JP7501985B2 - Endoscopic imaging device - Google Patents

Description

The present invention relates to an imaging optical system and an imaging device equipped with the same.

Depth of field is known as a physical quantity that represents the characteristics of an optical system. Depth of field is the range in which a clear image of an object can be obtained when the focus is on the object.

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

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

The depth of field in a normal optical system is explained below. A normal optical system is an optical system in which aberrations are well corrected. Hereinafter, the depth of field in a normal optical system is referred to as the "normal depth." In addition, an optical image corresponding to an object point located within the normal depth is referred to as the "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 far point. The third state is a state in which the position of the object point coincides with the position of the near point.

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

Normally, the sharpness of the point image is highest for point image N1. Therefore, the sharpness of point image N2 and point image N3 are lower than the sharpness of point image N1.

However, there is no difference in size between the clarity of point image N2 and the clarity of point image N1. There is also no significant difference between the clarity of point image N3 and the clarity of point image N1. Therefore, the clarity of point image N2 and the clarity of point image N3 can be considered to be approximately the same as the clarity of point image N1.

When an object point is located within the normal depth, the sharpness of the normal point image is the same or nearly the same as the sharpness of point image N1, regardless of the position of the object point. In contrast, when an object point is not located within the normal depth, the sharpness of the normal point image is lower than the sharpness of point image N1.

The surface of an object (hereinafter referred to as "surface OB") is formed by multiple object points. If the surface OB is non-planar and located in a range wider than the normal depth, the clarity of the optical image of the object points will vary depending on the position of the object points.

On the surface of an object located within normal depth (hereinafter referred to as "surface OBin"), an optical image of each object point is formed with the same sharpness (hereinafter referred to as "sharpness H") as the sharpness of the point image N1. Therefore, the optical image of the surface OBin has sharpness H. Sharpness H contains approximately the same sharpness as the sharpness of the point image N1.

In contrast, on the surface of an object that is not located within the normal depth (hereinafter referred to as "surface OBoff"), an optical image of each object point is formed with a sharpness (hereinafter referred to as "sharpness L") lower than the sharpness of the point image N1. Therefore, the optical image of the surface OBoff has sharpness L.

An image of the surface OB can be obtained by capturing an optical image of the surface OB with an imaging element. If the surface OB is located in a range wider than the normal depth, the image captured 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 with sharpness H. If a sharp image is an image with sharpness H, then the image of the surface OBin is a sharp image.

The image with the surface OBoff is an image with sharpness L. If a blurry image is an image with sharpness L, then the image with the surface OBoff is a blurry image.

In this way, when the surface OB is located in a range wider than the normal depth, the image acquired with the normal optical system includes clear and blurry images.

It is preferable to have as little difference in sharpness as possible within a single image. To reduce the difference in sharpness, you can increase the depth of field.

The depth of field is expressed, for example, by the following formula:
DOF = DOFf + DOFn
DOFf=(H×D)/(H−D)
DOFn=(H×D)/(H+D)
H = ^f2 / (F x c)
here,
DOF is depth of field,
DOFf is the depth of field on the far side,
DOFn is the depth of field on the near point side,
H is the hyperfocal distance,
D is the distance from the optical system to the focal point,
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,
It is.

Extended Depth of Field (EDoF) is known as a method for widening the depth of field. In an optical system using EDoF (hereinafter referred to as an "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 point image N1 can be considered as the diameter of the permissible circle of confusion. Therefore, by changing the size of point image N1, the depth of field can be changed.

When the sharpness of the point image N1 changes, the size of the point image N1 changes. When adding aberration, spherical aberration is added so that the sharpness of the point image N1 decreases.

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

In an EDoF optical system, aberration is added such that the size of the point image N1 becomes larger. As a result, the EDoF optical system has a depth of field that is wider than the normal depth. Hereinafter, the depth of field in an EDoF optical system will be referred to as the "extended depth." Also, an optical image corresponding to an object point located within the extended depth will be referred to as the "extended point image."

Point image E1 is an extended point image in a first state. Point image E2 is an extended point image in a second state. Point image E3 is an extended point image in a third state.

