JP2022052311A - Imaging optical system, and imaging apparatus with the same - Google Patents

Abstract

To provide an imaging optical system in which an optical system has a short overall length and has a wide depth of field.SOLUTION: An imaging optical system comprises, in order from the object side, a first lens group having negative refractive power, a second lens group and a third lens group each having positive refractive power, the second lens group has a diaphragm and an optical surface performing phase modulation. The optical surface is arranged at a position that matches the diaphragm or in the vicinity of the diaphragm, change occurs in the phase of wavefront by phase modulation, and the size of a permissible circle of confusion when the change occurs is larger than the size of a permissible circle of confusion when the change does not occur. The imaging optical system satisfies conditional expression (1): D2×cosα<D1.SELECTED DRAWING: Figure 1

Description

The present invention relates to an imaging optical system and an imaging device including the imaging optical system.

Depth of field is known as a physical quantity that expresses the characteristics of an optical system. Depth of field is the range in which an image of an object can be clearly obtained when the object is focused.

Depth of field is represented by the distance between two points in object space. The two points are located on the optical axis.

If one of the two points is the perigee and the other is the perigee, the perigee is closer to the optical system than the perigee. The position of the focused object (hereinafter referred to as "focus position") is included between the near point and the far point.

The depth of field in a normal optical system will be described. The ordinary optical system is an optical system in which aberrations are well corrected. Hereinafter, the depth of field in the normal optical system is referred to as "normal depth". Further, an optical image corresponding to an object point located within a normal depth is referred to as a "normal point image".

The first state is a state in which the position of the object point coincides with the in-focus position. The second state is a state in which the position of the object point coincides with the position of the apogee. The third state is a state in which the position of the object point coincides with the position of the near point.

The point image N1 is a normal point image in the first state. The point image N2 is a normal point image in the second state. The point image N3 is a normal point image in the third state.

The sharpness of the normal point image is the highest in the point image N1. Therefore, the sharpness of the point image N2 and the sharpness of the point image N3 are lower than the sharpness of the point image N1.

However, there is no difference in size between the sharpness of the point image N2 and the sharpness of the point image N1. There is no big difference between the sharpness of the point image N3 and the sharpness of the point image N1. Therefore, the sharpness of the point image N2 and the sharpness of the point image N3 can be regarded as substantially the same as the sharpness of the point image N1.

When the object point is located within the normal depth, the sharpness of the normal point image is the same as or almost the same as the sharpness of the point image N1 regardless of the position of the object point. On the other hand, when the object point is not located within the normal depth, the sharpness of the normal point image is lower than the sharpness of the point image N1.

The surface of an object (hereinafter referred to as "surface OB") is formed of a plurality of object points. When the surface OB is non-planar and is located in a range wider than the normal depth, the sharpness of the optical image of the object point depends on the position of the object point.

On the surface of an object normally located within the depth (hereinafter referred to as "surface OBin"), the optical image of each object point has the same sharpness as the sharpness of the point image N1 (hereinafter referred to as "sharpness H"). It is formed. Therefore, the optical image of the surface OBin has a sharpness H. The sharpness H includes substantially the same sharpness as the sharpness of the point image N1.

On the other hand, on the surface of an object that is not normally located within the depth (hereinafter referred to as "surface OBoff"), the sharpness is lower than the sharpness of the point image N1 (hereinafter referred to as "sharpness L"). An optical image of the object point is formed. Therefore, the optical image of the surface OBoff has a sharpness L.

By capturing the optical image of the surface OB with the image sensor, the image of the surface OB can be acquired. When the surface OB is located in a range wider than the normal depth, the image acquired by the normal optical system includes an image of the surface OBin and an image of the surface OBoff.

The image of the surface OBin is an image having a sharpness H. When the clear image is an image having sharpness H, the image of the surface OBin is a clear image.

The image of the surface OB off is an image having a sharpness L. If the unclear image is an image having sharpness L, the image of the surface OBoff is an unclear image.

As described above, when the surface OB is located in a range wider than the normal depth, the image acquired by the normal optical system includes a clear image and an unclear image.

In one image, it is preferable that the difference in sharpness is as small as possible. To reduce the difference in sharpness, increase the depth of field.

The depth of field is expressed by, for example, the following equation.
DOF = DOFf + DOFn
DOFf = (H × D) / (HD)
DOFn = (H × D) / (H + D)
H = f ² / (F × c)
here,
DOF is the depth of field,
DOFf is the depth of field on the apogee side.
DOFn is the depth of field on the near point side,
H is the hyperfocal length,
D is the distance from the optical system to the in-focus position,
f is the focal length of the optical system,
F is the F number of the optical system,
c is the diameter of the permissible circle of confusion,
Is.

EDoF (Extended of Depth of Field) is known as a method for increasing the depth of field. In an optical system using EDoF (hereinafter referred to as “EDoF optical system”), for example, aberration is intentionally added.

