JP4798529B2 - Imaging optics - Google Patents

Description

  The present invention relates to an imaging optical system used in a digital camera, a mobile phone with an imaging function, a scanner, and the like.

There is an increasing demand for a compact and high-resolution imaging device. Therefore, the imaging optical system used in such an imaging apparatus is also required to be small and have high resolution. Such an imaging optical system is described in Patent Documents 1 to 3, for example.

  Recently, as a means for reducing the size of the imaging optical system, a method of widening the angle tends to be used. In a solid-state image sensor, the light incident angle on the image sensor is limited in order to minimize the shielding of light rays by the wall of the wiring layer. It was necessary to control the range. In a general four-lens compact imaging optical system, correction of axial chromatic aberration is performed by a second lens having negative power, and at the same time, the direction of light rays is greatly changed toward the maximum image height, and the fourth lens is used. Control is made so that the angle of incidence of light on the image sensor becomes small. However, in such a method, the tolerance of the second lens having a large negative power becomes severe, and when the tolerance is exceeded, a large aberration occurs as the maximum visual field is approached, resulting in image degradation.

  In order to create such an optical system with a tight tolerance, it takes time to process and assemble, and the yield is reduced when only good products are obtained by inspection after the production. In any case, as a result, the manufacturing cost of the imaging optical system increases.

  On the other hand, with the development of backside illumination (BSI) technology for CMOS (Complementally Metal Oxide Semiconductor) image sensors, the allowable light incident angle on the sensor has been widened. The design and development that made the most of the advantages of this is still undeveloped.

Japanese Patent No. 3342030 Japanese Patent No. 4032667 Japanese Patent No. 4032668

  Therefore, there is a need for an imaging optical system that is small in size, has high resolution, and can reduce manufacturing costs.

が満たされる。
An imaging optical system according to the present invention includes a first lens having positive power from the object side to the image plane side, a second lens that is a convex meniscus lens on the image side, a third lens having positive power, and a negative lens. This is an imaging optical system composed of a fourth lens having power. The power of the third lens with respect to the principal ray in the meridional direction is positive in the paraxial region, and the power decreases as the distance from the optical axis decreases. The power of the fourth lens with respect to the principal ray in the meridional direction is negative in the paraxial region, and increases with increasing distance from the optical axis and becomes positive. The first lens includes a diffraction grating on the image side surface. The focal length of the second lens is f ₂ , the combined focal length of the imaging optical system is f _T , and the distance from the object side to the image plane among the vertices of the aperture surface or the object side surface of the first lens is TTL,

Is satisfied.

  According to the present invention, the axial chromatic aberration is corrected (achromatic) by the diffraction grating provided on the image plane side of the first lens, so that the absolute power of the second lens is satisfied so that the expression (1) is satisfied. The value is reduced. Therefore, the assembly tolerance can be increased and the manufacturing cost can be reduced.

  In addition, according to the present invention, a compact imaging optical system is realized by configuring so that the formula (2) is satisfied.

が満足される。 According to the embodiment of the present invention, the focal length of the fourth lens is f ₄ , and the refractive index of the d-line of the material of the fourth lens is n ₄ ,

Is satisfied.

  According to the present embodiment, by configuring so as to satisfy Expression (3), the Petzval sum is reduced, and an imaging optical system having a small field curvature in the vicinity of the optical axis is realized, and a high image height position is achieved. It is possible to smoothly and gently control the change in curvature of field up to.

  According to the embodiment of the present invention, the diffraction grating is composed of annular zones, and the number of annular zones is 10 or less.

  According to the present embodiment, since the number of ring zones is 10 or less, the influence on flare and ghosts derived from other light due to the processing limit can be minimized.

1 is a diagram illustrating a configuration of an imaging optical system according to Example 1. FIG. FIG. 6 is a diagram illustrating aberrations of the imaging optical system according to Example 1. 6 is a diagram illustrating a configuration of an imaging optical system according to Embodiment 2. FIG. FIG. 10 is a diagram illustrating aberrations of the imaging optical system according to Example 2. FIG. 6 is a diagram illustrating a configuration of an imaging optical system according to Example 3. FIG. 6 is a diagram illustrating aberrations of the imaging optical system according to Example 3. FIG. 6 is a diagram illustrating a configuration of an imaging optical system according to Example 4. FIG. 10 is a diagram illustrating aberrations of the imaging optical system according to Example 4. FIG. 10 is a diagram illustrating a configuration of an imaging optical system according to Example 5. FIG. 10 is a diagram illustrating aberrations of the imaging optical system according to Example 5. FIG. 10 is a diagram illustrating a configuration of an imaging optical system according to Example 6. FIG. 10 is a diagram illustrating aberrations of the imaging optical system according to Example 6.