The clarity of the extended point images is highest for point image E1. Therefore, the clarity of point image E2 and point image E3 are lower than the clarity of point image E1.

However, there is no difference in size between the clarity of point image E2 and the clarity of point image E1. There is no difference in size between the clarity of point image E3 and the clarity of point image E1. Therefore, the clarity of point image E2 and the clarity of point image E3 can be considered to be approximately the same as the clarity of point image E1.

When an object point is located within the extended depth, the sharpness of the extended point image is the same or nearly the same as the sharpness of point image E1, regardless of the position of the object point. In contrast, when an object point is not located within the extended depth, the sharpness of the extended point image is less than the sharpness of point image E1.

This explains the case where an object is located at a range wider than the normal depth.

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

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

A blurry image is an image that is out of focus. Therefore, a blurry image cannot be used for observation and diagnosis, for example. However, a clear image can be obtained by performing a restoration process on the blurry image.

In imaging by an optical system, the following relationship holds:
I(x,y)=O(x,y)*PSF(x,y) (A)
here,
O(x,y) is the object,
I(x,y) is the optical image of the object;
PSF(x,y) is the point spread function,
* is the operator representing convolution,
It is.

As can be seen from equation (A), O(x,y) can be calculated 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 calculated.

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

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

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

As mentioned above, in an EDoF optical system, the sharpness of point image E2 and the sharpness of point image E3 are approximately the same as the sharpness of point image E1. Therefore, when an object point is located within the extended depth, the intensity distribution of the extended point image can be considered to be the same regardless of the object position.

When the intensity distribution of the extended point image is the same at all object positions, the intensity distribution of the extended point image at all object positions can be expressed by a single PSF(x, y). Therefore, in an EDoF optical system, there is no need to calculate a PSF(x, y) for each object position.

In an EDoF optical system, the number of PSF(x, y) is one. In the restoration process, only one PSF(x, y) needs to be used, so O(x, y) can be easily calculated.

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

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

By satisfying this condition, the effect of field curvature on the extension of the depth of field can be essentially eliminated.

JP 2015-4883 A

In the imaging optical system of Patent Document 1, the field curvature is reduced in the imaging optical system before spherical aberration is added. In order to reduce the field curvature, for example, the number of lenses used in the imaging optical system can be increased. However, increasing the number of lenses increases the overall length of the optical system.

The present invention has been made in consideration of these problems, and aims to provide an imaging optical system with a short overall optical system length and a wide depth of field. It also aims to provide an imaging device that has a wide depth of field and can easily generate clear images.

In order to solve the above-mentioned problems and achieve the object, an endoscopic imaging device according to at least some embodiments of the present invention comprises:
An optical system;
an image sensor having an image pickup surface and converting an image formed on the image pickup surface by an optical system into an electrical signal;
The optical system is
From the object side,
a first lens group having negative refractive power;
A second lens group; and
a third lens group having a positive refractive power,
the first lens group has a plano-concave negative lens, the plano-concave lens being located closest to the object and having a flat surface facing the object;
the second lens group has a diaphragm and an optical surface that performs phase modulation;
the optical surface performing the phase modulation is provided on the image side of the phase modulation element, and the object side surface of the phase modulation element is a flat surface;
The optical surface is located coincident with or adjacent to the aperture;
Phase modulation causes a change in the phase of the wavefront,
The depth of field when a change occurs is greater than the depth of field when no change occurs.
Phase modulation adds spherical aberration,
Spherical aberration increases in the negative direction as the F-number decreases,
The imaging optical system is characterized in that it satisfies the following conditional expressions (1) , (2) and (3)' .
D2 × cos α < D1 (1)
D1 < D2 (2)
65°≦α (3) '
here,
The negative direction is the direction from the paraxial image plane toward the imaging optical system.
α is the half angle of view of the imaging optical system,
D1 is the diameter of the axial beam at the first lens surface,
D2 is the diameter of a given off-axis ray bundle at 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 light flux is a light flux defined by a half angle of view,
It is.

Moreover, the imaging device of the present invention comprises:
An optical system;
an image sensor having an image pickup surface and converting an image formed on the image pickup surface by an optical system into an electrical signal;
The optical system is characterized in that it is an imaging optical system.

