WO2021166027A1

WO2021166027A1 - Objective optical system, imaging device, and endoscope

Info

Publication number: WO2021166027A1
Application number: PCT/JP2020/006019
Authority: WO
Inventors: 中村稔
Original assignee: オリンパス株式会社
Priority date: 2020-02-17
Filing date: 2020-02-17
Publication date: 2021-08-26

Abstract

Provided is an objective optical system that, despite being small in size, has adequately corrected chromatic aberration of magnification.　An objective optical system comprising, in order from the object side to the image side, a first lens L1 having a negative refractive power, a second lens L2 having a positive refractive power, an aperture diaphragm S, and a third lens L3 having a positive refractive power, the second lens L2 being a biconvex lens, and the first lens L1, the second L2, and the third lens L3 being formed from a synthetic resin material and satisfying conditional expressions (1), (2), and (3).　(1): νd2<νd1. (2): -1.2<f/f1<-0.74. (3): 0.3<DZ2/f<0.9. A first intersection point is the point at which the object-side surface of the first lens and the optical axis intersect, a second intersection point is the point at which a principal ray at the maximum image height intersects the image-side surface of the first lens, and DZ2 is the distance parallel to the optical axis from the first intersection point to the second intersection point.

Description

Objective optical system, imaging device, and endoscope

The present invention relates to an objective optical system, an imaging device, and an endoscope.

An optical system having three lenses is disclosed in Patent Document 1 and Patent Document 2. The optical system includes a first lens having a negative power, a second lens having a positive power, an aperture diaphragm, and a third lens having a positive power.

Japanese Unexamined Patent Publication No. 2007-133324 JP-A-2010-231190

It cannot be said that the optical system disclosed in Patent Document 1 and the optical system disclosed in Patent Document 2 sufficiently correct the chromatic aberration of magnification. Therefore, good imaging performance cannot be obtained in the peripheral portion of the optical image.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an objective optical system which is small in size and whose chromatic aberration of magnification is sufficiently corrected. Another object of the present invention is to provide an imaging device capable of acquiring a clear image and an endoscope.

In order to solve the above-mentioned problems and achieve the object, the objective optical system according to at least some embodiments of the present invention is used.
From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens,
The first lens, the second lens, and the third lens are made of a synthetic resin material.
It is characterized in that the following conditional expressions (1), (2), and (3) are satisfied.
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens,
νd2 is the Abbe number of the second lens,
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is the point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
If the second intersection is closer to the image than the first intersection, the distance value is a positive value,
Is.

Further, the image pickup apparatus according to at least some embodiments of the present invention may be used.
From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens,
The first lens, the second lens, and the third lens are made of a synthetic resin material.
It is characterized by providing an objective optical system that satisfies the following conditional expressions (1), (2), and (3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens,
νd2 is the Abbe number of the second lens,
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is the point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
If the second intersection is closer to the image than the first intersection, the distance value is a positive value,
Is.

Further, the endoscope according to at least some embodiments of the present invention is
From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens,
The first lens, the second lens, and the third lens are made of a synthetic resin material.
It is characterized by providing an objective optical system that satisfies the following conditional expressions (1), (2), and (3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens,
νd2 is the Abbe number of the second lens,
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is the point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
If the second intersection is closer to the image than the first intersection, the distance value is a positive value,
Is.

According to the present invention, it is possible to provide an objective optical system that is compact but has sufficiently corrected chromatic aberration of magnification. Further, it is possible to provide an imaging device and an endoscope capable of acquiring a clear image.

It is a figure which shows the cross-sectional view and the parameter of the objective optical system of this embodiment. It is sectional drawing and aberration diagram of the objective optical system of Example 1. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 2. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 3. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 4. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 5. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 6. It is sectional drawing and aberration diagram of the objective optical system of Example 7. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 8. FIG. It is sectional drawing and aberration diagram of the objective optical system of Example 9. FIG. It is a figure which shows the optical apparatus of this embodiment.

Hereinafter, the reason and operation of the objective optical system according to the present embodiment, the imaging device according to the present embodiment, and the endoscope according to the present embodiment will be described. The present invention is not limited to these embodiments.

The objective optical system of the present embodiment has a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture aperture, and a first lens having a positive refractive power in this order from the object side to the image side. The second lens is a biconvex lens, the first lens, the second lens, and the third lens are made of a synthetic resin material, and the following conditional equations (1), (2), and ( It is characterized by satisfying 3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens,
νd2 is the Abbe number of the second lens,
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is the point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
If the second intersection is closer to the image than the first intersection, the distance value is a positive value,
Is.

The objective optical system of this embodiment will be described. FIG. 1 is a diagram showing a cross-sectional view and parameters of the objective optical system of the present embodiment.