The formula for depth of field includes the diameter of the permissible circle of confusion. The size of the point image N1 can be regarded as the diameter of the permissible circle of confusion. Therefore, the depth of field can be changed by changing the size of the point image N1.

When the sharpness of the point image N1 changes, the size of the point image N1 changes. In addition of aberration, spherical aberration is added so that the sharpness of the point image N1 is reduced.

When the sharpness of the point image N1 decreases, the size of the point image N1 increases. As the size of the point image N1 increases, the diameter of the permissible circle of confusion increases. Therefore, the depth of field can be widened.

In the EDoF optical system, aberrations are added so that the magnitude of the point image N1 becomes large. As a result, the EDoF optical system has a wider depth of field than normal depth. Hereinafter, the depth of field in the EDoF optical system is referred to as "extended depth". Further, an optical image corresponding to an object point located within the extended depth is referred to as an "extended point image".

The point image E1 is an extended point image in the first state. The point image E2 is an extended point image in the second state. The point image E3 is an extended point image in the third state.

The sharpness of the extended point image is the highest in the point image E1. Therefore, the sharpness of the point image E2 and the sharpness of the point image E3 are lower than the sharpness of the point image E1.

However, there is no difference in size between the sharpness of the point image E2 and the sharpness of the point image E1. There is no size difference between the sharpness of the point image E3 and the sharpness of the point image E1. Therefore, the sharpness of the point image E2 and the sharpness of the point image E3 can be regarded as substantially the same as the sharpness of the point image E1.

When the object point is located within the extension depth, the sharpness of the extended point image is the same as or almost the same as the sharpness of the point image E1 regardless of the position of the object point. On the other hand, when the object point is not located within the extended depth, the sharpness of the extended point image is lower than the sharpness of the point image E1.

The case where the object is located in a range wider than the normal depth will be described.

The extended depth is wider than the normal depth. Therefore, even if the surface OBoff is located, if the surface OBoff is located within the expansion depth, an optical image of the surface OBoff is formed with the same sharpness as the point image E1.

However, the size of the point image E1 is larger than the size of the point image N1. Therefore, the sharpness of the point image E1 is lower than the sharpness of the point image N1. As a result, in the image acquired by the EDoF optical system, the image of the surface OBin and the image of the surface OBoff become unclear images.

Blurred images are out-of-focus images. Therefore, the unclear image cannot be used for observation and diagnosis, for example. However, a clear image can be obtained by performing a recovery process on an unclear image.

The following relationship holds in the image formation by the optical system.
I (x, y) = O (x, y) * PSF (x, y) (A)
here,
O (x, y) is an object,
I (x, y) is an optical image of an object,
PSF (x, y) is the point spread function,
* Is an operator that represents convolution,
Is.

As can be seen from the equation (A), O (x, y) can be obtained from I (x, y) and PSF (x, y). In the recovery process, deconvolution is performed using I (x, y) and PSF (x, y). As a result, O (x, y) can be obtained.

Equation (A) expresses the relationship between an object and an optical image of the object. Therefore, when an object is located in a wide range in the optical axis direction, PSF (x, y) must be obtained for each position of the object in the optical axis direction (hereinafter referred to as "object position").

PSF (x, y) represents the intensity distribution of the point image. The intensity distribution of the point image can be obtained based on the pupil function of the optical system. Therefore, the PSF (x, y) can be obtained from the pupil function of the optical system.

However, the intensity distribution of the point image differs depending on the position of the object. Therefore, PSF (x, y) must be obtained for each object position. However, it is very difficult to obtain PSF (x, y) for each object position.

As described above, in the EDoF optical system, the sharpness of the point image E2 and the sharpness of the point image E3 are almost the same as the sharpness of the point image E1. Therefore, when the object point is located within the expansion depth, the intensity distribution of the expansion point image can be regarded as the same regardless of the object position.

When the intensity distribution of the extended point image is the same at all the object positions, the intensity distribution of the extended point image at all the object positions can be represented by one PSF (x, y). Therefore, in the EDoF optical system, it is not necessary to obtain PSF (x, y) for each object position.

In the EDoF optical system, the number of PSFs (x, y) is one. Since one PSF (x, y) may be used in the recovery process, O (x, y) can be easily obtained.

Patent Document 1 discloses an imaging optical system having a wide depth of field. In this imaging optical system, spherical aberration is intentionally added to the optical system in order to increase the depth of field. Spherical aberration is added by the phase plate.

This imaging optical system satisfies the following conditions.
A / 10 ≧ B
here,
A is the amount of spherical aberration after the addition of spherical aberration,
B is the maximum value of curvature of field of the imaging optical system before spherical aberration is added.
Is.

By satisfying this condition, the influence of curvature of field on the expansion of the depth of field can be substantially eliminated.