The present invention can provide an imaging optical system with a short overall optical system length and a wide depth of field. It can also provide an imaging device that can easily generate clear images with a wide depth of field.

1 is a cross-sectional view of a lens of an imaging optical system according to an embodiment of the present invention. FIG. 4 is a diagram illustrating a light beam incident on a first lens surface. 1 is a lens sectional view of an imaging optical system according to a first embodiment. 4 is a diagram showing the MTF of the imaging optical system of Example 1. FIG. FIG. 1 illustrates an embodiment of an imaging device.

Before describing the examples, the effects of an embodiment relating to a certain aspect of the present invention will be described. When specifically describing the effects of this embodiment, specific examples will be shown. However, as in the case of the examples described below, these exemplified aspects are merely a portion of the aspects included in the present invention, and there are many variations in these aspects. Therefore, the present invention is not limited to the exemplified aspects.

The imaging optical system of this embodiment has, in order from the object side, a first lens group having negative refractive power, a second lens group, and a third lens group having positive refractive power, the second lens group having an aperture and an optical surface that performs phase modulation, the optical surface being disposed at a position coinciding with the aperture or in the vicinity of the aperture, a change in the phase of the wavefront occurs due to the phase modulation, the size of the permissible circle of confusion when the change occurs is larger than the size of the permissible circle of confusion when no change occurs, and is characterized in satisfying the following conditional expression (1):
D2 × cos α < D1 (1)
here,
α is the half angle of the imaging optical system,
D1 is the diameter of the axial beam at the first lens surface,
D2 is the diameter of a given off-axis ray bundle at 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 light flux is a light flux defined by a half angle of view,
It is.

Figure 1 is a cross-sectional view of the lenses of the imaging optical system of this embodiment. The imaging 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 negative refractive power. The second lens group G2 has positive refractive power. However, the refractive power of the second lens group G2 is not limited to positive refractive power. The third lens group G3 has 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 imaging optical system OBJ, the negative lens L1 is positioned closest to the object. This allows a wide angle of view to be ensured.

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

In an optical system with a wide angle of view, the focal length is short. 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 ensure a long back focus while also ensuring a wide angle of view.

Retrofocus optical systems are known as optical systems that can ensure a long back focus. In retrofocus optical systems, the order of refractive power is negative and positive.

In the imaging optical system OBJ, the first lens group G1 and the third lens group G3 have negative and positive refractive powers. In this case, the imaging optical system OBJ can be considered to have a retrofocus optical system. Therefore, a sufficient back focus can be ensured. As a result, a cover glass can be placed on the object side of the image plane. Also, an optical filter can be placed on the object side of the image plane.

In the second lens group G2, phase modulation is performed. Phase modulation means, for example, changing the phase of the wavefront. When a change occurs in the phase of the wavefront, the aberration changes. Therefore, phase modulation can be considered as the addition of aberration.

Phase modulation can be performed by an optical surface. To perform phase modulation, an optical surface that performs phase modulation (hereinafter referred to as the "phase modulation surface") is disposed 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 flat surface.

The aperture stop S is disposed at the pupil position of the imaging optical system OBJ. However, the aperture stop S may also be disposed near the pupil position. In this case, the aperture stop S can be considered as an aperture stop.

The phase modulation surface PMS is positioned at a position coincident with the aperture stop S or in the vicinity of the aperture stop S. As described above, the aperture stop S is positioned at the pupil position of the imaging optical system OBJ. Therefore, the same phase modulation can be performed on both the on-axis light beam and the off-axis light beam.

In the imaging optical system OBJ, a change occurs in the phase of the wavefront due to phase modulation. When a change occurs in the phase of the wavefront, the phase of the wavefront emerging from the phase modulation surface PMS differs from the phase of the wavefront incident on the phase modulation surface PMS.

When a change occurs in the phase of the wavefront, 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.

When the size of the permissible circle of confusion increases, the change in the size of the permissible circle of confusion with respect to a change in object distance (hereinafter referred to as the "predetermined change") becomes smaller. A small predetermined change means that the depth of field is wide. Therefore, the depth of field when a change occurs in the phase of the wavefront is wider than the depth of field when no change occurs in the phase of the wavefront.