  FIG. 1 is a diagram showing a configuration of an imaging optical system according to an embodiment of the present invention. The imaging optical system according to the present embodiment includes a first lens 101, a second lens 102, a third lens 103, and a fourth lens 104 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 101 and on the image side of the apex of the object side surface of the first lens 101. Specifically, the stop is on the object side surface of the first lens 101. The light that has passed through the first lens 101, the second lens 102, the third lens 103, and the fourth lens 104 passes through the glass plate 105 and reaches the image plane 106.

とし、結像光学系の合成焦点距離及び絞り面または第１レンズの物体側面の頂点のうち、物体側のものから像面までの距離（以下、光学長とも呼称する）を、それぞれ

とする。 Hereinafter, features of the imaging optical system according to the present invention will be described. In the following, i is an integer of 1 to 4, and the focal length of the i-th lens and the refractive index of the d-line (wavelength 587.6 nm) of the lens material are

And the distance from the object side to the image plane (hereinafter also referred to as the optical length) among the combined focal length of the imaging optical system and the apex of the object side surface of the aperture surface or the first lens, respectively,

And

Types of Four Lenses An imaging optical system according to an embodiment of the present invention includes a first lens having a positive power from the object side to the image plane side, a second lens that is a convex meniscus lens on the image side, and a positive power. And a fourth lens having negative power. Here, the lens having positive or negative power means that the lens has positive or negative power near its paraxial axis.

  The power of the third lens with respect to the principal ray in the meridional direction is positive in the paraxial region, and the power decreases as the distance from the optical axis decreases. The power of the fourth lens with respect to the principal ray in the meridional direction is negative in the paraxial region, and increases with increasing distance from the optical axis and becomes positive. By combining the third lens and the fourth lens having the respective powers, it is possible to control the field curvature from the paraxial region to the maximum field of view to be small.

Diffraction Grating An imaging optical system according to an embodiment of the present invention includes an annular diffraction grating for correcting axial chromatic aberration (hereinafter also referred to as achromatic) on the image side surface of the first lens. The number of ring zones is 10 or less. If the number of ring zones is greater than 10, the width of the ring zone at the peripheral edge portion becomes small, and the influence on flare and ghosts derived from other light due to the processing limit increases.

The absolute value of the ratio between the focal length of the second lens and the combined focal length The imaging optical system according to the embodiment of the present invention satisfies the following expression.

  In the conventional imaging optical system of the same type of the present invention, the first lens is made of a low-dispersion material when performing achromaticity using the difference in Abbe number (dispersion rate) of the lens material. The lens has a positive power, and the second lens is a lens having a negative power made of a highly dispersed material. In this case, in order to realize a predetermined achromatic performance, the negative power of the second lens needs to be greater than or equal to a predetermined magnitude. In addition, even when a diffraction grating is used for achromatism, in order to improve the achromatization performance, achromatism using the difference in the dispersion of the lens material and achromatism using the diffraction grating are combined. It was.

  According to the conventional method, the design resolution can be sufficiently improved. However, in practice, the processing tolerance or assembly tolerance of the lens in the manufacturing process becomes strict, which makes it difficult to manufacture or increases the manufacturing cost.

  As a result of analyzing the manufacturing process, the inventor found that the first lens has a positive power, whereas the second lens has a negative power of a predetermined magnitude or more. New knowledge that tight tolerances were obtained. Therefore, if the negative power of the second lens can be reduced while maintaining high resolution, the manufacturing cost can be reduced. Based on the above knowledge, the present invention realizes an imaging optical system in which the negative power of the second lens is reduced so as to satisfy Expression (1) while maintaining high resolution. In the present invention, the negative power of the second lens is reduced by providing the main achromatic function to the diffraction grating.

光学長と合成焦点距離が式（２）の上限値を超えると、コンパクトな結像光学系を実現するのが困難となる。 Ratio of optical length to combined focal length The imaging optical system according to the embodiment of the present invention satisfies the following expression.

If the optical length and the combined focal length exceed the upper limit value of Expression (2), it becomes difficult to realize a compact imaging optical system.

一般的に、複数のレンズからなる結像光学系の像面湾曲を小さくするには、以下の式で表せるペッツヴァル和Ｐをゼロに近づける必要がある。

Ratio of product of d-line refractive index and focal length of material of fourth lens and combined focal length The imaging optical system according to the embodiment of the present invention satisfies the following expression.

In general, in order to reduce the curvature of field of an imaging optical system composed of a plurality of lenses, it is necessary to bring the Petzval sum P expressed by the following equation close to zero.