Japanese Unexamined Patent Publication No. 2015-4883

In the imaging optical system of Patent Document 1, the curvature of field is reduced in the imaging optical system before spherical aberration is added. In order to reduce the curvature of field, for example, the number of lenses used in the imaging optical system may be increased. However, if the number of lenses is increased, the total length of the optical system becomes long.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an imaging optical system having a short overall length and a wide depth of field. Another object of the present invention is to provide an image pickup apparatus capable of easily generating a clear image having a wide depth of field.

In order to solve the above-mentioned problems and achieve the object, the imaging optical system according to at least some embodiments of the present invention is used.
From the object side,
The first lens group with negative refractive power,
The second lens group and
With a third lens group having a positive refractive power,
The second lens group has an aperture and an optical surface for phase modulation.
The optical surface is placed at a position that coincides with the aperture or near the aperture.
Phase modulation causes a change in the phase of the wavefront,
The size of the permissible circle of confusion when a change occurs is larger than the size of the permissible circle of confusion when no change occurs.
It is characterized by satisfying the following conditional expression (1).
D2 × cosα <D1 (1)
here,
α is the half angle of view of the imaging optical system,
D1 is the diameter of the axial luminous flux on the first lens surface,
D2 is the diameter of a predetermined off-axis luminous flux on the first lens surface,
The first lens surface is a lens surface located closest to the object in the first lens group.
A predetermined off-axis luminous flux is a luminous flux defined by a half angle of view,
Is.

Further, the image pickup apparatus of the present invention is
Optical system and
It has an image pickup surface and has an image pickup element that converts an image formed on the image pickup surface by an optical system into an electric signal.
The optical system is an imaging optical system.

According to the present invention, it is possible to provide an imaging optical system having a short overall length and a wide depth of field. Further, it is possible to provide an image pickup apparatus capable of easily generating a clear image having a wide depth of field.

It is a lens sectional view of the image pickup optical system of this embodiment. It is a figure which shows the light flux incident on the 1st lens surface. It is a lens sectional view of the image pickup optical system of Example 1. FIG. It is a figure which shows the MTF of the image pickup optical system of Example 1. FIG. It is a figure which shows the Example of the image pickup apparatus.

Prior to the description of the examples, the effects of the embodiments according to certain aspects of the present invention will be described. In addition, when concretely explaining the action and effect of this embodiment, a concrete example will be shown and explained. However, as in the case of the examples described later, those exemplary embodiments are merely a part of the embodiments included in the present invention, and there are many variations in the embodiments. Therefore, the present invention is not limited to the exemplary embodiments.

The image pickup optical system of the present embodiment has a first lens group having a negative refractive force, a second lens group, and a third lens group having a positive refractive force in order from the object side, and has a second lens group. The lens group has an aperture and an optical surface for phase modulation, and the optical surface is arranged at a position corresponding to the aperture or in the vicinity of the aperture, and the phase modulation causes a change in the phase of the wave surface. The size of the permissible circle of confusion when a change occurs is larger than the size of the permissible circle of confusion when no change occurs, and the following conditional expression (1) is satisfied.
D2 × cosα <D1 (1)
here,
α is the half angle of view of the imaging optical system,
D1 is the diameter of the axial luminous flux on the first lens surface,
D2 is the diameter of a predetermined off-axis luminous flux on the first lens surface,
The first lens surface is a lens surface located closest to the object in the first lens group.
A predetermined off-axis luminous flux is a luminous flux defined by a half angle of view,
Is.

FIG. 1 is a cross-sectional view of a lens of the imaging optical system of the present embodiment. The image pickup optical system OBJ has a first lens group G1, a second lens group G2, and a third lens group G3.

The first lens group G1 has a negative refractive power. The second lens group G2 has a positive refractive power. However, the refractive power of the second lens group G2 is not limited to the positive refractive power. The third lens group G3 has a positive refractive power.

The first lens group G1, the second lens group G2, and the third lens group G3 can be formed, for example, as follows.

The first lens group G1 can have a negative lens L1. The second lens group G2 can have a positive lens L2, a phase modulation element PM, and a diaphragm S. The third lens group G3 can have a positive lens L3, a positive lens L4, a negative lens L5, and a positive lens L6.

In the image pickup optical system OBJ, the negative lens L1 is arranged on the object side most. Therefore, a wide angle of view can be secured.

An optical image is formed on the image plane of the imaging optical system OBJ. The optical image can be imaged using, for example, an image pickup device. The image pickup device has a cover glass. A long back focus is required to allow placement of the cover glass.

An optical system having a wide angle of view has a short focal length. In an optical system with a short focal length, the back focus is short. Since the imaging optical system OBJ has a wide angle of view, the back focus is short. Therefore, it is necessary to secure a long back focus while ensuring a wide angle of view.

A retrofocus type optical system is known as an optical system that can secure a long back focus. In the retrofocus type optical system, the arrangement of the refractive powers is a negative refractive power and a positive refractive power.