The objective optical system OBJ has a first lens L1 having a negative refractive power, a second lens L2 having a positive refractive power, an aperture aperture S, and a first lens having a positive refractive power in this order from the object side to the image side. It has three lenses L3 and.

The first lens L1 is a biconcave negative lens. However, the shape of the first lens L1 is not limited to the biconcave shape. The second lens L2 is a biconvex positive lens. The third lens L3 is a biconvex positive lens. However, the shape of the third lens L3 is not limited to the biconvex shape.

In order for the objective optical system to have a wide angle of view, for example, an angle of view of 130 ° or more, it is preferable that the refractive power of the lens located closest to the object is negative. In the objective optical system OBJ, both concave and negative lenses are arranged as the first lens L1 on the most object side. Therefore, a wide angle of view can be obtained.

A biconvex lens is used for the second lens L2. By using a biconvex lens for the second lens L2, the second lens L2 can have an appropriate refractive power. Therefore, the outer diameter of the first lens L1 can be reduced while satisfactorily correcting the astigmatic difference and the curvature of field. Further, the total length of the optical system can be shortened. Therefore, the shape of the second lens L2 is preferably a biconvex shape.

As the shape of the second lens L2, for example, a meniscus shape can be considered. However, if the shape of the second lens L2 is a meniscus shape that is convex toward the image side, the refractive power of the side surface of the object has a negative refractive power. In this case, the height of the main ray becomes too high in the first lens L1. Therefore, the outer diameter of the first lens L1 becomes large.

If the shape of the second lens L2 is a meniscus shape that is convex toward the object, the curvature of the side surface of the object becomes large. Therefore, the refractive power of the side surface of the object becomes too large. As a result, it becomes difficult to satisfactorily correct astigmatism and curvature of field. Therefore, it is not preferable that the shape of the second lens L2 is a meniscus shape.

In the objective optical system OBJ, the aperture diaphragm S is arranged between the second lens L2 and the third lens L3. By doing so, the correction effect of the chromatic aberration of magnification obtained by the first lens L1 and the second lens L2 can be strengthened.

A synthetic resin material is used for the first lens L1, the second lens L2, and the third lens L3. By using a synthetic resin material for each lens, each lens can be manufactured by injection molding. If injection molding is used, the lens can be manufactured at low cost. Therefore, the cost can be reduced while ensuring good imaging performance.

The objective optical system of this embodiment satisfies the conditional equations (1), (2), and (3). Before explaining the conditional expression, the parameters used in the conditional expression will be described.

DZ2 is used in the above-mentioned conditional expression (3) and the later-described conditional expression (4). Further, in the conditional expression (5) described later, DY2 is used. DZ2 and DY2 will be described with reference to FIG.

FIG. 1 shows a distance DZ2, a distance DY2, a lens surface R2, a surface apex VR2, an intersection point CP1, an intersection point CP2, a light ray CRmax, an optical axis AX, a straight line PL, and an optical image IM.

(Distance DZ2)
As described above, DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection. The first intersection is a point where the image side surface of the first lens intersects the optical axis. The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.

The surface top VR2 is a point where the lens surface R2 intersects the optical axis AX. The lens surface R2 is an image side surface of the first lens L1. Therefore, the surface top VR2 represents the first intersection.

The intersection point CP1 is the point where the light ray CRmax intersects the lens surface R2. Since the ray CRmax has reached the highest position of the optical image IM, the ray CRmax is the main ray of maximum image height. The lens surface R2 is an image side surface of the first lens L1. Therefore, the intersection CP1 represents the second intersection.

However, the surface top VR2 is located on the optical axis AX, but the intersection CP1 is not located on the optical axis AX. Therefore, the distance DZ2 is calculated based on the distance parallel to the optical axis AX from the surface top VR2 to the intersection CP1.

In calculating the distance parallel to the optical axis AX from the surface top VR2 to the intersection CP1, the point when the intersection CP1 is projected on the optical axis AX may be used instead of the intersection CP1.

The intersection point CP2 is the point where the straight line PL intersects the optical axis AX. The straight line PL is a virtual straight line that passes through the intersection CP1 and is orthogonal to the optical axis AX. Therefore, the intersection point CP2 represents a point when the intersection point CP1 is projected on the optical axis AX.

The distance on the optical axis AX from the surface apex VR2 to the intersection CP2 represents the distance parallel to the optical axis AX from the surface apex VR2 to the intersection CP1. Therefore, the distance DZ2 may be calculated based on the distance on the optical axis AX from the surface top VR2 to the intersection CP2.