When there is no change in the phase of the wavefront, it is said that the aberration is well corrected. An optical system in which the aberration is well corrected is a normal optical system. In the imaging optical system OBJ, a change occurs in the phase of the wavefront. Therefore, the depth of field in the imaging optical system OBJ is wider than the depth of field in a normal optical system.

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

The imaging optical system of this embodiment is an EDoF optical system and satisfies conditional expression (1). Conditional expression (1) is a conditional expression relating to the diameter of the on-axis light beam and the diameter of the predetermined off-axis light beam. The diameter of the on-axis light beam and the diameter of the predetermined off-axis light beam will be described below.

Figure 2 shows a light beam incident on the first lens surface. Figure 2 shows the negative lens L1, light beam Lon, and light beam Loff.

The negative lens L1 has a first lens surface S1. In the first lens group G1, the negative lens L1 is disposed closest to the object. Therefore, the first lens surface S1 is the lens surface in the first lens group G1 that is located closest to the object.

Light beams are incident on the first lens surface S1 at various angles. The light beams incident on the first lens surface S1 include on-axis light beams and off-axis light beams. For on-axis light beams, the central ray coincides with the optical axis AX. For off-axis light beams, the central ray intersects with the optical axis AX.

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

The principal ray CRmax of the light beam Loff intersects with the optical axis AX. Therefore, the light beam Loff represents an off-axis light beam.

For the light beam Loff, the angle between the chief ray CRmax and the optical axis AX is α. If the angle of view of the imaging optical system OBJ is ω, then α = ω/2. Therefore, α represents the half angle of view of the imaging optical system OBJ. The angle of view is the angle that represents the maximum range of the object space that can be imaged by the optical system.

The light beam when the angle between the chief ray and the optical axis is half the angle of view is defined as the light beam defined by the half angle of view. In addition, the light beam defined by the half angle of view is defined as a specified off-axis light beam.

For light beam Loff, the angle between the chief ray CRmax and the optical axis AX is the half angle of view. Therefore, light beam Loff is a light beam defined by the half angle of view. Since the light beam defined by the half angle of view is a specified off-axis light beam, light beam Loff represents a specified off-axis light beam.

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

Diameter D1 is the diameter of the light beam Lon at the first lens surface S1. Since the light beam Lon is an on-axis light beam, diameter D1 is the diameter of the on-axis light beam at the first lens surface S1. Diameter D2 is the diameter of the light beam Loff at the first lens surface S1. Since the light beam Loff is a predetermined off-axis light beam, diameter D2 is the diameter of the predetermined off-axis light beam at the first lens surface S1.

The diameter of the light beam Lon is the diameter of the light beam in a plane perpendicular to the central ray. For the light beam Lon, the plane perpendicular to the central ray is parallel to the first lens surface S1. Therefore, the diameter of the light beam Lon is equal to the diameter D1.

The diameter of the light beam Loff is the diameter of the light beam in a plane perpendicular to the chief ray CRmax. For the light beam Loff, the plane perpendicular to the chief ray CRmax is non-parallel to the first lens surface S1. Therefore, the diameter of the light beam Loff is different from the diameter D2. The diameter of the light beam Loff is D2 x cos α.

Explains the relationship between the diameter of the light beam and the amount of aberration.

The imaging optical system of this embodiment can be considered as an optical system in which a phase modulation surface is disposed in a normal optical system. As described above, phase modulation can be considered as the addition of aberration. Therefore, the aberrations that occur in the imaging optical system of this embodiment include aberrations that occur in normal optical systems (hereinafter referred to as "basic aberrations") and aberrations that occur at the phase modulation surface (hereinafter referred to as "additional aberrations").

The width of the depth of field changes depending on the size of the permissible circle of confusion. To change the depth of field by phase modulation, the size of the permissible circle of confusion only needs to be changed mainly by additional aberrations. To change the size of the permissible circle of confusion by additional aberrations, the amount of basic aberrations (hereinafter referred to as "basic aberration amount") needs to be much smaller than the amount of additional aberrations.