In the image pickup optical system OBJ, the arrangement of the refractive powers of the first lens group G1 and the third lens group G3 is a negative refractive power and a positive refractive power. In this case, it can be considered that the image pickup optical system OBJ has a retrofocus type optical system. Therefore, the back focus can be sufficiently secured. As a result, the cover glass can be arranged on the object side of the image plane. Further, the optical filter can be arranged on the object side of the image plane.

In the second lens group G2, phase modulation is performed. Phase modulation is, for example, changing the phase of the wavefront. When the phase of the wavefront changes, the aberration changes. Therefore, phase modulation can be regarded as the addition of aberrations.

Phase modulation can be done on the optical surface. In order to perform phase modulation, an optical surface for performing phase modulation (hereinafter referred to as "phase modulation surface") is arranged in the second lens group G2.

The second lens group G2 has a diaphragm S and a phase modulation surface PMS. The phase modulation surface PMS is provided on the image side surface of the phase modulation element PM. The object side surface of the phase modulation element PM is a plane.

The diaphragm S is arranged at the pupil position of the imaging optical system OBJ. However, the diaphragm S may be arranged in the vicinity of the pupil position. In this case, the aperture S can be regarded as an aperture stop.

The phase modulation surface PMS is arranged at a position corresponding to the aperture S or in the vicinity of the aperture S. As described above, the diaphragm S is arranged at the pupil position of the imaging optical system OBJ. Therefore, the same phase modulation can be performed for both the on-axis luminous flux and the off-axis luminous flux.

In the imaging optical system OBJ, the phase of the wavefront changes due to phase modulation. When the phase of the wavefront changes, the phase of the wavefront ejected from the phase modulation plane PMS is different from the phase of the wavefront incident on the phase modulation plane PMS.

As the phase of the wavefront changes, the size of the permissible circle of confusion changes. The size of the permissible circle of confusion when a change occurs is larger than the size of the permissible circle of confusion when no change occurs.

As the size of the permissible circle of confusion increases, the change in the size of the permissible circle of confusion (hereinafter referred to as "predetermined change") with respect to the change in the object distance becomes smaller. A small change means that the depth of field is wide. Therefore, the depth of field when the phase of the wavefront changes is wider than the depth of field when the phase of the wavefront does not change.

When there is no change in the phase of the wavefront, it is assumed that the aberration is well corrected. An optical system in which aberrations are well corrected is an ordinary optical system. In the imaging optical system OBJ, the phase of the wavefront is changed. Therefore, the depth of field in the imaging optical system OBJ is wider than the depth of field in the normal optical system.

The depth of field in a normal optical system is a normal depth. The imaging optical system OBJ has a depth of field wider than the normal depth. As mentioned above, the EDoF optical system has a wider depth of field than normal depth. Therefore, the image pickup optical system OBJ is an EDoF optical system.

The image pickup optical system of the present embodiment is an EDoF optical system and satisfies the conditional expression (1). The conditional expression (1) is a conditional expression relating to the diameter of the on-axis luminous flux and the diameter of the predetermined off-axis luminous flux. The diameter of the on-axis luminous flux and the diameter of a predetermined off-axis luminous flux will be described.

FIG. 2 is a diagram showing a luminous flux incident on the first lens surface. FIG. 2 illustrates the negative lens L1, the luminous flux Lon, and the luminous flux Loff.

The negative lens L1 has a first lens surface S1. In the first lens group G1, the negative lens L1 is arranged on the object side most. Therefore, the first lens surface S1 is the lens surface located closest to the object in the first lens group G1.

Luminous flux is incident on the first lens surface S1 at various angles. The luminous flux incident on the first lens surface S1 includes an on-axis luminous flux and an off-axis luminous flux. In the on-axis luminous flux, the central ray coincides with the optical axis AX. In the off-axis luminous flux, the central ray intersects the optical axis AX.

In the luminous flux Lon, the central ray coincides with the optical axis AX. Therefore, the luminous flux Lon is an axial luminous flux.

In the luminous flux Loff, the main ray CRmax intersects the optical axis AX. Therefore, the luminous flux Loff represents an off-axis luminous flux.

In the luminous flux Loff, the angle formed by the main ray CRmax and the optical axis AX is α. Assuming that the angle of view of the imaging optical system OBJ is ω, α = ω / 2. Therefore, α represents the half angle of view of the imaging optical system OBJ. The angle of view is an angle representing the maximum range of the object space that can be imaged by the optical system.

The luminous flux when the angle between the main ray and the optical axis is a half angle of view is defined as the luminous flux defined by the half angle of view. Further, the luminous flux defined by the half angle of view is defined as a predetermined off-axis luminous flux.

In the luminous flux Loff, the angle formed by the main ray CRmax and the optical axis AX is a half angle of view. Therefore, the luminous flux Loff is a luminous flux defined by a half angle of view. Since the luminous flux defined by the half angle of view is a predetermined off-axis luminous flux, the luminous flux Loff represents a predetermined off-axis luminous flux.