The intersection point CP1 and the intersection point CP2 are located on the straight line PL. Since the position of the intersection CP1 is known, the position of the intersection CP2 can be calculated from the position of the intersection CP1. By calculating the position of the intersection CP2, the distance on the optical axis AX from the surface top VR2 to the intersection CP2 can be calculated. As a result, the distance DZ2 can be calculated.

The distance DZ2 is calculated based on the surface top VR2. Regarding the positive / negative of the distance value, the traveling direction of light is positive and the opposite direction is negative. When the intersection point CP1 is located on the image side of the surface top VR2, the distance value is a positive value. That is, when the second intersection is located closer to the image than the first intersection, the distance value is a positive value.

(Distance DY2)
As will be described later, DY2 is the shortest distance from the optical axis to the second intersection.

As described above, the intersection CP1 represents the second intersection. Since the intersection CP2 is located on the optical axis AX, the position of the intersection CP2 can be regarded as the position of the optical axis AX.

The intersection point CP1 and the intersection point CP2 are located on the straight line PL. The straight line PL is a virtual straight line that passes through the intersection CP1 and is orthogonal to the optical axis AX. Since the position of the intersection CP2 can be regarded as the position of the optical axis AX, the distance from the intersection CP2 to the intersection CP1 represents the shortest distance from the optical axis AX to the intersection CP1. Therefore, the distance DY2 may be calculated based on the distance from the intersection CP2 to the intersection CP1.

In the calculation of the distance DY2, the distance is calculated with reference to the optical axis AX. The intersection point CP1 is located on the lens surface R2. The lens surface R2 is a rotationally symmetric surface. Therefore, since there is only one direction from the optical axis AX to the intersection CP1, the distance value is always a positive value.

The conditional expressions (1), (2), and (3) will be explained.

In an optical system with a wide angle of view, it is necessary to improve the imaging performance off the axis. In order to improve the imaging performance off the axis, it is sufficient to suppress the occurrence of chromatic aberration of magnification. In order to suppress the occurrence of chromatic aberration of magnification, it is preferable to use a synthetic resin material having a larger dispersion than that of the first lens for the second lens.

Conditional expression (1) is a conditional expression that defines the Abbe number of the second lens. When the conditional expression (1) is satisfied, the Abbe number of the second lens is different from the Abbe number of the first lens. Therefore, the occurrence of chromatic aberration of magnification can be suppressed.

Conditional expression (2) is a conditional expression that defines the refractive power of the first lens. In the conditional expression (2), the refractive power of the first lens is defined with respect to the refractive power of the objective optical system.

If it falls below the lower limit of the conditional expression (2), the refractive power of the first lens becomes too large. In this case, it is easily affected by manufacturing errors. For example, if the first lens is eccentric due to a manufacturing error, the aberration increases. Therefore, the imaging performance is greatly reduced.

In order to prevent deterioration of imaging performance, manufacturing error must be reduced. However, if an attempt is made to reduce the manufacturing error, the manufacturing cost will increase.

If the upper limit of the conditional expression (2) is exceeded, the refractive power of the first lens becomes too small. Therefore, it becomes difficult to secure a wide angle of view, or the outer diameter of the first lens becomes large.

The conditional expression (3) is a conditional expression that defines the passing position of the main ray having the maximum image height on the image side surface of the first lens.

If it is below the lower limit of the conditional expression (3), the point where the main ray of the maximum image height intersects the image side surface of the first lens is too close to the optical axis. In this case, the height of the off-axis light rays becomes too low between the first lens and the second lens. Further, the distance between the first lens and the second lens becomes too narrow. Therefore, it becomes difficult to widen the optical system. The distance between the first lens and the second lens is the distance on the optical axis.

If the upper limit of the conditional expression (3) is exceeded, the outer diameter of the first lens becomes large and the distance between the first lens and the second lens becomes too wide. Therefore, it becomes difficult to sufficiently correct the chromatic aberration of magnification. In addition, the distance from the lens surface located closest to the image side to the image surface becomes too short.

Instead of the conditional expression (3), it is preferable to satisfy the following conditional expression (3').
0.59 <DZ2 / f <0.9 (3')

In the objective optical system of the present embodiment, the image side surface of the first lens is an aspherical surface, and it is preferable that the following conditional expression (4) is satisfied.
0.8 <DZ2 / R2 <1.4 (4)
here,
The first intersection is the point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
If the second intersection is closer to the image than the first intersection, the distance value is a positive value,
R2 is the radius of curvature of the image side of the first lens,
Is.

In the objective optical system of this embodiment, the image side surface of the first lens is an aspherical surface. The shape of the aspherical surface is a shape in which the curvature decreases from the center to the periphery.

Conditional expression (4) is a conditional expression that defines the aspherical shape of the image side surface of the first lens.

