberration (SA), an astigmatism (AS), a distortion (DT), and a chromatic aberration of magnification (CC) respectively, of the example 1;

FIG. 4A and FIG. 4B are diagrams showing a cross-sectional arrangement of an objective optical system for endoscope according to an example 2 of the present invention, where, FIG. 4A is a cross-sectional view in a normal observation state, and FIG. 4B is a cross-sectional view in a close observation state;

FIG. 5A , FIG. 5B , FIG. 5C , FIG. 5D , FIG. 5E , FIG. 5F , FIG. 5G , and FIG. 5H are aberration diagrams showing a spherical aberration (SA), an astigmatism (AS), a distortion (DT), and a chromatic aberration of magnification (CC) respectively, of the example 2;

FIG. 6A and FIG. 6B are diagrams showing a cross-sectional arrangement of an objective optical system for endoscope according to an example 3 of the present invention, where, FIG. 6A is a cross-sectional view in a normal observation state, and FIG. 6B is a cross-sectional view in a close observation state;

FIG. 7A , FIG. 7B , FIG. 7C , FIG. 7D , FIG. 7E , FIG. 7F , FIG. 7G , and FIG. 7H are aberration diagrams showing a spherical aberration (SA), an astigmatism (AS), a distortion (DT), and a chromatic aberration of magnification (CC) respectively, of the example 3;

FIG. 8A and FIG. 8B are diagrams showing a cross-sectional arrangement of an objective optical system for endoscope according to an example 4 of the present invention, where, FIG. 8A is a cross-sectional view in a normal observation state, and FIG. 8B is a cross-sectional view in a close observation state; and

FIG. 9A , FIG. 9B , FIG. 9C , FIG. 9C , FIG. 9E , FIG. 9F , FIG. 9G , and FIG. 9H are aberration diagrams showing a spherical aberration (SA), an astigmatism (AS), a distortion (DT), and a chromatic aberration of magnification (CC) respectively, of the example 4.

Reasons for adopting such arrangement for an objective optical system for endoscope according to the present embodiment and effects thereof will be described below by using diagrams. However, the present invention is not restricted to the following embodiments.

FIG. 1A and FIG. 1B are diagrams showing a cross-sectional arrangement of the objective optical system for endoscope according to the present embodiment. Here, FIG. 1A is a diagram showing a cross-sectional arrangement of the objective optical system for endoscope in a normal observation state, and FIG. 1B is a diagram showing a cross-sectional arrangement of the objective optical system for endoscope in a close observation state.

In the objective optical system for endoscope (hereinafter, referred to as ‘objective optical system’) according to the present embodiment, for realizing an objective optical system with a small size and a superior performance which can be used in an endoscope, first of all, a first negative lens L 1 is disposed nearest to an object. Accordingly, it is made possible to adopt an arrangement of a retro focus type as an arrangement of the objective optical system.

Besides, the first negative lens L 1 is let to be a planoconcave lens, and also, the first negative lens L 1 is disposed such that a flat surface is directed toward an object side. By making such arrangement, removal of water during observation is improved, and breaking of a lens due to an impact is reduced.

Moreover, a second meniscus lens L 2 is disposed on the image side of the first negative lens L 1 . Furthermore, the shape of the second meniscus lens L 2 is let to be a meniscus shape with a convex surface directed toward the image side. By making such arrangement, a lens diameter is suppressed from becoming large, while correcting an aberration of the first negative lens L 1 .

Furthermore, a focusing unit in which, a lens moves is disposed on the image side of the second meniscus lens L 2 . More specifically, a third positive meniscus lens L 3 is disposed as the focusing unit. At the time of focusing, the third positive meniscus lens L 3 is moved along an optical axis. Moreover, to suppress an aberration fluctuation due to the movement of the lens at the time of focusing, a shape of the third positive meniscus lens L 3 is let to be a meniscus shape having a convex surface directed toward the object side.

