Diffractive optical element, optical system having the same, and imaging apparatus

Abstract

A diffractive optical element includes a first lens having a convex surface, a second lens having a concave surface, disposed in such a manner that the concave surface of the second lens faces the convex surface of the first lens, and a diffraction grating section formed between the first and the second lenses and having positive optical power through diffraction. The diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from a side closer to the first lens; the second diffraction grating has a refractive index larger than that of the first diffraction grating, and an inner diameter of a grating wall surface of the diffraction grating section decreases as approaching to the second lens from the first lens.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a diffractive optical element used for an optical system such as a digital camera.

Description of Related Art

A diffractive optical element having a saw-tooth diffraction grating is known as an optical element used to reduce the chromatic aberration of an optical system. When a diffractive optical element is used for the optical system, it is important to reduce flare arising when light is reflected or refracted by a wall surface of the diffraction grating.

Japanese Patent Application Laid-Open No. 2008-170594 discusses a technique for reducing flare by forming a diffractive optical element in such a manner that a grating wall surface of the diffractive optical element becomes parallel with incident light.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a diffractive optical element includes a first lens having a convex surface, a second lens having a concave surface, disposed in such a manner that the concave surface of the second lens faces the convex surface of the first lens, and a diffraction grating section formed between the first and the second lenses and having positive optical power through diffraction, wherein the diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from a side closer to the first lens, the second diffraction grating having a larger refractive index than the first diffraction grating, and wherein an inner diameter of an arbitrary grating wall surface of the diffraction grating section decreases as approaching to the second lens from the first lens.

According to an aspect of the present invention, a diffractive optical element includes a first lens having a convex surface, a second lens having a concave surface, disposed in such a manner that the concave surface of the second lens faces the convex surface of the first lens, and a diffraction grating section formed between the first and the second lenses and having positive optical power through diffraction, wherein the diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from a side closer to the first lens, the second diffraction grating having a larger refractive index than the first diffraction grating, and wherein the following conditional expression is satisfied:θ_H×θ_M<0

where θ_H is an angle formed by an arbitrary grating wall surface and an optical axis, and θ_M is an angle formed by the optical axis and a normal to an enveloping surface of the first diffraction grating, the enveloping surface formed by connecting apical portions of the first diffraction grating, at a position where the arbitrary grating wall surface contacts the enveloping surface, wherein the angle is negative when measured in the clockwise direction with respect to the optical axis, and positive when measured in the counterclockwise direction with respect to the optical axis, such that angles θ_H and θ_M have different signs.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views illustrating a diffractive optical element according to a first exemplary embodiment.

FIGS. 2A, 2B, and 2C are schematic views illustrating a diffractive optical element according to a first comparative example.

FIGS. 3A, 3B, and 3C are schematic views illustrating a diffractive optical element according to a second comparative example.

FIG. 4 is a cross-sectional view illustrating an optical system according to a second exemplary embodiment in a state of being focused at infinity.

FIG. 5 illustrates θ_M, θ_H, and θ_HM in a diffractive optical element of the optical system according to the second exemplary embodiment.

FIG. 6 illustrates aberrations of the optical system according to the second exemplary embodiment in a state of being focused at infinity.

FIG. 7 is a cross-sectional view illustrating an optical system according to a third exemplary embodiment in a state of being focused at infinity.

FIG. 8 illustrates θ_M, θ_H, and θ_HM in a diffractive optical element of the optical system according to the third exemplary embodiment.

FIG. 9 illustrates aberrations of the optical system according to the third exemplary embodiment in a state of being focused at infinity.

FIG. 10 is a cross-sectional view illustrating an optical system according to a fourth exemplary embodiment when focusing at infinity.

FIG. 11 illustrates θ_M, θ_H, and θ_HM in a diffractive optical element of the optical system according to the fourth exemplary embodiment.

FIG. 12 illustrates aberrations of the optical system of according to the fourth exemplary embodiment in a state of being focused at infinity.

FIG. 13 is a cross-sectional view illustrating an optical system according to a fifth exemplary embodiment in a state of being focused at infinity.

FIG. 14 illustrates θ_M, θ_H, and θ_HM in a diffractive optical element of the optical system according to the fifth exemplary embodiment.

FIG. 15 illustrates aberrations of the optical system according to the fifth exemplary embodiment in a state of being focused at infinity.

FIG. 16 is a cross-sectional view illustrating an optical system according to a sixth exemplary embodiment in a state of being focused at infinity.