If it is less than the lower limit of the conditional expression (4), the radius of curvature of the image side surface of the first lens becomes large, so that the correction effect due to the aspherical surface weakens. Further, the point where the main ray having the maximum image height intersects the image side surface of the first lens is too close to the optical axis. In this case, the height of the off-axis ray becomes low. Therefore, the correction effect of the aspherical surface is weakened with respect to the off-axis light rays. Also, the distance between the first lens and the second lens becomes too narrow. Therefore, it becomes difficult to widen the optical system.

If the upper limit of the conditional expression (4) is exceeded, the distance between the first lens and the second lens becomes too wide. Therefore, it becomes difficult to sufficiently correct the chromatic aberration of magnification.

Instead of the conditional expression (4), it is preferable to satisfy the following conditional expression (4').
1.05 <DZ2 / R2 <1.35 (4')

The objective optical system of the present embodiment preferably satisfies the following conditional expression (5).
0.6 <D2 / DY2 <1.35 (5)
here,
D2 is the distance between the image side surface of the first lens and the object side surface of the second lens on the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DY2 is the shortest distance from the optical axis to the second intersection,
Is.

If it falls below the lower limit of the conditional expression (5), the distance between the first lens and the second lens becomes too narrow. Therefore, it becomes difficult to secure a wide angle of view.

If the upper limit of the conditional expression (5) is exceeded, the distance between the first lens and the second lens becomes too wide. Therefore, it becomes difficult to sufficiently correct the chromatic aberration of magnification.

The objective optical system of the present embodiment preferably satisfies the following conditional expression (6).
0.25 <f / f2 <0.6 (6)
here,
f is the focal length of the objective optical system,
f2 is the focal length of the second lens,
Is.

Conditional formula (6) is a conditional formula that defines the refractive power of the second lens. In the conditional expression (6), the refractive power of the second lens is defined with respect to the refractive power of the objective optical system.

By giving the first lens an appropriate amount of refractive power, light rays with a wide angle of view can be incident on the first lens. The light beam incident on the first lens finally incidents on the third lens. At this time, it is preferable that the light rays incident on the first lens are incident on the third lens in good condition.

The image formed by the first lens and the second lens is imaged on the image plane by the third lens. In order to suppress the occurrence of various aberrations in the third lens, it is better to reduce the imaging magnification in the third lens. For that purpose, the axial light rays incident on the center of the image plane are as parallel as possible to the optical axis. It is better to enter the third lens so as to be. A good state is a state in which the imaging magnification of the third lens is small, or a state in which the axial light beam incident on the third lens is substantially parallel to the optical axis.

A second lens is arranged between the first lens and the third lens. In order for the light beam incident on the first lens to be incident on the third lens in a good state, it is necessary to give the second lens an appropriate magnitude of refractive power.

If it falls below the lower limit of the conditional expression (6), the refractive power of the second lens becomes too small. In this case, the distance between the first lens and the second lens becomes too wide. Therefore, the total length of the optical system becomes long, and the outer diameter of the first lens becomes large. In addition, the chromatic aberration of magnification cannot be sufficiently corrected.

If the upper limit of the conditional expression (6) is exceeded, the refractive power of the second lens becomes too large. In this case, it is easily affected by manufacturing errors. For example, if the second lens is eccentric due to a manufacturing error, the aberration increases. Therefore, the imaging performance is greatly reduced.

The objective optical system of the present embodiment preferably satisfies the following conditional expression (7).
-3.0 <f2 / f1 <-1.9 (7)
here,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
Is.

The conditional expression (7) is a conditional expression that defines the refractive power of the first lens. In the conditional expression (7), the refractive power of the first lens is defined with respect to the refractive power of the second lens.

If it is less than the lower limit of the conditional expression (7), the refractive power of the first lens becomes too large as compared with the refractive power of the second lens. Therefore, the chromatic aberration of magnification cannot be sufficiently corrected.

If the upper limit of the conditional expression (7) is exceeded, the refractive power of the first lens becomes too small as compared with the refractive power of the second lens. Therefore, the outer diameter of the first lens becomes large.

The objective optical system of this embodiment preferably satisfies the following conditional expressions (8) and (9).
50 <νd1 (8)
νd2 <35 (9)
here,
νd1 is the Abbe number of the first lens,
νd2 is the Abbe number of the second lens,
Is.

By satisfying the conditional expressions (8) and (9), it is possible to sufficiently correct the chromatic aberration of magnification while reducing the size of the optical system.

The objective optical system of the present embodiment preferably satisfies the following conditional expressions (10), (11), and (12).
nd1 <1.55 (10)
nd2 <1.7 (11)
nd3 <1.55 (12)
here,
nd1 is the refractive index of the first lens on the e-line.
nd2 is the refractive index of the second lens on the e-line.
nd3 is the refractive index of the third lens on the e-line.
Is.