The luminous flux Lon and the luminous flux Loff represent the luminous flux when the object point is located at infinity. Since the object point is located at infinity, both the luminous flux Lon and the luminous flux Loff are parallel fluxes. The luminous flux Lon and the luminous flux Loff are incident on the first lens surface S1.

The diameter D1 is the diameter of the luminous flux Lon in the first lens surface S1. Since the luminous flux Lon is an axial luminous flux, the diameter D1 is the diameter of the axial luminous flux in the first lens surface S1. The diameter D2 is the diameter of the luminous flux Loff on the first lens surface S1. Since the luminous flux Loff is a predetermined off-axis luminous flux, the diameter D2 is the diameter of the predetermined off-axis luminous flux on the first lens surface S1.

The diameter of the luminous flux Lon is the diameter of the luminous flux in a plane orthogonal to the central ray. In the luminous flux Lon, the plane orthogonal to the central ray is parallel to the first lens surface S1. Therefore, the diameter of the luminous flux Lon is equal to the diameter D1.

The diameter of the luminous flux Loff is the diameter of the luminous flux in a plane orthogonal to the main ray CRmax. In the luminous flux Loff, the plane orthogonal to the main ray CRmax is non-parallel to the first lens surface S1. Therefore, the diameter of the luminous flux Loff is different from the diameter D2. The diameter of the luminous flux Loff is D2 × cosα.

The relationship between the diameter of the luminous flux and the amount of aberration generated will be described.

The image pickup optical system of the present embodiment can be regarded as an optical system in which a phase modulation surface is arranged on a normal optical system. As mentioned above, phase modulation can be seen as the addition of aberrations. Therefore, the aberrations generated in the imaging optical system of the present embodiment include the aberrations generated in the normal optical system (hereinafter referred to as "basic aberrations") and the aberrations generated in the phase modulation surface (hereinafter referred to as "additional aberrations"). And, are included.

The depth of field varies depending on the size of the permissible circle of confusion. In order to change the depth of field by phase modulation, the size of the permissible circle of confusion may change mainly due to additional aberrations. In order to change the size of the permissible circle of confusion by the additional aberration, it is sufficient that the aberration amount of the fundamental aberration (hereinafter referred to as "basic aberration amount") is very small as compared with the aberration amount of the additional aberration.

The basic aberration amount is determined by the aberration amount of on-axis aberration (hereinafter referred to as "on-axis aberration amount") and the aberration amount of off-axis aberration (hereinafter referred to as "off-axis aberration amount").

When the value of the F number of the off-axis luminous flux is the same as the value of the F number of the off-axis luminous flux, the amount of off-axis aberration is generally larger than the amount of on-axis aberration. In order to reduce the amount of fundamental aberration, it is necessary to reduce the amount of off-axis aberration. Normally, in order to reduce the amount of off-axis aberration in an optical system, it is necessary to use many lenses.

When the conditional expression (1) is satisfied, the diameter of the predetermined off-axis luminous flux is smaller than the diameter of the on-axis luminous flux. Therefore, the value of the F number in the predetermined off-axis luminous flux is larger than the value of the F number in the on-axis luminous flux.

The depth of field increases as the F-number value increases. Therefore, in the imaging optical system of the present embodiment, the depth of field in the predetermined off-axis luminous flux is wider than the depth of field in the on-axis luminous flux.

The amount of aberration decreases as the value of the F number increases. Therefore, in the imaging optical system of the present embodiment, the amount of off-axis aberration is smaller than that in the case where the value of the F number of the off-axis luminous flux is the same as the value of the F number of the off-axis luminous flux.

As described above, in order to reduce the amount of off-axis aberration, it is usually necessary to use many lenses in an optical system. However, in the imaging optical system of the present embodiment, the amount of off-axis aberration is small. Therefore, it is not usually necessary to use many lenses in the optical system. Therefore, the total length of the optical system can be shortened.

Further, the size of the permissible circle of confusion increases as the value of the F number increases. Therefore, in the imaging optical system of the present embodiment, the size of the permissible circle of confusion in the off-axis luminous flux is larger than that in the case where the value of the F number of the off-axis luminous flux is the same as the value of the F number of the off-axis luminous flux.

The larger the size of the permissible circle of confusion, the smaller the influence of the amount of off-axis aberration on the permissible circle of confusion. Therefore, in the imaging optical system of the present embodiment, a larger off-axis aberration is allowed to occur in the normal optical system as compared with the case where the F number value of the off-axis luminous flux is the same as the F number value of the off-axis luminous flux. be able to. As a result, for example, a wide depth of field can be obtained even if the curvature of field is large.

Further, in the manufacture of an optical system, for example, processing of a lens and assembly of an optical system are performed. Processing errors may occur in the processing of lenses, and assembly errors may occur in the assembly of optical systems. When these errors occur, for example, a phenomenon called one-sided blur occurs. When one-sided blur occurs, in the acquired image, the peripheral portion on the right side is clear, but the peripheral portion on the left side becomes unclear.