FIG. 17 illustrates θ_M, θ_H, and θ_HM in a diffractive optical element of the optical system according to the sixth exemplary embodiment.

FIG. 18 illustrates aberrations of the optical system according to the sixth exemplary embodiment in a state of being focused at infinity.

FIG. 19 is a schematic view illustrating an imaging apparatus.

FIG. 20 is a schematic view illustrating an inner diameter of a grating wall surface.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the present invention will be described below.

FIGS. 1A and 1B are schematic views illustrating a diffractive optical element (hereinafter, referred to as a DOE) according to a first exemplary embodiment.

FIG. 1A is a cross-sectional view illustrating a DOE 10 according to the present exemplary embodiment. As illustrated in FIG. 1A, the DOE 10 according to the present exemplary embodiment has a first lens 12 and a second lens 13. The first lens 12 has a convex surface, and the second lens 13 has a concave surface. The second lens 13 is disposed in such a manner that the concave surface of the second lens 13 faces the convex surface of the first lens 12. The first lens 12 has an optical axis OL.

A diffraction grating section 14 is formed between the first lens 12 and the second lens 13 in a space where the concave surface of the second lens 13 faces the convex surface of the first lens 12. When the DOE 10 is used for an optical system, the DOE 10 is disposed in such a manner that the first lens 12 is disposed on the object side of the second lens 13. More specifically, incident light enters the DOE 10 from the side of the first lens 12, such that the incident light is diffracted by the diffraction grating section 14.

FIG. 1B is an enlarged view illustrating the portion enclosed in dotted lines illustrated in FIG. 1A. In the DOE 10 according to the present exemplary embodiment, the diffraction grating section 14 is formed of a first diffraction grating 15 and a second diffraction grating 16 which are closely stacked in this order from the side of the first lens 12 to the second lens 13. The interface between the first diffraction grating 15 and the second diffraction grating 16 has a saw-tooth shape (sawtooth profile), where a grating surface 14a and a grating wall surface 14b are alternately formed.

Both of the first diffraction grating 15 and the second diffraction grating 16 are concentrically shaped centered on the optical axis OL. A lens action can be given to the diffraction grating section 14 by changing the grating pitch (distance between adjoining grating wall surfaces 14b) of the first diffraction grating 15 and the second diffraction grating 16. In the DOE 10 according to the present exemplary embodiment, the optical power by the diffraction of the diffraction grating section 14 is positive.

Now, the grating wall surface 14b of the diffraction grating section 14 will be described. As illustrated in FIG. 1B, the grating wall surface 14b is shaped to be inclined relative to a straight line 17 parallel to the optical axis OL. That is, the grating wall surface 14b of the diffractive optical element 10 is not parallel to the optical axis OL. More specifically, the grating wall surface 14b is inclined in such a manner that the inner diameter of the grating wall surface 14b gradually decreases as the grating wall surface 14b goes from a point on the first lens 12 to a point on the second lens 13, as shown in FIG. 20. The grating wall surface of which inner diameter is illustrated in FIG. 20 is a rotationally symmetric surface with respect to the optical axis. The grating wall surface of which inner diameter is illustrated in FIG. 20 can be also said to be a part of a conical surface formed about the optical axis as a rotational axis.

From another viewpoint, the grating wall surface 14b satisfies the following formula (1).θ_H×θ_M<0  (1)where θ_H is the angle formed by the grating wall surface 14b and the optical axis OL, and θ_M is the angle formed by the optical axis OL and a normal 2 to an enveloping surface 19 of the first grating (the enveloping surface is formed by connecting apical portions of the first diffraction grating 15) at the position where the grating wall surface 14b contacts the enveloping surface 19.

Referring to the formula (1), the angle is negative when measured in the clockwise direction with respect to the optical axis, and positive when measured in the counterclockwise direction with respect to the optical axis. Formula (1) means that angles θ_H and θ_M have different signs.

Inclining the grating wall surface 14b with respect to the optical axis OL in this manner enables reducing the angle formed by incident light 18 and the grating wall surface 14b when the incident light 18 enters the diffraction grating section 14 as convergence light from the side of the first lens 12. This enables reducing the amount of light incident on the grating wall surface 14b out of the incident light 18 and reducing flare generation caused by the grating wall surface 14b.

Next, the diffraction efficiency of light in the diffraction grating section 14 of the DOE 10 will be described.