By satisfying the conditional expressions (10), (11), and (12), it is possible to sufficiently correct various aberrations while making the optical system compact and inexpensive.

The image pickup apparatus of the present embodiment has a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture aperture, and a third lens having a positive refractive power in this order from the object side to the image side. The second lens is a biconvex lens, and the first lens, the second lens, and the third lens are made of a synthetic resin material, and the above-mentioned conditional equations (1), (2), and (3). ) Is provided.

The endoscope of the present embodiment has a first lens having a negative refractive power, a second lens having a positive refractive power, an aperture aperture, and a first lens having a positive refractive power in this order from the object side to the image side. The second lens is a biconvex lens, and the first lens, the second lens, and the third lens are made of a synthetic resin material. It is characterized by providing an objective optical system that satisfies 3).

The imaging device of the present embodiment and the endoscope of the present embodiment may satisfy at least one of the above-mentioned conditional expressions (4) to (12).

Hereinafter, examples of the objective optical system will be described in detail based on the drawings. The present invention is not limited to this embodiment.

The lens cross-sectional view of each embodiment will be described. In FIGS. 2 to 10, (a) shows a cross-sectional view of the lens. The image plane is indicated by I.

The aberration diagram of each embodiment will be described. In FIGS. 2 to 10, (a) shows a cross-sectional view of the lens. (B) is spherical aberration (SA), (c) is astigmatism (AS), (d) is chromatic aberration of magnification (CC), (e) is distortion (DT), and (f) is coma (CM). Is shown. Coma (CM) is an aberration at a position 0.8 times the maximum image height.

In each aberration diagram, the horizontal axis represents the amount of aberration. For spherical aberration, astigmatism, chromatic aberration of magnification, and coma, the unit of the amount of aberration is mm. For distortion, the unit of aberration is%. FYI is the image height, the unit is mm, and FNO is the F number. The unit of wavelength of the aberration curve is nm.

The objective optical system of the first embodiment has a biconcave negative lens L1, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The aspherical surface is provided on a total of three surfaces, that is, the image side surface of the biconcave negative lens L1, the object side surface of the biconvex positive lens L2, and the image side surface of the biconvex positive lens L3.

The objective optical system of the second embodiment has a biconcave negative lens L1, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of the third embodiment has a negative meniscus lens L1 having a convex surface facing the object side, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The aspherical surface is provided on a total of three surfaces, that is, the image side surface of the negative meniscus lens L1, the object side surface of the biconvex positive lens L2, and the image side surface of the biconvex positive lens L3.

The objective optical system of the fourth embodiment has a negative meniscus lens L1 having a convex surface facing the object side, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of the fifth embodiment has a negative meniscus lens L1 having a convex surface facing the object side, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of the sixth embodiment has a negative meniscus lens L1 having a convex surface facing the object side, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of Example 7 has a biconcave negative lens L1, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of Example 8 has a biconcave negative lens L1, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The objective optical system of the ninth embodiment has a negative meniscus lens L1 having a convex surface facing the object side, a biconvex positive lens L2, and a biconvex positive lens L3 in order from the object side. The aperture diaphragm S is arranged between the biconvex positive lens L2 and the biconvex regular lens L3.

The numerical data of each of the above examples is shown below. In the surface data, r is the radius of curvature of each lens surface, d is the distance between each lens surface, ne is the refractive index of the e-line of each lens, and νd is the Abbe number of each lens.

In various data, OBJ is the object point distance, f is the focal length of the objective optical system on the e-line, and Fno. Is the F number, ω is the half angle of view, and IH is the image height. The diaphragm is an aperture diaphragm.

The aspherical shape has the following equation when the optical axis direction is z, the direction orthogonal to the optical axis is y, the conical coefficient is k, and the aspherical coefficient is A4, A6, A8, A10, A12, and so on. expressed.
z = (y ² / r) / [1 + {1- (1 + k) (y / r) ² } ^1/2 ]
+ A4y ⁴ + A6y ⁶ + A8y ⁸ + A10y ¹⁰ + A12y ¹² + ...
Further, in the aspherical coefficient, "En" (n is an integer) indicates "10 ^-n ". The symbols of these specification values are also common to the numerical data of the examples described later.