The PSF (x, y) when one-sided blur occurs is different from the PSF (x, y) at the time of design. Therefore, even if the recovery process is performed using the PSF (x, y) at the time of design, it is not possible to clearly acquire an image having a wide depth of field.

In the image pickup optical system of the present embodiment, it is possible to tolerate the generation of larger off-axis aberrations. If one-sided blur is regarded as an off-axis aberration, the influence of one-sided blur can be minimized in the imaging optical system of the present embodiment. Therefore, even if the acquired image is out of focus, an image with a wide depth of field can be clearly acquired by performing the recovery process using the PSF (x, y) at the time of design.

When the object point is not located at infinity, the luminous flux Lon and the luminous flux Loff are both divergent light. In this case, the comparison between the luminous flux Lon and the luminous flux Loff can be performed by the numerical aperture on the object side. When the conditional expression (1) is satisfied, the value of the numerical aperture on the object side in the predetermined off-axis luminous flux is smaller than the value of the numerical aperture on the object side in the on-axis luminous flux.

The depth of field increases as the numerical aperture of the object side decreases. Therefore, in the imaging optical system of the present embodiment, the depth of field in the predetermined off-axis luminous flux is wider than the depth of field in the on-axis luminous flux.

Therefore, regardless of whether the object point is located at infinity or the object point is not located at infinity, in the imaging optical system of the present embodiment, the depth of field at a predetermined off-axis luminous flux is on the axis. Wider than the depth of field in luminous flux.

In the phase modulation element PM, the side surface of the object is a plane. However, the side surface of the object can be spherical. By making the side surface of the object spherical, the side surface of the object can be used for aberration correction of a normal optical system.

The image side surface of the phase modulation element PM may be used for aberration correction of a normal optical system. In this case, on the image side of the phase modulation element PM, phase modulation and aberration correction of a normal optical system are performed. Further, phase modulation may be added to the lens surface of a lens that normally forms an optical system.

The imaging optical system of the present embodiment preferably satisfies the following conditional expression (2).
D1 <D2 (2)
here,
D1 is the diameter of the axial luminous flux on the first lens surface,
D2 is the diameter of a predetermined off-axis luminous flux on the first lens surface,
The first lens surface is a lens surface located closest to the object in the first lens group.
A predetermined off-axis luminous flux is a luminous flux defined by a half angle of view,
Is.

By satisfying the conditional expression (2), it is possible to prevent a decrease in the resolving power around the visual field. As a result, according to the image pickup optical system of the present embodiment, substantially the same resolution can be maintained from the center of the field of view to the periphery.

The imaging optical system of the present embodiment preferably satisfies the following conditional expression (3).
40 ° ≤ α (3)
here,
α is the half angle of view of the imaging optical system,
Is.

A wide field of view can be obtained by satisfying the conditional expression (3).

The image pickup optical system of this embodiment can be used for an image pickup device, for example, an objective optical system of an endoscope. In an endoscope, the objective optical system is inserted, for example, into a body cavity. According to the imaging optical system of the present embodiment, a wide range in the body cavity can be clearly observed.

The imaging optical system of the present embodiment has a predetermined spatial frequency band satisfying the following conditional expression (4) at all object positions, and the maximum spatial frequency of the predetermined spatial frequency band is in the vicinity of the Nyquist frequency. Spatial frequency is preferred.
0.08 ≤ VMTF (4)
here,
VMTF is the value of MTF when the object height is zero,
The object position is any position between perigee and perigee,
The near point is the point located closest to the imaging optical system in the depth of field of the imaging optical system.
The far point is the point located farthest from the imaging optical system in the depth of field,
Is.

According to the image pickup optical system of the present embodiment, high resolution can be maintained from the center to the periphery of the field of view.

In the imaging optical system of the present embodiment, it is preferable that spherical aberration is added by phase modulation.

According to the image pickup optical system of the present embodiment, a wide depth of field can be obtained.

In the imaging optical system of the present embodiment, the spherical aberration increases in the negative direction as the F number value decreases, and the negative direction is the direction from the near-axis image plane toward the imaging optical system, and the aberration curve of the spherical aberration. Preferably have a turning point.

According to the image pickup optical system of the present embodiment, a wide depth of field can be obtained.

In the imaging optical system of the present embodiment, the first lens surface is preferably a convex surface or a flat surface.

The first lens surface faces an object. Therefore, the first lens surface may come into contact with an object. By making the first lens surface convex or flat, the impact at the time of contact can be alleviated.

Further, the image pickup optical system of the present embodiment can be used as an objective optical system of an endoscope. By making the first lens surface convex or flat, dirt on the first lens surface can be easily removed.