The condition under which the diffraction efficiency of the m-th order diffraction light in the DOE 10 is maximized is given by the formula (2).φ(λ)=(N_R(λ)−N_L(λ))×d=mλ  (2)where λ is the wavelength, N_L(λ) is the refractive index of the first diffraction grating 15, and N_R(λ) is the refractive index of the second diffraction grating 16.

φ(λ) denotes the optical path length difference in the diffraction grating section 14. d denotes the distance between the enveloping surface 19 of the apical portions of the first diffraction grating 15 and the enveloping surface of the apical portions of the second diffraction grating 16, and is equivalent to the grating height of the diffraction grating section 14. Referring to the formula (2), m denotes the diffraction order represented by an arbitrary integral value. The diffraction order of the diffraction light diffracted in the direction toward the optical axis with respect to the 0th order diffraction light is positive, and the diffraction order of the diffraction light diffracted in the direction away from the optical axis with respect to the 0th order diffraction light is negative.

Referring to the formula (2), forming the first diffraction grating 15 and the second diffraction grating 16 by using materials having different refractive indices and suitably designing the grating height d enable improving the diffraction efficiency at an arbitrary wavelength. To improve the diffraction efficiency in a wider wavelength band, it is necessary to form the diffraction grating section 14 by combining a material having a relatively high refractive index and low dispersion with a material having a relatively low refractive index and high dispersion.

From the viewpoint of improving the diffraction efficiency, whichever the first diffraction grating 15 or the second diffraction grating 16 in the diffraction grating section 14 may be provided with a relatively high refractive index. However, in the DOE 10 according to the present exemplary embodiment, the refractive index of the second diffraction grating 16 is made larger than the refractive index of the first diffraction grating 15 to facilitate the manufacturing of the DOE 10 while reducing flare generation. The configuration of the present invention will be described below with reference to a comparative example.

When positive optical power is given to the diffraction grating section 14, the inclination of the grating surface 14a is determined by the magnitude relationship between the refractive indices of the first diffraction grating 15 and the second diffraction grating 16. When the refractive index of the second diffraction grating 16 is made larger than the refractive index of the first diffraction grating 15, the grating surface 14a of the diffraction grating section 14 having positive optical power has a shape as illustrated in FIG. 1B.

The grating wall surface 14b of the diffraction grating section 14 has a shape inclined relative to the optical axis as described above to reduce flare. Therefore, as illustrated in FIG. 1B, an angle θ_top formed by the grating wall surface 14b and the grating surface 14a can be increased to enable easily forming the first diffraction grating 15 and the second diffraction grating 16.

Although a diffraction grating is generally manufactured by forming resin using a mold, the DOE 10 according to the present exemplary embodiment illustrated in FIG. 1B is shaped to minimize interference between the mold and the diffraction grating when the mold is released by moving in the direction parallel to the optical axis. Therefore, a mold releasing process in the manufacturing of the DOE 10 can be performed easily.

FIGS. 2A to 2C are schematic views illustrating a DOE 20 according to a first comparative example. Similar to the DOE 10 according to the first exemplary embodiment, the DOE 20 according to the first comparative example is formed of a first lens 22 having a convex surface, a second lens 23 having a concave surface facing the convex surface of the first lens 22, and a diffraction grating section 24 formed between the first lens 22 and the second lens 23.

FIG. 2A is a side view illustrating the DOE 20 according to the first comparative example. FIG. 2B is an enlarged view illustrating a cross-section of the area drawn by broken lines illustrated in FIG. 2A. In the DOE according to the first comparative example, the diffraction grating section 24 is formed of the first diffraction grating 25 and the second diffraction grating 26 stacked in this order from the side closer to the first lens 22. The diffraction grating section 24 of the DOE 20 has positive optical power like the DOE 10 according to the first exemplary embodiment. The DOA 20 differs from the DOE 10 according to the first exemplary embodiment in that the refractive index of the second diffraction grating 26 is made smaller than the refractive index of the first diffraction grating 25. That is, in the comparative example, the refractive index of the second diffraction grating 26 is made smaller than the refractive index of the first diffraction grating 25. Therefore, the inclination of the grating surface 24a in the DOE 20 according to the first comparative example is inclined in the direction opposite to the grating surface 14a of the DOE 10 according to the first exemplary embodiment.