Numerical Example 1
Unit mm

Surface data Surface number r d ne ν d
1 -67.5778 0.3800 1.53336 56.00
2 * 0.3644 0.6446 1.
3 * 1.2135 0.9791 1.64117 23.90
4-2.1149 0.0400 1.
5 (Aperture) ∞ 0.0940 1.
6 13.3727 0.8000 1.53336 56.00
7 * -0.8892 1.3628 1.
Image plane ∞

Second surface of aspherical data
k = -0.8112
Third side
k = 0.
A2 = 0.000E + 00, A4 = -2.2107E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 2.6525E-01, A6 = 6.6785E-01

Various data OBJ 10.0
f 0.654
Fno. 4.31
ω 70.5
IH 0.975

Numerical Example 2
Unit mm

Surface data Surface number r d ne ν d
1 -35.5588 0.3800 1.53336 56.00
2 * 0.3645 0.6703 1.
3 * 1.3190 0.8967 1.64117 23.90
4-2.0172 0.0400 1.
5 (Aperture) ∞ 0.0940 1.
6 13.0628 0.9000 1.53336 56.00
7 * -0.8450 1.3223 1.
Image plane ∞

Second surface of aspherical data
k = -0.7942
Third side
k = 0.
A2 = 0.000E + 00, A4 = -1.7926E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.0360E-01, A6 = 6.6355E-01

Various data OBJ 10.0
f 0.621
Fno. 4.19
ω 78.3
IH 0.975

Numerical Example 3
Unit mm

Surface data Surface number r d ne ν d
1 453.6508 0.3800 1.53336 56.00
2 * 0.3810 0.4796 1.
3 * 1.2280 1.1074 1.64117 23.90
4-4.3141 0.0400 1.
5 (Aperture) ∞ 0.1922 1.
6 2.2176 0.8013 1.53336 56.00
7 * -0.7871 1.2582 1.
Image plane ∞

Second surface of aspherical data
k = -0.8490
Third side
k = 0.
A2 = 0.000E + 00, A4 = -1.6622E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.1722E-01, A6 = 8.1801E-01

Various data OBJ 10.0
f 0.631
Fno. 4.32
ω 69.8
IH 0.975

Numerical Example 4
Unit mm

Surface data Surface number r d ne ν d
1 41.4725 0.3800 1.53336 56.00
2 * 0.3805 0.4800 1.
3 * 1.1458 1.1373 1.64117 23.90
4 -11.1625 0.0400 1.
5 (Aperture) ∞ 0.2672 1.
6 1.8503 0.7191 1.53336 56.00
7 * -0.7898 1.2764 1.
Image plane ∞

Second surface of aspherical data
k = -0.7753
Third side
k = 0.
A2 = 0.000E + 00, A4 = -1.0391E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 2.6615E-01, A6 = 9.5022E-01

Various data OBJ 10.0
f 0.632
Fno. 4.28
ω 70.0
IH 0.975

Numerical Example 5
Unit mm

Surface data Surface number r d ne ν d
1 43.8711 0.3800 1.53336 56.00
2 * 0.3502 0.5000 1.
3 * 1.1934 1.1000 1.64117 23.90
4 -3.0438 0.0400 1.
5 (Aperture) ∞ 0.3540 1.
6 2.1804 0.8000 1.53336 56.00
7 * -0.7625 1.1271 1.
Image plane ∞

Second surface of aspherical data
k = -0.7076
Third side
k = 0.
A2 = 0.000E + 00, A4 = 1.9535E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.7211E-01, A6 = 9.8200E-01

Various data OBJ 10.0
f 0.560
Fno. 4.31
ω 70.5
IH 0.975

Numerical Example 6
Unit mm

Surface data Surface number r d ne ν d
1 83.3059 0.3800 1.53336 56.00
2 * 0.3826 0.4213 1.
3 * 1.3628 1.0673 1.64117 23.90
4-5.9748 0.0400 1.
5 (Aperture) ∞ 0.1632 1.
6 1.9784 0.8290 1.53336 56.00
7 * -0.7634 1.3452 1.
Image plane ∞

Second surface of aspherical data
k = -0.8909
Third side
k = 0.
A2 = 0.000E + 00, A4 = -2.6726E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 2.9122E-01, A6 = 1.0184E + 00

Various data OBJ 10.0
f 0.649
Fno. 4.46
ω 69.8
IH 0.975

Numerical Example 7
Unit mm

Surface data Surface number r d ne ν d
1 -34.3216 0.3800 1.53336 56.00
2 * 0.4148 0.3810 1.
3 * 1.2461 1.1455 1.64117 23.90
4 -30.6935 0.0400 1.
5 (Aperture) ∞ 0.2053 1.
6 1.8472 0.7349 1.53336 56.00
7 * -0.7585 1.3869 1.
Image plane ∞

Second surface of aspherical data
k = -1.0117
A2 = 0.000E + 00, A4 = -7.5223E-02, A6 = -2.1668E-01
Third side
k = 0.
A2 = 0.000E + 00, A4 = -3.5265E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.6153E-01, A6 = 7.8683E-01