A fourth positive lens unit L 4 is disposed on the image side of the third positive meniscus lens L 3 . The fourth positive lens unit L 4 contributes mainly to image formation. Therefore, a light beam is converged by a positive refractive power of the fourth positive lens unit L 4 .

On the image side of the fourth positive lens unit L 4 , an axial light beam and an off-axis light beam get separated. Therefore, a cemented lens is disposed on the image side of the fourth positive lens unit L 4 . The cemented lens includes a fifth positive lens L 5 and a sixth negative lens L 6 . When such an arrangement is made, the cemented lens including the fifth positive lens L 5 and the sixth negative lens L 6 is disposed at a position at which, a height of a light ray of the off-axis light beam becomes high. Accordingly, it is possible to correct a chromatic aberration favorably. It is possible to let the sixth negative lens L 6 to be a negative meniscus lens having a convex surface directed toward the image side.

Generally, when an Fno of an optical system becomes fast, an amount of a spherical aberration and a coma that occur, becomes large. Consequently, when the Fno of an optical system is made fast, it is disadvantageous for correction of the spherical aberration and the coma.

Here, in the objective optical system of the present embodiment, the main positive refractive power is secured as a refractive power of lens units disposed on the image side of the third positive meniscus lens L 3 , or in other words, the positive refractive power of the fourth positive lens unit L 4 and the positive refractive power of the fifth positive lens L 5 in the cemented lens. Therefore, when the Fno of the optical system is made fast, an aberration is susceptible to occur in the fourth positive lens unit L 4 and the fifth positive lens L 5 .

Moreover, also the positive refractive power that is necessary for realizing the arrangement of the retro focus type is achieved by the fourth positive lens unit L 4 and the fifth positive lens L 5 . Consequently, the positive refractive power of the fourth positive lens unit L 4 and the positive refractive power of the fifth positive lens L 5 are related to the size of the objective optical system.

Particularly, the fourth positive lens unit L 4 is disposed on the object side of the fifth positive lens L 5 , here, the height of an axial light ray becomes higher at the fourth positive lens unit L 4 than the height at the fifth positive lens L 5 . Consequently, an effect of aberration is more at the fourth positive lens unit L 4 where the height of the axial light ray is high.

On the other hand, for a positive lens which is not cemented, it is easy to secure the necessary refractive power, and a contribution to the size of the optical system is also high. Therefore, it is preferable to set appropriately a focal length of the fourth positive lens unit L 4 .

For such reasons, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (1)

2.3≦ fp/Ih≦ 5  (1)

where,

fp denotes a focal length of the fourth positive lens unit L 4 , and

Ih denotes the maximum image height of the objective optical system for endoscope.

When exceeding an upper limit value of conditional expression (1), the positive refractive power of the fourth positive lens unit L 4 becomes excessively small. In this case, the overall length of the optical system becomes large. Consequently, small-sizing of the optical system cannot be met.

When falling below a lower limit value of conditional expression (1), the positive refractive power of the fourth positive lens unit L 4 becomes excessively large with respect to the Fno. In this case, the spherical aberration and the coma are deteriorated. Consequently, it is not possible to secure a superior optical performance.

It is desirable to satisfy the following conditional expression (1′) instead of conditional expression (1).

2.4≦ fp/Ih≦ 4  (1′)

Furthermore, it is desirable to satisfy the following conditional expression (1″) instead of conditional expression (1).

2.5≦ fp/Ih≦ 3.5(1″)

As a refractive power of the first negative lens L 1 , for realizing the arrangement of retro focus type, a comparatively large negative refractive power is necessary. Here, when the negative refractive power is made too large, the coma is deteriorated. Therefore, it is preferable to set the refractive power of the first negative lens unit L 1 appropriately. By making such arrangement, even in an objective optical system with a comparatively fast Fno, such as the objective optical system according to the present embodiment, it is possible to have a favorable aberration balance in the overall optical system.

For such reasons, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (2).

−2≦ f 1/ Ih≦− 0.9  (2)

where,

f 1 denotes a focal length of the first negative lens L 1 , and

Ih denotes the maximum image height of the objective optical system for endoscope.