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

When the F-number of the off-axis light beam is the same as the F-number of the on-axis light beam, the amount of off-axis aberration is generally greater than the amount of on-axis aberration. In order to reduce the amount of basic aberration, it is necessary to reduce the amount of off-axis aberration. In a normal optical system, many lenses must be used to reduce the amount of off-axis aberration.

When conditional expression (1) is satisfied, the diameter of the specified off-axis light beam is smaller than the diameter of the on-axis light beam. Therefore, the F-number value for the specified off-axis light beam is larger than the F-number value for the on-axis light beam.

The greater the F-number, the wider the depth of field. Therefore, in the imaging optical system of this embodiment, the depth of field for a given off-axis light beam is wider than the depth of field for an on-axis light beam.

The amount of aberration decreases as the F-number increases. Therefore, in the imaging optical system of this embodiment, the amount of off-axis aberration is smaller than when the F-number of the off-axis light beam is the same as the F-number of the on-axis light beam.

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

The size of the permissible circle of confusion also increases as the F-number increases. Therefore, in the imaging optical system of this embodiment, the size of the permissible circle of confusion for the off-axis light beam is larger than when the F-number for the off-axis light beam is the same as the F-number for the on-axis light beam.

The larger the size of the permissible circle of confusion, the smaller the effect of the amount of off-axis aberration on the permissible circle of confusion. Therefore, in the imaging optical system of this embodiment, the occurrence of larger off-axis aberrations can be tolerated in a normal optical system compared to when the F-number value of the off-axis light beam is the same as the F-number value of the on-axis light beam. As a result, for example, even if there is a large curvature of field, a wide depth of field can be obtained.

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

The PSF(x, y) when one-sided blur occurs differs from the PSF(x, y) at the time of design. Therefore, even if the restoration process is performed using the PSF(x, y) at the time of design, it is not possible to obtain a clear image with a wide depth of field.

The imaging optical system of this embodiment can tolerate larger off-axis aberrations. If one-sided blur is considered to be off-axis aberration, the imaging optical system of this embodiment can minimize the effect of one-sided blur. Therefore, even if one-sided blur occurs in the acquired image, a clear image with a wide depth of field can be acquired by performing recovery processing using the PSF(x, y) at the time of design.

When the object point is not located at infinity, both the light beams Lon and Loff are diverging light. In this case, the comparison of the light beams Lon and Loff can be made in terms of the object-side numerical aperture. When conditional expression (1) is satisfied, the value of the object-side numerical aperture for a given off-axis light beam is smaller than the value of the object-side numerical aperture for an axial light beam.

The depth of field becomes wider as the object-side numerical aperture becomes smaller. Therefore, in the imaging optical system of this embodiment, the depth of field for a given off-axis light beam is wider than the depth of field for an on-axis light beam.

Therefore, in the imaging optical system of this embodiment, whether the object point is located at infinity or not, the depth of field for a given off-axis light beam is wider than the depth of field for an on-axis light beam.

In the phase modulation element PM, the object side surface is a plane. However, the object side surface can be made spherical. By making the object side surface spherical, the object side surface can be used to correct aberrations in a normal optical system.

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

It is preferable that the imaging optical system of this embodiment satisfies the following conditional expression (2).
D1 < D2 (2)
here,
D1 is the diameter of the axial beam at the first lens surface,
D2 is the diameter of a given off-axis ray bundle at 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 light flux is a light flux defined by a half angle of view,
It is.

By satisfying conditional expression (2), it is possible to prevent a decrease in resolution at the periphery of the field of view. As a result, the imaging optical system of this embodiment can maintain approximately the same resolution from the center to the periphery of the field of view.

It is preferable that the imaging optical system of this embodiment satisfies the following conditional expression (3).
40°≦α (3)
here,
α is the half angle of the imaging optical system,
It is.

By satisfying conditional expression (3), a wide field of view can be obtained.

The imaging optical system of this embodiment can be used in an imaging 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. The imaging optical system of this embodiment allows clear observation of a wide area inside the body cavity.