Various data OBJ 10.0
f 0.686
Fno. 4.45
ω 69.6
IH 0.975

Numerical Example 8
Unit mm

Surface data Surface number r d ne ν d
1 -11.6179 0.3800 1.53336 56.00
2 * 0.3436 0.4385 1.
3 * 1.1116 0.9207 1.64117 23.90
4 -2.6054 0.0400 1.
5 (Aperture) ∞ 0.0940 1.
6 3.1350 0.8000 1.53336 56.00
7 * -0.9314 1.4907 1.
Image plane ∞

Second surface of aspherical data
k = -0.7735
Third side
k = 0.
A2 = 0.000E + 00, A4 = -1.4315E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.2902E-01, A6 = 5.2510E-01

Various data OBJ 10.0
f 0.734
Fno. 4.56
ω 69.5
IH 0.975

Numerical Example 9
Unit mm

Surface data Surface number r d ne ν d
1 20.1287 0.3800 1.53336 56.00
2 * 0.3992 0.7593 1.
3 * 1.3349 1.0270 1.64117 23.90
4 -10.6598 0.0400 1.
5 (Aperture) ∞ 0.0940 1.
6 1.9296 0.8000 1.53336 56.00
7 * -0.8065 1.1953 1.
Image plane ∞

Second surface of aspherical data
k = -0.9226
A2 = 0.000E + 00, A4 = 7.4369E-02
Third side
k = 0.
A2 = 0.000E + 00, A4 = -2.7080E-01
7th page
k = 0.
A2 = 0.000E + 00, A4 = 3.5316E-01, A6 = 1.020E + 00

Various data OBJ 10.0
f 0.576
Fno. 3.83
ω 69.9
IH 0.975

The values of the conditional expressions in each embodiment are listed below.
Conditional expression Example 1 Example 2 Example 3
(2) f / f1 -0.965 -0.921 -0.882
(3) DZ2 / f 0.60 0.62 0.65
(4) DZ2 / R2 1.07 1.06 1.07
(5) D2 / DY2 1.27 1.34 0.90
(6) f / f2 0.482 0.447 0.390
(7) f2 / f1 -2.003 -2.062 -2.261

Conditional expression Example 4 Example 5 Example 6
(2) f / f1 -0.875 -0.844 -0.899
(3) DZ2 / f 0.72 0.82 0.56
(4) DZ2 / R2 1.20 1.32 0.95
(5) D2 / DY2 0.88 0.98 0.82
(6) f / f2 0.376 0.376 0.354
(7) f2 / f1 -2.328 -2.241 -2.542

Conditional expression Example 7 Example 8 Example 9
(2) f / f1 -0.896 -1.186 -0.750
(3) DZ2 / f 0.51 0.38 0.88
(4) DZ2 / R2 0.84 0.81 1.27
(5) D2 / DY2 0.69 1.05 1.23
(6) f / f2 0.362 0.545 0.301
(7) f2 / f1 -2.474 -2.174 -2.49

FIG. 11 is a diagram showing an optical device of this embodiment. The optical device 1 is an image pickup device or an endoscope. The optical device 1 has an objective optical system OBJ.

The image sensor IS can be used in the optical device 1. In this case, the optical image formed by the objective optical system OBJ can be imaged by the image sensor IS. As a result, the image can be acquired.

In the imaging device, for example, the objective optical system OBJ can be used for a digital camera, a surveillance camera, and a mobile terminal. In the endoscope, the objective optical system OBJ can be used for the flexible endoscope, the rigid endoscope, and the capsule endoscope.

In the objective optical system OBJ, chromatic aberration of magnification is sufficiently corrected. Therefore, according to the optical device of the present embodiment, a clear image can be acquired. That is, a clear image can be acquired with an imaging device or an endoscope.

As described above, the present invention is suitable for an objective optical system that is compact but has sufficiently corrected chromatic aberration of magnification. It is also suitable for imaging devices and endoscopes that can acquire clear images.