In the DOE 20 according to the first comparative example, similar to the DOE 10 according to the first exemplary embodiment, the grating wall surface 24b is inclined to decrease the angle formed by the incident light and the grating wall surface 24b. However, as illustrated in FIG. 2B, the angle θ_top formed by the grating surface 24a and the grating wall surface 24b of the DOE 20 is smaller than θ_top of the DOE 10 according to the first exemplary embodiment. As understood from FIG. 2B, when the first diffraction grating 25 or the second diffraction grating 26 is formed of the DOE 20 using a mold, the shape of the grating (25 or 26) will prevent easy separation between the diffraction grating and the mold when the mold is released in the direction parallel to the optical axis. Accordingly, the DOE 20 is more difficult to manufacture than the DOE 10.

On the other hand, in the DOE 20 according to the first comparative example, when the grating wall surface 24b is inclined as illustrated in FIG. 2C to prevent interference between the mold and the diffraction grating, mold release operations could be improved but the angle formed by the incident light 18 and the grating wall surface 24b will increase. As a result, the amount of light incident on the grating wall surface 24b increases, and notable flare is generated.

A case where the shapes of the first and the second lenses are different from those in the DOE 10 with reference to a second comparative example. FIG. 3A is a cross-sectional view illustrating a DOE 30 according to the second comparative example. The DOE 30 according to the second comparative example includes a first lens 32 having a concave surface, a second lens 33 having a convex surface facing the concave surface of the first lens 32, and a diffraction grating section 34 formed between the first lens 32 and the second lens 33. The diffraction grating section 34 includes a first diffraction grating 35 and a second diffraction grating 36 stacked in this order from the side closer to the first lens 32. Similar to the DOE according to the first exemplary embodiment, the diffraction grating section 34 of the DOE 30 has positive optical power.

FIG. 3B is an enlarged view illustrating the area enclosed by dotted lines illustrated in FIG. 3A when the refractive index of the second diffraction grating 36 is made larger than the refractive index of the first diffraction grating 35 in the DOE 30. Similar to the DOE 10 according to the first exemplary embodiment, also in the DOE 30 according to the second comparative example, the grating wall surface 34b is inclined to decrease the angle formed by the incident light 18 and the grating wall surface 34b. Therefore, flare caused by the grating wall surface 34b can also be reduced in the DOE 30.

However, as illustrated in FIG. 3B, the angle θ_top formed by the grating surface 34a and the grating wall surface 34b of the DOE 30 is smaller than θ_top of the DOE 10 according to the first exemplary embodiment. Therefore, the DOE 30 is more difficult to manufacture than the DOE 10.

FIG. 3C is an enlarged view illustrating the area enclosed by dotted lines illustrated in FIG. 3A when the refractive index of the first diffraction grating 35 is made larger than the refractive index of the second diffraction grating 36 in the DOE 30. In this case, the angle θ_top formed by the grating surface 34a and the grating wall surface 34b can be made larger than θ_top in the case illustrated in FIG. 3B. However, in the configuration illustrated in FIG. 3C, the shape of the DOE 30 will cause interference between the diffraction grating and the mold when the mold for forming the first diffraction grating 35 or the second diffraction grating 36 is released in the direction parallel to the optical axis. Therefore, the DOE 30 is more difficult to manufacture than the DOE 10.

As described above, unlike the DOE 20 according to the first comparative example and the DOE 30 according to the second comparative example, the DOE 10 according to the first exemplary embodiment can be easily manufactured while reducing flare generation. In the diffraction grating section 14 having positive optical power formed on a curved surface having a center of curvature on the light incident side, the grating wall surface 14b is inclined in such a manner that the refractive index of the second diffraction grating 16 is made larger than the refractive index of the first diffraction grating 15 and accordingly the angle formed by the grating wall surface 4b and the incident light decreases.

To facilitate the manufacturing of the DOE 10 while reducing flare generation, it is desirable that the inner diameter of the grating wall surface 14b gradually decreases as the grating wall surface 14b comes closer from the first lens 12 to the second lens 13 with at least 50 percent of all the annulars of the diffraction grating section 14. It is more desirable that the inner diameter of the grating wall surface 14b gradually decreases as the grating wall surface 14b comes closer from the first lens 12 to the second lens 13 with at least 70 percent of all the annulars of the diffraction grating section 14, still more desirably, with all of the annulars of the diffraction grating section 14.