When exceeding an upper limit value of conditional expression (2), the negative refractive power of the first negative lens L 1 becomes excessively large. In this case, the coma and an astigmatism are not corrected adequately. Consequently, the optical performance is degraded.

When falling below a lower limit value of conditional expression (2), the negative refractive power of the first negative lens unit L 1 becomes excessively small. In this case, the overall length of the optical system becomes excessively large. Consequently, a condition of small-sizing which is necessary as an objective optical system for endoscope cannot be fulfilled.

It is desirable to satisfy the following conditional expression (2′) instead of conditional expression (2).

−1.6≦ f 1/ Ih≦− 0.9  (2′)

Furthermore, it is desirable to satisfy the following conditional expression (2″) instead of conditional expression (2).

−1.2≦ f 1/ Ih≦− 0.9  (2″)

Moreover, the fifth positive lens L 5 in the cemented lens, while contributing to securing the overall positive refractive power, also contributes to the correction of chromatic aberration by being cemented with the sixth negative lens L 6 .

For such reason, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (3) in at least one of a normal observation state and a close observation state.

−3≦( fc 1× fc 2)/( fc×f )≦−0.85  (3)

where,

fc 1 denotes a focal length of the fifth positive lens L 5 in the cemented lens,

fc 2 denotes a focal length of the sixth negative lens L 6 in the cemented lens,

fc denotes a combined focal length of the cemented lens, and

f denotes a focal length of the overall objective optical system for endoscope.

When exceeding an upper limit value of conditional expression (3), either the spherical aberration and the comma aberration are deteriorated, or correction of a chromatic aberration of magnification becomes insufficient.

When falling below a lower limit value of conditional expression (3), either the overall length of the optical system becomes excessively large, or the correction of the chromatic aberration of magnification becomes excessive.

It is desirable to satisfy the following conditional expression (3′) instead of conditional expression (3)

−2.3≦( fc 1× fc 2)/( fc×f )≦−0.88  (3′)

Furthermore, it is desirable to satisfy the following conditional expression (3″) instead of conditional expression (3).

−1.7≦( fc 1× fc 2)/( fc×f )≦−0.9  (3″)

Moreover, when an attempt is made to secure the negative refractive power, which is necessary for realizing the arrangement of the retro focus type, only by the first negative lens L 1 , since the first negative lens L 1 has a large negative refractive power, a substantial aberration occurs. Therefore, it is preferable to make the refractive power of the second meniscus lens L 2 to be disposed on the image side thereof, to be a negative refractive power. Thus, it is necessary to divide the negative refractive power between the first negative lens L 1 and the second meniscus lens L 2 , as well as to correct favorably the astigmatism and the coma in a peripheral area of screen.

For such reasons, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expressions (4-1) and (4-2)

−1.5≦ r 2/ r 3≦−0.28  (4-1)

−0.2≦ r 2/ r 4≦−0.08  (4-2)

where,

r 2 denotes an image-side radius of curvature of the first negative lens L 1 ,

r 3 denotes an object-side radius of curvature of the second meniscus lens L 2 , and

r 4 denotes an image-side radius of curvature of the second meniscus lens L 2 .

When exceeding upper limit values of conditional expressions (4-1) and (4-2), either a correction effect with respect to the astigmatism and the coma is degraded, or an amount of aberration that occurs in the first negative lens L 1 becomes excessively large, and all the aberrations are deteriorated.

When falling below lower limit values of conditional expressions (4-1) and (4-2), either the correction of the astigmatism and the coma becomes excessive, or the overall length of the optical system becomes excessively large.

It is desirable to satisfy the following conditional expressions (4-1′) and (4-2′) instead of conditional expressions (4-1) and (4-2).

−1≦ r 2/ r 3≦−0.3  (4-1′)

−0.15≦ r 2/ r 4≦−0.09  (4-2′)

Furthermore, it is desirable to satisfy the following conditional expressions (4-1″) and (4-2″) instead of conditional expressions (4-1) and (4-2).