  In the present embodiment, since the first and third lenses have positive power and the absolute value of the power of the second lens is small, only the fourth lens has a substantial negative power. Therefore, in order to bring the Petzval sum of Equation (4) closer to 0, Equation (3) needs to be satisfied.

  Examples 1 to 6 of the present invention will be described below.

An imaging optical system characteristic table 1 according to the embodiment is a table showing the characteristics of the first to sixth embodiments. In the table below, the unit of length is millimeters unless otherwise specified.

  According to Table 1, Examples 1 to 6 satisfy Expressions (1) to (3). Further, the number of ring zones of the diffraction grating provided on the image plane side of the first lens is 10 or less.

ここで、ｚは、レンズ面と光軸との交点を基準とし、像側を正とした、レンズ面上の点の光軸方向の位置を示す座標である。ｒは、レンズ面上の点の光軸からの距離を示す。Ｒは、レンズ面の頂点における曲率半径である。ｋは、円錐定数である。Ａｉは、多項式の係数である。 The expression of the lens surface of the embodiment and the phase function of the diffraction grating The surface of each lens of the embodiment can be expressed by the following expression.

Here, z is a coordinate indicating the position of the point on the lens surface in the optical axis direction, with the image side being positive with the intersection of the lens surface and the optical axis as a reference. r represents the distance from the optical axis of a point on the lens surface. R is the radius of curvature at the apex of the lens surface. k is a conic constant. Ai is a polynomial coefficient.

ここで、Ｂｉは、多項式の係数である。 The phase function of the diffraction grating provided on the image plane side of the first lens can be expressed by the following equation.

Here, Bi is a coefficient of a polynomial.

Example 1
FIG. 1 is a diagram illustrating a configuration of an imaging optical system according to the first embodiment. The imaging optical system according to the first embodiment includes a first lens 101, a second lens 102, a third lens 103, and a fourth lens 104 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 101 and on the image side of the apex of the object side surface of the first lens 101. Specifically, the stop is on the object side surface of the first lens 101. The light that has passed through the first lens 101, the second lens 102, the third lens 103, and the fourth lens 104 passes through the glass plate 105 and reaches the image plane 106.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of ring zones of the diffraction grating is ten.

  FIG. 2 is a diagram illustrating aberrations of the imaging optical system according to the first embodiment. FIG. 2 shows aberrations for three wavelengths in the visible region. FIG. 2A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 2A indicates the focal position in the optical axis direction with the image plane position as a reference (unit: millimeter). The vertical axis in FIG. 2 (a) indicates the light passage position at the stop. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 2B is a diagram showing astigmatism and curvature of field. The horizontal axis of FIG.2 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.2 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 2B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG. 2C is a diagram showing distortion. The horizontal axis of FIG.2 (c) shows a distortion aberration (a unit is a percentage). The vertical axis | shaft of FIG.2 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 2 is a table showing lens data of the imaging optical system according to Example 1. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 2, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 3 is a table showing the radius of curvature R and the conic constant k at the apex of the expression (5) of the first to eighth lens surfaces.

  Table 4 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the first surface to the fourth surface.

  Table 5 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 6 is a table showing the coefficient Bi of the polynomial in Expression (6) representing the phase function of the diffraction grating.

Example 2
FIG. 3 is a diagram illustrating a configuration of the imaging optical system according to the second embodiment. The imaging optical system according to Example 2 includes a first lens 201, a second lens 202, a third lens 203, and a fourth lens 204 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 201 and on the image side of the apex of the object side surface of the first lens 201. Specifically, the stop is on the object side surface of the first lens 201. The light that has passed through the first lens 201, the second lens 202, the third lens 203, and the fourth lens 204 passes through the glass plate 205 and reaches the image plane 206.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of ring zones of the diffraction grating is nine.

  FIG. 4 is a diagram illustrating aberrations of the imaging optical system according to the second embodiment. FIG. 4 shows aberrations for three wavelengths in the visible region. FIG. 4A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 4A indicates the focal position in the optical axis direction with the image plane position as a reference (unit: millimeter). The vertical axis in FIG. 4 (a) indicates the light passage position at the stop. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 4B is a diagram showing astigmatism and field curvature. The horizontal axis of FIG.4 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.4 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 4B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG. 4C is a diagram showing distortion. The horizontal axis of FIG.4 (c) shows a distortion aberration (distortion) (a unit is a percentage). The vertical axis | shaft of FIG.4 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 7 is a table showing lens data of the imaging optical system according to Example 2. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 7, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 8 is a table showing the radius of curvature R and the conic constant k at the apex of Expression (5) of the first to eighth lens surfaces.