It is preferable that the imaging optical system of this embodiment has a predetermined spatial frequency band that satisfies the following conditional expression (4) at all object positions, and the maximum spatial frequency of the predetermined spatial frequency band is a spatial frequency close to the Nyquist frequency.
0.08≦VMTF (4)
here,
VMTF is the MTF value when the object height is zero.
The object position is any position between the near point and the far point,
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.
It is.

The imaging optical system of this embodiment can maintain high resolution from the center to the periphery of the field of view.

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

The imaging optical system of this embodiment can provide a wide depth of field.

In the imaging optical system of this embodiment, the spherical aberration increases in the negative direction as the F-number value decreases, and the negative direction is the direction from the paraxial image surface toward the imaging optical system, and it is preferable that the aberration curve of the spherical aberration has an inflection point.

The imaging optical system of this embodiment can provide a wide depth of field.

In the imaging optical system of this embodiment, it is preferable that the first lens surface is a convex surface or a flat surface.

The first lens surface faces the object. Therefore, there is a possibility that the first lens surface may come into contact with the object. By making the first lens surface convex or flat, it is possible to reduce the impact of contact.

The imaging optical system of this embodiment can also be used in the 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.

If the first lens surface is convex, it is preferable that the convex surface be a smooth surface. It is also preferable that the radius of curvature of the convex surface is large. By increasing the radius of curvature of the convex surface, it is possible to reduce the amount of protrusion toward the object side.

In the imaging optical system of this embodiment, it is preferable that the MTF of light rays at an object height of 0 is 8% or more in the frequency band from 0 to near the Nyquist frequency and in the range from close objects to far objects.

The imaging optical system of this embodiment can maintain high resolution from the center to the periphery of the field of view.

The imaging device of this embodiment has an optical system and an imaging element that has an imaging surface and converts an image formed on the imaging surface by the optical system into an electrical signal, and is characterized in that the optical system is the imaging optical system of this embodiment.

The imaging device of this embodiment can easily generate clear images with a wide depth of field.

Below, an embodiment of the imaging optical system will be described in detail with reference to the drawings. Note that the present invention is not limited to this embodiment.

Figure 3 is a lens cross-sectional view of the imaging optical system of Example 1. The first lens group is indicated as G1, the second lens group as G2, the third lens group as G3, the aperture as S, and the image plane (imaging plane) as I.

The imaging optical system of Example 1 has, in order from the object side, a first lens group G1 having negative refractive power, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

The first lens group G1 has a plano-concave negative lens L1 with its flat surface facing the object side.

The second lens group G2 has a positive meniscus lens L2 with a convex surface facing the image side, and a phase modulation element PM. The object side surface of the phase modulation element PM is flat, and the image side surface is aspheric.

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 flat surface facing the image side. The biconvex positive lens L4 and the plano-concave negative lens L5 are cemented together.

The aperture stop S is disposed in the second lens group G2. More specifically, it is disposed on the image side of the phase modulation element PM.

Figure 4 shows the MTF of the imaging optical system of Example 1. Figure 4(a) shows the MTF when the object distance is 3 mm. Figure 4(b) shows the MTF when the object distance is 6.6 mm. Figure 4(c) shows the MTF when the object distance is 150 mm.

The solid line is the MTF for an on-axis beam. The dashed and dashed lines are the MTF for a given off-axis beam. The dashed line is the MTF in the radial direction, and the dashed line is the MTF in the tangential direction. The dotted line shows the maximum MTF at the diffraction limit.

For an axial light beam, the position of the peak of the MTF curve changes significantly as the object distance changes. On the other hand, even for a given off-axis light beam, the position of the peak of the MTF curve changes as the object distance changes.

However, the change in the position of the peak for a given off-axis light beam is smaller than the change in the position of the peak for an on-axis light beam. In particular, the change in the position of the peak is very small for the MTF in the tangential direction.

This means that for a given off-axis light beam, the effect of defocus when the object distance changes is smaller than for an on-axis light beam. A smaller effect of defocus means a smaller change in the size of the permissible circle of confusion.

As mentioned 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 this embodiment, the depth of field for a given off-axis light beam is wider than the depth of field for an on-axis light beam.