When the first lens surface is a convex surface, the convex surface is preferably a smooth surface. Further, it is preferable that the radius of curvature of the convex surface is large. By increasing the radius of curvature of the convex surface, the amount of protrusion toward the object can be reduced.

In the imaging optical system of the present embodiment, it is preferable that the MTF of the light beam having an object height of 0 is 8% or more in the frequency band from 0 to the vicinity of the Nyquist frequency and in the region from the short-range object to the long-range object.

According to the image pickup optical system of the present embodiment, high resolution can be maintained from the center to the periphery of the field of view.

The image pickup apparatus of the present embodiment includes an optical system and an image pickup element having an image pickup surface and converting an image formed on the image pickup surface by the optical system into an electric signal, and the optical system is the present embodiment. It is characterized by being an imaging optical system.

According to the image pickup apparatus of the present embodiment, it is possible to easily generate a clear image having a wide depth of field.

Hereinafter, examples of the imaging optical system will be described in detail with reference to the drawings. The present invention is not limited to this embodiment.

FIG. 3 is a cross-sectional view of the lens of the imaging optical system of the first embodiment. The first lens group is indicated by G1, the second lens group is indicated by G2, the third lens group is indicated by G3, the aperture is indicated by S, and the image plane (imaging plane) is indicated by I.

In the imaging optical system of the first embodiment, the first lens group G1 having a negative refractive power, the second lens group G2 having a positive refractive power, and the third lens group having a positive refractive power are arranged in this order from the object side. It has G3 and.

The first lens group G1 has a plano-concave negative lens L1 having a plane facing the object side.

The second lens group G2 includes a positive meniscus lens L2 having a convex surface facing the image side, and a phase modulation element PM. In the phase modulation element PM, the object side is a plane and the image side surface is an aspherical surface.

The third lens group G3 has a positive meniscus lens L3 with a convex surface facing the image side, a biconvex positive lens L4, a plano-concave negative lens L5, and a plano-convex positive lens L6 with a plane facing the image side. .. The biconvex positive lens L4 and the plano-concave negative lens L5 are joined.

The diaphragm S is arranged in the second lens group G2. More specifically, it is arranged on the image side surface of the phase modulation element PM.

FIG. 4 is a diagram showing an MTF of the imaging optical system of the first embodiment. FIG. 4A is an MTF when the object distance is 3 mm. FIG. 4B is an MTF when the object distance is 6.6 mm. FIG. 4C is an MTF when the object distance is 150 mm.

The solid line is the MTF in the on-axis luminous flux. The dashed line and the alternate long and short dash line are the MTFs at a given off-axis luminous flux. The dashed line is the MTF in the radial direction, and the alternate long and short dash line is the MTF in the tangential direction. The dotted line indicates the maximum value of MTF at the diffraction limit.

In the on-axis luminous flux, the position of the peak of the MTF curve changes greatly with the change of the object distance. On the other hand, even with a predetermined off-axis luminous flux, the position of the peak of the MTF curve changes as the object distance changes.

However, the change in the position of the peak in the predetermined off-axis luminous flux is smaller than the change in the position of the peak in the on-axis luminous flux. In particular, in the MTF in the tangier direction, the change in the peak position is very small.

This means that for a predetermined off-axis luminous flux, the effect of defocus when the object distance changes is smaller than that of the on-axis luminous flux. The fact that the effect of defocus is small means that the change in the size of the permissible circle of confusion is small.

As described above, when the change in the size of the permissible circle of confusion is small, the depth of field is wide. Therefore, in the imaging optical system of the present embodiment, the depth of field in the predetermined off-axis luminous flux is wider than the depth of field in the on-axis luminous flux.

Further, the peak value of the MTF curve is substantially the same for the on-axis luminous flux and the predetermined off-axis luminous flux. Therefore, in the image pickup optical system of this embodiment, substantially the same resolution can be maintained from the center to the periphery of the field of view.

The numerical data of Example 1 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, νd is the Abbe number of each lens, and * is an aspherical surface.

In various data, OB is the object distance, f is the focal length of the whole system, and FNO. Is the F number and α is the half angle of view.

The aspherical shape is as follows 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, A14 ... It is expressed by an expression.
z = (y ² / r) / [1 + {1- (1 + k) (y / r) ² } ^1/2 ]
＋ A4y ⁴ ＋ A6y ⁶ ＋ A8y ⁸ ＋ A10y ¹⁰ ＋ A12y ¹² ＋ A14y ¹⁴ ＋…
Further, in the aspherical coefficient, "E + n" (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 rd ne νd
1 ∞ 0.220 1.88815 40.76
2 0.6260 0.549
3 -3.1353 0.472 1.97188 17.47
4 -1.8636 0.033
5 ∞ 0.801 1.51825 64.14
6 * ∞ 0
7 (Aperture) ∞ 0.187
8 -7.8277 0.582 1.88815 40.76
9 -1.3859 0.088
10 2.0766 0.692 1.69979 55.53
11 -1.3376 0.297 1.97188 17.47
12 ∞ 0.362
13 1.6055 0.953 1.51825 64.14
14 ∞ 0
Image plane ∞

6th surface of aspherical data
k = 0.000
A4 = -2.7933E + 01, A6 = 3.0550E + 03, A8 = -1.5491E + 05,
A10 = 4.0813E + 06, A12 = -5.7054E + 07, A14 = 3.3699E + 08

Various data OB 6.6
f 0.5
FNO. 3.8 3.8
α 65

The values of the conditional expression in Example 1 are listed below.