  Table 9 is a table showing the coefficient Ai of the polynomial in Expression (5) for the lens surfaces of the first surface to the fourth surface.

  Table 10 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 11 is a table showing the coefficient Bi of the polynomial in Expression (6) representing the phase function of the diffraction grating.

Example 3
FIG. 5 is a diagram illustrating a configuration of the imaging optical system according to the third embodiment. The imaging optical system according to Example 3 includes a first lens 301, a second lens 302, a third lens 303, and a fourth lens 304 from the object side to the image side. The stop is located closer to the object side than the image side surface of the first lens 301 and closer to the image side than the vertex of the object side surface of the first lens 301. Specifically, the stop is on the object side surface of the first lens 301. The light that has passed through the first lens 301, the second lens 302, the third lens 303, and the fourth lens 304 passes through the glass plate 305 and reaches the image plane 306.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of ring zones of the diffraction grating is ten.

  FIG. 6 is a diagram illustrating aberrations of the imaging optical system according to Example 3. FIG. 6 shows aberrations for three wavelengths in the visible region. FIG. 6A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 6A indicates the focal position in the optical axis direction with the image plane position as a reference (unit: millimeter). The vertical axis in FIG. 6 (a) indicates the light passage position at the stop. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 6B is a diagram showing astigmatism and field curvature. The horizontal axis of FIG.6 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.6 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 6B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG. 6C is a diagram showing distortion. The horizontal axis of FIG.6 (c) shows a distortion aberration (distortion) (a unit is a percentage). The vertical axis | shaft of FIG.6 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 12 is a table showing lens data of the imaging optical system according to Example 3. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 7, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 13 is a table showing the radius of curvature R and the conic constant k at the apex of the expression (5) of the first to eighth lens surfaces.

  Table 14 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the first surface to the fourth surface.

  Table 15 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 16 is a table showing the coefficient Bi of the polynomial in Expression (6) representing the phase function of the diffraction grating.

Example 4
FIG. 7 is a diagram illustrating a configuration of the imaging optical system according to the fourth embodiment. The imaging optical system according to Example 4 includes a first lens 401, a second lens 402, a third lens 403, and a fourth lens 404 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 401 and on the image side of the apex of the object side surface of the first lens 401. Specifically, the stop is on the object side surface of the first lens 401. The light that has passed through the first lens 401, the second lens 402, the third lens 403, and the fourth lens 404 passes through the glass plate 405 and reaches the image plane 406.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of ring zones of the diffraction grating is eight.

  FIG. 8 is a diagram illustrating aberrations of the imaging optical system according to the fourth example. FIG. 8 shows aberrations for three wavelengths in the visible region. FIG. 8A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 8A indicates the focal position in the optical axis direction with respect to the image plane position (unit: millimeter). The vertical axis | shaft of Fig.8 (a) shows the passage position of the light ray in an aperture_diaphragm | restriction. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 8B is a diagram showing astigmatism and field curvature. The horizontal axis of FIG.8 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.8 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 8B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG. 8C is a diagram showing distortion. The horizontal axis of FIG.8 (c) shows a distortion aberration (distortion) (a unit is a percentage). The vertical axis | shaft of FIG.8 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 17 is a table showing lens data of the imaging optical system according to Example 4. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 7, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 18 is a table showing the radius of curvature R and the conic constant k at the apex of the expression (5) of the first to eighth lens surfaces.

  Table 19 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the first surface to the fourth surface.

  Table 20 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 21 is a table showing the coefficient Bi of the polynomial in Expression (6) representing the phase function of the diffraction grating.

Example 5
FIG. 9 is a diagram illustrating a configuration of the imaging optical system according to the fifth embodiment. The imaging optical system according to Example 5 includes a first lens 501, a second lens 502, a third lens 503, and a fourth lens 504 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 501 and on the image side of the apex of the object side surface of the first lens 501. Specifically, the stop is on the object side surface of the first lens 501. The light that has passed through the first lens 501, the second lens 502, the third lens 503, and the fourth lens 504 passes through the glass plate 505 and reaches the image plane 506.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of ring zones of the diffraction grating is five.