In addition, the peak value of the MTF curve is approximately the same for the on-axis light beam and a specified off-axis light beam. Therefore, the imaging optical system of this embodiment can maintain approximately the same resolution from the center to the periphery of the field of view.

The numerical data for 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 each lens for the e-line, νd is the Abbe number of each lens, and * denotes an aspheric surface.

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

In addition, the aspheric shape is expressed by the following equation, where the optical axis direction is z, the direction perpendicular to the optical axis is y, the conic coefficient is k, and the aspheric coefficients are A4, A6, A8, A10, A12, A14 . . .
z = (y ² /r)/[1 + {1 - (1 + k) (y/r) ² } ^1/2 ]
+ ^A4y4 + ^A6y6 + ^A8y8 + ^A10y10 + ^A12y12 + ^A14y14 +...
In addition, in the aspheric coefficient, "E+n" (n is an integer) indicates "10 ⁿ ". The symbols of these specification values are common to the numerical data of the examples described later.

Numerical Example 1
Unit: mm

Surface data surface number rd ne νd
1∞0.2201.8881540.76
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 ∞

Aspheric data No. 6
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
Alpha 65

The values of the conditional expressions in the first embodiment are given below.

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

5 is a diagram showing an embodiment of an imaging device. The imaging device 1 has an imaging optical system 2 and an imaging element 3. The imaging optical system 2 is, for example, the imaging optical system of the first embodiment.
The imaging optical system 2 forms an optical image of an object 4 on an image plane.

The imaging surface of the imaging element 3 is located on the image plane. The imaging element 3 captures an optical image. The optical image is converted into an electrical signal. As a result, an image of the object 4 is obtained.

The imaging optical system 2 is an EDoF optical system. Images acquired by an EDoF optical system are blurred even if the object is located within the extended depth. Therefore, the imaging device 1 acquires a blurred image of the object 4.

The imaging device 1 can be combined with an image processing device 5. An image of the object 4 is input to the image processing device 5. The image processing device 5 can perform a restoration process 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 that has a short overall optical system length and a wide depth of field. The present invention is also suitable for an imaging device that has a wide depth of field and can easily generate clear images.

G1 First lens group G2 Second lens group G3 Third lens group PM Phase modulation element PMS Phase modulation surface S Aperture L1 Negative lens Lon, Loff Light flux S1 First lens surface AX Optical axis CRmax Chief ray α Half angle of view ω Angle of view D1, D2 Diameter I Image surface 1 Imaging device 2 Imaging optical system 3 Imaging element 4 Object 5 Image processing device

Claims (2)

An optical system;
an image sensor having an image pickup surface and converting an image formed on the image pickup surface by the optical system into an electrical signal;
The optical system comprises:
From the object side,
a first lens group having negative refractive power;
A second lens group; and
a third lens group having a positive refractive power,
the first lens group includes a plano-concave negative lens having a flat surface facing the object side, the plano-concave negative lens being located closest to the object side;
the second lens group has a diaphragm and an optical surface that performs phase modulation;
the optical surface performing the phase modulation is provided on an image side surface of a phase modulation element, and the object side surface of the phase modulation element is a flat surface;
the optical surface is positioned coincident with or adjacent to the aperture;
The phase modulation causes a change in the phase of the wavefront,
The magnitude of the depth of field when the change occurs is greater than the magnitude of the depth of field when the change does not occur.
The phase modulation adds spherical aberration,
The spherical aberration increases in the negative direction as the F-number value decreases,
An endoscopic imaging apparatus characterized in that it has an imaging optical system which satisfies the following conditional expressions (1) , (2) and (3)' .
D2 × cos α < D1 (1)
D1 < D2 (2)
65°≦α (3) '
here,
the negative direction is a direction from a paraxial image plane toward the imaging optical system,
α is the half angle of view of the imaging optical system,
D1 is the diameter of the axial beam at the first lens surface,
D2 is the diameter of a given off-axis ray bundle at the first lens surface,
the first lens surface being a lens surface located closest to an object in the first lens group;
The predetermined off-axis light flux is a light flux defined by the half angle of view,
It is. 2. The endoscopic imaging apparatus according to claim 1, wherein the aberration curve of the spherical aberration has an inflection point .

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