From another viewpoint, to facilitate the manufacturing of the DOE 10 while reducing flare generation, it is desirable that the grating wall surface 14b is inclined to satisfy the formula (1) with at least 50 percent of all the annulars of the diffraction grating section 14. It is more desirable that the grating wall surface 14b is inclined to satisfy the formula (1) with at least 70 percent of all the annulars of the diffraction grating section 14, still more desirably, with all of the annulars of the diffraction grating section 14.

With the DOE 10, it is desirable that the first lens 12 is a positive lens. A positive lens refers to a convex lens of which the thickness decreases with increasing distance from the optical axis. As described above, the first lens 12 has a convex surface. Therefore, when a negative lens is used as the first lens 12, the absolute value of the curvature of the lens surface of the first lens 12 on the side where the diffraction grating section 14 is not formed will be too large. In this case, the amount of aberration on the lens surface of the first lens 12 on the side where the diffraction grating section 14 is not formed will increase.

With the DOE 10, it is desirable that the second lens 13 is a negative lens. A negative lens refers to a concave lens of which the thickness increases with increasing distance from the optical axis. As described above, the second lens 13 has a concave surface. Therefore, when a positive lens is used as the second lens 13, the absolute value of the curvature of the lens surface of the second lens 13 on the side where the diffraction grating section 14 is not formed will be too large. In this case, the amount of aberration on the lens surface of the second lens 13 on the side where the diffraction grating section 14 is not formed will increase.

It is desirable that the following conditional expression (3) is satisfied.20<υ_LL−υ_LR<60  (3)where υ_LL is the Abbe number of the first lens 12, and υ_LR is the Abbe number of the second lens 13.

The Abbe number υ_d is given by υ_d=(N_d−1)/(N_F−N_C), where N_g, N_F, N_d, and N_C are refractive indices for the Fraunhofer g line (435.8 nm), F line (486.1 nm), d line (587.6 nm), and C line (656.3 nm), respectively.

When the difference between υ_LL and υ_LR is small, to sufficiently correct the chromatic aberration by using the DOE 10, it is necessary to increase the absolute value of the curvature of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed. When the difference between υ_LL and υ_LR, (υ_LL×υ_LR), falls below the lower limit of the formula (3), the absolute values of the curvatures of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed will be too large, and the incident angle of light incident on the diffraction grating section 14 will be too large. In this case, the amount of light incident on the grating wall surface 14b increases, making it difficult to sufficiently reduce flare.

On the other hand, when the difference between υ_LL and υ_LR, (υ_LL−υ_LR), exceeds the upper limit of the formula (3), the curvatures of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed will be too small, making it difficult to correct the spherical aberration.

It is desirable that the range of the formula (3) is set within the range of the following formula (3a), more desirably, set within the range of the formula (3b).27<υ_LL−υ_LR<55  (3a)30<υ_LL−_LR<53  (3b)

It is desirable that the following formula (4) is satisfied.0.8<N_L/N_LL<1.2  (4)where N_LL is the refractive index of the first lens 12 for the d line, and N_L is the refractive index of the first diffraction grating 15 for the d line.

When N_L is smaller than N_LL, the interface between the first lens 12 and the first diffraction grating 15 has positive refractive power. When N_L is smaller than N_LL to such an extent that N_L/N_LL falls below the lower limit of the formula (4), light that entered the DOE 10 is largely diffracted at the interface between the first lens 12 and the first diffraction grating 15. As a result, the incident angle of light incident on the diffraction grating section 14 will be too large. In this case, the amount of light incident on the grating wall surface 14b increases, making it difficult to sufficiently reduce flare.

On the other hand, when the refractive index N_L of the first diffraction grating 15 is larger than the refractive index N_LL of the first lens 12 to such an extent that N_L/N_LL exceeds the upper limit of the formula (4), the range of options for the materials to be used for the first diffraction grating 15 and the second diffraction grating 16 will be narrowed. As a result, it becomes difficult to acquire high diffraction efficiency in a wide wavelength range.

It is desirable that the range of the formula (4) is set within the range of the following formula (4a).0.9<N_L/N_LL<1.1  (4a)

The angle formed by the arbitrary grating wall surface 14b and the surface normal 2 of an enveloping surface 19 (formed by connecting apical portions of the first diffraction grating 15) at the position where the grating wall surface 14b contacts the enveloping surface 19 has an absolute angle value θ_HM. In this case, it is desirable that the following conditional expression (5) is satisfied.5 degrees<Δθ_HM<45 degrees  (5)where Δθ_HM is the absolute value of the difference between the maximum and the minimum values of θ_HM in the diffraction grating section 14.