  FIG. 10 is a diagram illustrating aberrations of the imaging optical system according to the fifth example. FIG. 10 shows aberrations for three wavelengths in the visible region. FIG. 10A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 10A indicates the focal position in the optical axis direction with the image plane position as a reference (unit: millimeter). The vertical axis | shaft of Fig.10 (a) shows the passage position of the light beam in an aperture_diaphragm | restriction. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 10B is a diagram showing astigmatism and field curvature. The horizontal axis of FIG.10 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.10 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 10B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG.10 (c) is a figure which shows a distortion aberration. The horizontal axis of FIG.10 (c) shows a distortion aberration (distortion) (a unit is a percentage). The vertical axis | shaft of FIG.10 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 22 shows lens data of the imaging optical system according to Example 5. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 7, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 23 is a table showing the radius of curvature R and the conic constant k at the apex of Expression (5) of the first to eighth lens surfaces.

  Table 24 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the first surface to the fourth surface.

  Table 25 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 26 is a table showing the coefficient Bi of the polynomial in Expression (6) that represents the phase function of the diffraction grating.

Example 6
FIG. 11 is a diagram illustrating a configuration of the imaging optical system according to the sixth embodiment. The imaging optical system according to Example 6 includes a first lens 601, a second lens 602, a third lens 603, and a fourth lens 604 from the object side to the image side. The stop is located on the object side of the image side surface of the first lens 601 and on the image side of the apex of the object side surface of the first lens 601. Specifically, the stop is on the object side surface of the first lens 601. The light that has passed through the first lens 601, the second lens 602, the third lens 603, and the fourth lens 604 passes through the glass plate 605 and reaches the image plane 606.

  In this embodiment, a diffraction grating for achromatic color is provided on the image plane side of the first lens. In the present embodiment, the main part of the achromatic function is carried by the diffraction grating, and the number of rings of the diffraction grating is nine.

  FIG. 12 is a diagram illustrating aberrations of the imaging optical system according to Example 6. FIG. 12 shows aberrations for three wavelengths in the visible region. FIG. 12A is a diagram showing spherical aberration and axial chromatic aberration. The horizontal axis in FIG. 12A indicates the focal position in the optical axis direction with respect to the image plane position (unit: millimeter). The vertical axis | shaft of Fig.12 (a) shows the passage position of the light ray in a stop. 0 on the vertical axis indicates that the light beam passes through the center of the stop, and the maximum value on the vertical axis indicates that the light beam passes through the end of the stop. FIG. 12B is a diagram showing astigmatism and curvature of field. The horizontal axis of FIG.12 (b) shows the focus position of an optical axis direction (a unit is a millimeter). The vertical axis | shaft of FIG.12 (b) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle. In FIG. 12B, T represents the shape of the meridional image plane, and S represents the shape of the sagittal image plane. FIG. 12C is a diagram showing distortion. The horizontal axis of FIG.12 (c) shows a distortion aberration (distortion) (a unit is a percentage). The vertical axis | shaft of FIG.12 (c) shows a visual field. 0 on the vertical axis indicates a viewing angle of 0, and the maximum value on the vertical axis indicates the maximum viewing angle.

  Table 27 is a table showing lens data of the imaging optical system according to Example 6. The 1st thru | or 8th surface shows the surface of the 1st thru | or 4th lens, and the 9th thru | or 10th surface shows the surface of a glass plate. In Table 7, as an example, the surface interval of the first surface (the object side surface of the first lens) is the interval between the first surface and the second surface (the image side surface of the first lens).

  Table 28 is a table showing the radius of curvature R and the conic constant k at the apex of Expression (5) of the first to eighth lens surfaces.

  Table 29 is a table showing the coefficient Ai of the polynomial in Expression (5) for the first to fourth lens surfaces.

  Table 30 is a table showing the coefficient Ai of the polynomial in Expression (5) of the lens surfaces of the fifth surface to the eighth surface.

Table 31 is a table showing the coefficient Bi of the polynomial in Expression (6) representing the phase function of the diffraction grating.

Claims (3)

The image surface side from the object side, a first lens having a positive power, a third lens and a fourth lens having a negative power with the second lens is a convex meniscus lens on the image side, the positive power The power of the third lens with respect to the principal ray in the meridional direction is positive in the paraxial region, the power decreases and becomes negative as the distance from the optical axis increases, and the fourth lens in the meridional direction. The power with respect to the chief ray is negative in the paraxial region, and the power increases as the distance from the optical axis increases. The first lens has a diffraction grating on the image side surface, and the focal length of the second lens is f ₂ , The composite focal length of the imaging optical system is f _T , and the distance from the object side vertex to the image plane among the vertices of the object side surface of the aperture plane or the first lens is TTL,

An imaging optical system that satisfies The focal length of the fourth lens is f ₄ , and the refractive index of the d-line of the material of the fourth lens is n ₄ .

The imaging optical system according to claim 1, wherein:   The imaging optical system according to claim 1, wherein the diffraction grating is formed of a ring body, and the number of ring zones is 10 or less.

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