1 Optical device L1 1st lens L2 2nd lens L3 3rd lens S Aperture aperture I Image plane IM Optical image OBJ Objective optical system IS Imaging element

Claims

From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens.
The first lens, the second lens, and the third lens are made of a synthetic resin material.
An objective optical system characterized by satisfying the following conditional expressions (1), (2), and (3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is a point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
When the second intersection is located closer to the image than the first intersection, the distance value is a positive value.
Is.
The image side surface of the first lens is an aspherical surface.
The objective optical system according to claim 1, wherein the objective optical system satisfies the following conditional expression (4).
0.8 <DZ2 / R2 <1.4 (4)
here,
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
R2 is the radius of curvature of the image side surface of the first lens.
Is.
The objective optical system according to claim 1, wherein the objective optical system satisfies the following conditional expression (5).
0.6 <D2 / DY2 <1.35 (5)
here,
D2 is the distance between the image side surface of the first lens and the object side surface of the second lens on the optical axis.
DY2 is the shortest distance from the optical axis to the second intersection.
Is.
The objective optical system according to claim 1, wherein the objective optical system satisfies the following conditional expression (6).
0.25 <f / f2 <0.6 (6)
here,
f is the focal length of the objective optical system,
f2 is the focal length of the second lens,
Is.
The objective optical system according to claim 4, wherein the objective optical system satisfies the following conditional expression (7).
-3.0 <f2 / f1 <-1.9 (7)
here,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
Is.
The objective optical system according to claim 1, wherein the following conditional expressions (8) and (9) are satisfied.
50 <νd1 (8)
νd2 <35 (9)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
Is.
From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens.
The first lens, the second lens, and the third lens are made of a synthetic resin material.
An imaging device including an objective optical system that satisfies the following conditional expressions (1), (2), and (3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is a point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
When the second intersection is located closer to the image than the first intersection, the distance value is a positive value.
Is.
The image side surface of the first lens is an aspherical surface.
The imaging device according to claim 7, wherein the imaging device satisfies the following conditional expression (4).
0.8 <DZ2 / R2 <1.4 (4)
here,
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
R2 is the radius of curvature of the image side surface of the first lens.
Is.
The imaging device according to claim 7, wherein the imaging device satisfies the following conditional expression (5).
0.6 <D2 / DY2 <1.35 (5)
here,
D2 is the distance between the image side surface of the first lens and the object side surface of the second lens on the optical axis.
DY2 is the shortest distance from the optical axis to the second intersection.
Is.
The imaging device according to claim 7, wherein the imaging device satisfies the following conditional expression (6).
0.25 <f / f2 <0.6 (6)
here,
f is the focal length of the objective optical system,
f2 is the focal length of the second lens,
Is.
The imaging device according to claim 10, wherein the imaging device satisfies the following conditional expression (7).
-3.0 <f2 / f1 <-1.9 (7)
here,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
Is.
The imaging device according to claim 7, wherein the following conditional expressions (8) and (9) are satisfied.
50 <νd1 (8)
νd2 <35 (9)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
Is.
From the object side to the image side, in order
The first lens with negative refractive power and
A second lens with positive refractive power and
Aperture aperture and
It consists of a third lens with positive refractive power.
The second lens is a biconvex lens.
The first lens, the second lens, and the third lens are made of a synthetic resin material.
An endoscope comprising an objective optical system satisfying the following conditional expressions (1), (2), and (3).
νd2 <νd1 (1)
-1.2 <f / f1 <-0.74 (2)
0.3 <DZ2 / f <0.9 (3)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
f is the focal length of the objective optical system,
f1 is the focal length of the first lens,
The first intersection is a point where the image side surface of the first lens intersects the optical axis.
The second intersection is the point where the main ray with the maximum image height intersects the image side surface of the first lens.
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
When the second intersection is located closer to the image than the first intersection, the distance value is a positive value.
Is.
The image side surface of the first lens is an aspherical surface.
The endoscope according to claim 13, wherein the endoscope satisfies the following conditional expression (4).
0.8 <DZ2 / R2 <1.4 (4)
here,
DZ2 is the distance parallel to the optical axis from the first intersection to the second intersection.
R2 is the radius of curvature of the image side surface of the first lens.
Is.
The endoscope according to claim 13, wherein the endoscope satisfies the following conditional expression (5).
0.6 <D2 / DY2 <1.35 (5)
here,
D2 is the distance between the image side surface of the first lens and the object side surface of the second lens on the optical axis.
DY2 is the shortest distance from the optical axis to the second intersection.
Is.
The endoscope according to claim 13, wherein the endoscope satisfies the following conditional expression (6).
0.25 <f / f2 <0.6 (6)
here,
f is the focal length of the objective optical system,
f2 is the focal length of the second lens,
Is.
The endoscope according to claim 16, wherein the endoscope satisfies the following conditional expression (7).
-3.0 <f2 / f1 <-1.9 (7)
here,
f1 is the focal length of the first lens,
f2 is the focal length of the second lens,
Is.
The endoscope according to claim 13, wherein the endoscope satisfies the following conditional expressions (8) and (9).
50 <νd1 (8)
νd2 <35 (9)
here,
νd1 is the Abbe number of the first lens.
νd2 is the Abbe number of the second lens.
Is.
The endoscope according to claim 13, wherein the endoscope is a capsule endoscope.