−0.75≦ r 2/ r 3≦−0.32  (4-1″)

−0.13≦ r 2/ r 4≦−0.1  (4-2″)

Moreover, in the third positive meniscus lens 13 or in an optical system disposed on the image side of an aperture stop S, it s necessary to secure the positive refractive power which is associated mainly with image formation. For this, for the fourth positive lens unit L 4 and the cemented lens, it is desirable to set a ratio of focal lengths that would enable to correct favorably the spherical aberration and the coma, while taking into consideration shortening the overall length and securing back focus.

For such reasons, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (5).

0.45≦ fp/fc≦ 1.3  (5)

where,

fp denotes the focal length of the fourth positive lens unit 14 , and

fc denotes a combined focal length of the cemented lens.

When exceeding an upper limit value of conditional expression (5), either the overall length of the optical system becomes excessively large, or occurrence of the coma due to the cemented lens cannot be corrected fully.

When falling below a lower limit value of conditional expression (5), either the back focus is excessively short thereby making the assembling impossible, or the spherical aberration which occurs due to the fourth positive lens unit L 4 cannot be corrected fully.

It is desirable to satisfy the following conditional expression (5′) instead of conditional expression (5).

0.47≦ fp/fc≦ 1.1  (5′)

Furthermore, it is desirable to satisfy the following conditional expression (5″) instead of conditional expression (5)

0.45≦ fp/fc≦ 0.9  (5″)

As mentioned above, in the objective optical system according to the present embodiment, the arrangement of the retro focus type is realized by the lens having a negative refractive power which is disposed on the object side and the lens having a positive refractive power which is disposed on the image side. Accordingly, small-sizing of the optical system is facilitated while correcting the astigmatism and the coma.

Here, particularly, an arrangement of the first negative lens L 1 and a predetermined optical system is significant for small-sizing and for maintaining the aberration balance of the overall objective optical system. The predetermined optical system refers to an optical system formed by combining the fourth positive lens unit L 4 and the cemented lens (the fifth positive lens L 5 and the sixth negative lens L 6 )

For such reasons, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (6)

−5≦ fpc/f 1≦−2  (6)

where,

fpc denotes a combined focal length of the fourth positive lens unit L 4 and the cemented lens, and

f 1 denotes a focal length of the first negative lens L 1 .

When exceeding an upper limit value of conditional expression (6), the refractive power of the first negative lens unit L 1 becomes excessively small. Accordingly, either a diameter of the optical system becomes large or the refractive power of the predetermined optical system becomes excessively large, and the spherical aberration and the coma are deteriorated.

When falling below a lower limit value of conditional expression (6), the refractive power of the first negative lens unit becomes excessively large. Accordingly, either the aberration as a whole is deteriorated, or the refractive power of the predetermined optical system becomes excessively small, and the overall length of the optical system becomes large,

It is desirable to satisfy the following conditional expression (6′) instead of conditional expression (6).

−4≦ fpc/f 1≦−2.1  (6′)

Furthermore, it is desirable to satisfy the following conditional expression (6″) instead of conditional expression (6).

−3.2≦ fpc/f 1≦−2.2  (6″)

Moreover, the third positive meniscus lens L 3 moves as a focusing lens at the time of focusing. Consequently, in the third positive meniscus lens L 3 , an aberration fluctuation due focusing is susceptible to occur. Moreover, when the lens moves, the lens is tilted and shifted, and deviation of the practical optical system from an original ideal design state cannot be avoided. Consequently, it becomes necessary to set appropriately a focal length of the focusing lens with respect to an amount of movement of lens due to focusing.

For such reason, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (7).

22≦ f 3/ df≦ 50  (7)

where,

f 3 denotes a focal length of the third positive meniscus lens L 3 , and

df denotes the maximum amount of movement of the third positive meniscus lens L 3 .

When exceeding an upper limit value of conditional expression (7), an amount of movement of the focusing lens becomes excessively large. Consequently, the objective . optical system becomes large-sized.