Example 1
(1) D2 × cosα 0.063
(2) D1 <D2
D1 0.121
D2 0.148
(3) α 65

FIG. 5 is a diagram showing an embodiment of an image pickup apparatus. The image pickup apparatus 1 includes an image pickup optical system 2 and an image pickup element 3. For the image pickup optical system 2, for example, the image pickup optical system of Example 1 is used.
The image pickup optical system 2 forms an optical image of the object 4 on the image plane.

The image pickup surface of the image pickup element 3 is located on the image plane. The optical image is imaged by the image sensor 3. The optical image is converted into an electrical signal. As a result, the image of the object 4 is acquired.

The image pickup optical system 2 is an EDoF optical system. The image acquired by the EDoF optical system is an unclear image even if the object is located within the extended depth. Therefore, the image pickup apparatus 1 acquires an unclear image of the object 4.

The image pickup device 1 can be combined with the image processing device 5. The image of the object 4 is input to the image processing device 5. The image processing device 5 can perform recovery processing on the image of the object 4. As a result, a clear image of the object 4 can be obtained.

As described above, the present invention is suitable for an imaging optical system having a short overall length and a wide depth of field. Further, the present invention is suitable for an image pickup apparatus capable of easily generating a clear image with a wide depth of field.

G1 1st lens group G2 2nd lens group G3 3rd lens group PM Phase modulation element PMS Phase modulation surface S Aperture L1 Negative lens Lon, Loff Luminous S1 1st lens surface AX Optical axis CRmax Main ray α Half angle of view ω Angle of view D1, D2 Diameter I Image plane 1 Image pickup device 2 Image pickup optical system 3 Image pickup element 4 Object 5 Image processing device

Claims (7)

From the object side,
The first lens group with negative refractive power,
The second lens group and
With a third lens group having a positive refractive power,
The second lens group has an aperture and an optical surface for phase modulation.
The optical surface is arranged at a position corresponding to the diaphragm or in the vicinity of the diaphragm.
The phase modulation causes a change in the phase of the wavefront.
The size of the permissible circle of confusion when the change occurs is larger than the size of the permissible circle of confusion when the change does not occur.
An imaging optical system characterized by satisfying the following conditional expression (1).
D2 × cosα <D1 (1)
here,
α is the half angle of view of the imaging optical system.
D1 is the diameter of the axial luminous flux on the first lens surface,
D2 is the diameter of the predetermined off-axis luminous flux on the first lens surface,
The first lens surface is a lens surface located closest to the object in the first lens group.
The predetermined off-axis luminous flux is a luminous flux defined by the half angle of view.
Is. The imaging optical system according to claim 1, wherein the image pickup optical system is characterized by satisfying the following conditional expression (2).
D1 <D2 (2)
here,
D1 is the diameter of the axial luminous flux on the first lens surface,
D2 is the diameter of the predetermined off-axis luminous flux on the first lens surface,
The first lens surface is a lens surface located closest to the object in the first lens group.
The predetermined off-axis luminous flux is a luminous flux defined by the half angle of view.
Is. The imaging optical system according to claim 1, wherein the image pickup optical system is characterized by satisfying the following conditional expression (3).
40 ° ≤ α (3)
here,
α is the half angle of view of the imaging optical system.
Is. All object positions have a predetermined spatial frequency band that satisfies the following conditional expression (4).
The imaging optical system according to claim 1, wherein the maximum spatial frequency in the predetermined spatial frequency band is a spatial frequency in the vicinity of the Nyquist frequency.
0.08 ≤ VMTF (4)
here,
VMTF is the value of MTF when the object height is zero,
The object position is an arbitrary position between the near point and the far point.
The near point is a point located closest to the image pickup optical system in the depth of field of the image pickup optical system.
The far point is the point located farthest from the imaging optical system in the depth of field.
Is. The imaging optical system according to claim 1, wherein spherical aberration is added by the phase modulation. The spherical aberration increases in the negative direction as the F number value decreases, and becomes larger.
The negative direction is a direction from the paraxial image plane toward the imaging optical system.
The imaging optical system according to claim 5, wherein the aberration curve of the spherical aberration has an inflection point. Optical system and
It has an image pickup surface and has an image pickup element that converts an image formed on the image pickup surface by an optical system into an electric signal.
The image pickup apparatus according to any one of claims 1 to 6, wherein the optical system is an image pickup optical system.

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