When falling below a lower limit value of conditional expression (7), an amount of aberration occurring due to focusing and variability, or in other words, a manufacturing error becomes large. Consequently, the optical performance of the optical system is degraded.

It is desirable to satisfy the following conditional expression (7′) instead of conditional expression (7).

22≦ f 3/ df≦ 40  (7′)

Furthermore, it is desirable to satisfy the following conditional expression (7″) instead of conditional expression (7).

22≦ f 3/ df≦ 35  (7″)

As mentioned above, when an attempt is made to secure the negative refractive power which is necessary for realizing the arrangement of retro focus type only by the first negative lens L 1 , the first negative lens L 1 has to bear a large negative refractive power. Therefore, in the objective optical system according to the present embodiment, the negative refractive power is divided between the first negative lens L 1 and the second meniscus lens L 2 as mentioned above.

Here, although the negative refractive power is divided, the first negative lens L 1 still has a large negative refractive power, and an optical system of the retro focus type is formed. However, since the first negative lens L 1 has a large negative refractive power, a substantial aberration occurs in the first negative lens L 1 .

In such case, correcting an aberration such as the coma by the third positive meniscus lens L 3 having a positive refractive power becomes effective. Particularly, a difference in an image ray height at the first negative lens L 1 and at the third positive meniscus lens L 3 affects the aberration correction. Therefore, it is preferable to maintain an appropriate distance between the first negative lens L 1 and the third positive meniscus lens L 3 .

For such reasons, in the objective optical lens according to the present embodiment, it is desirable to satisfy the following conditional expression (8).

1.35≦Σ d ( L 1− L 3)/ Ih≦ 3  (8)

where,

Σd (L 1 −L 3 ) denotes the minimum axial distance from an object-side surface of the first negative lens L 1 up to an object-side surface of the third positive meniscus lens L 3 , and

Ih denotes the maximum image height of the objective optical system for endoscope.

When exceeding an upper limit value of conditional expression (8), either a diameter of the first negative lens L 1 becomes excessively large, or an amount of movement of the third positive meniscus lens L 3 becomes large. Consequently, small-sizing of the first negative lens L 1 and the third positive meniscus lens L 3 cannot be achieved.

When falling below a lower limit value of conditional expression (8), the refractive power of the first negative lens L 1 and the third positive meniscus lens L 3 becomes excessively large. Consequently, the coma is deteriorated.

It is desirable to satisfy the following conditional expression (8′) instead of conditional expression (8)

1.4≦Σ d ( L 1− L 3)/ Ih≦ 2.5  (8′)

Furthermore, it is desirable to satisfy the following conditional expression (8″) instead of conditional expression (8).

1.5≦Σ d ( L 1− L 3)/ Ih≦ 2  (8″)

Moreover, in the objective optical system according to the present embodiment, it is desirable to satisfy the following conditional expression (9)

4≦Fno≦7  (9)

where,

Fno denotes an F-number for the objective optical system for endoscope.

When exceeding an upper limit value of conditional expression (9), there is an effect of diffraction with respect to a pixel pitch of a CCD (charge coupled device).

When falling below a lower limit value of conditional expression (9), the aberration remained becomes excessively large, and also superior optical performance cannot be achieved.

It is desirable to satisfy the following conditional expression (9′) instead of conditional expression (9).

4.5≦Fno≦6.6  (9′)

Furthermore, it is desirable to satisfy the following conditional expression (9″) instead of conditional expression (9).

5≦ Fno≦ 6.3  (9″)

Moreover, in the objective optical system according to the present embodiment, it is desirable that only the third positive meniscus lens L 3 moves at the time of focusing. It is preferable to make such arrangement, as a mechanical arrangement becomes simple.

Moreover, in the objective optical system according to the present embodiment, it is desirable that an aperture stop S is disposed on the image side of the third positive meniscus lens L 3 . It is preferable to make such arrangement, as it enables to have an aberration balance of the coma and the chromatic aberration of magnification.

Moreover, in the objective optical system according to the present embodiment, the fourth positive lens unit L 4 may include a plurality of lenses.