A decrease in the value of the formula (5), Δθ_HM, is equivalent to an increase in the curvature radius of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed. When the curvature radii of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed are large to such an extent that Δθ_HM falls below the lower limit of the formula (5), it becomes difficult to correct aberrations such as the spherical aberration.

On the other hand, when the curvature radii of the lens surfaces of the first lens 12 and the second lens 13 on the sides where the diffraction grating section 14 is formed are small to such an extent that Δθ_HM exceeds the upper limit of the formula (5), the incident angle of light incident on the diffraction grating section 14 will be too large. In this case, it becomes difficult to reduce flare generation in the diffraction grating section 14.

It is desirable that the range of the value of the formula (5) is set within the range of the following formula (5a).10 degrees<Δθ_HM<40 degrees  (5a)

It is desirable that at least one of the first diffraction grating 15 and the second diffraction grating 16 is formed of resin. Since it is easy to form resin by using a mold, forming at least one of the first diffraction grating 15 and the second diffraction grating 16 with resin facilitates the forming of the diffraction grating section 14.

Although, in the DOE 10 illustrated in FIGS. 1A and 1B, the first diffraction grating 15 and the second diffraction grating 16 are closely bonded at the diffraction grating section 14, a thin film may be formed between the first diffraction grating 15 and the second diffraction grating 16. For example, a thin film may be formed only at the grating wall surface 14b, or a thin film may be formed at both of the grating surface 14a and the grating wall surface 14b. When a thin film is formed at the portion equivalent to the grating surface 14a illustrated in FIG. 1B, the grating surface 14a serves as the interface between the first diffraction grating 15 and the thin film. When a thin film is formed at the portion equivalent to the grating wall surface 14b illustrated in FIG. 1B, the grating wall surface 14b serves as the interface between the first diffraction grating 15 and the thin film.

Optical systems according to second to sixth exemplary embodiments of the present invention will be described below.

An optical system according to each exemplary embodiment is an imaging optical system used for an imaging apparatus such as a video camera, digital still camera, and silver-halide film camera.

An optical system according to the second exemplary embodiment will be described below. FIG. 4 is a cross-sectional view illustrating an optical system 100 according to the second exemplary embodiment in a state of focusing at infinity. Referring to FIG. 4, the left side is the object side, and the right side is the image side. The optical system 100 includes a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, an aperture diaphragm SP, and a third lens group L3 having negative refractive power, which are disposed in this order from the object side to the image side. In focusing from infinity to the shortest distance, the second lens group L2 moves to the image side. The first lens group L1 and the third lens group L3 do not move while focusing.

FIG. 4 includes an image plane (IP). When using the optical system 100 as the imaging optical system of a digital camera, a solid-state image sensor such as a complementary metal-oxide semiconductor (CMOS) sensor or charge-coupled device (CCD) sensor is disposed on the image plane IP. When the optical system 100 is used as the imaging optical system of a silver-halide film camera, the image plane IP is equivalent to a film plane.

The first lens group L1 includes a DOE 110 arranged on the image side of one or more lenses. Using the DOE 110 as an optical system enables suitably correcting aberrations such as the chromatic aberration.

As illustrated in FIG. 4, the DOE 110 includes a first lens 112 having a convex surface on the image side, and a second lens 113 having a concave surface on the object side. A diffraction grating section 114 is formed between the first lens 112 and the second lens 113. In the optical system 100, the DOE 110 is disposed in such a manner that the first lens 112 is disposed on the object side of the second lens 113.

The configuration of the diffraction grating section 114 is similar to the configuration of the DOE 10 according to the first exemplary embodiment. More specifically, the diffraction grating section 114 includes a first diffraction grating and a second diffraction grating having a larger refractive index than the first diffraction grating in this order from the first lens 112. The optical power by the diffraction of the diffraction grating section 114 is positive.

Each grating wall surface of the diffraction grating section 114 is inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface gradually decreases as the grating wall surface comes closer from the first lens 112 to the second lens 113. From another viewpoint, the grating wall surface of the diffraction grating section 114 is inclined to satisfy the formula (1).

For each grating wall surface of the diffraction grating section 114 of the DOE 110, relations between the distance from the grating wall surface to the optical axis, and respective θ_H (degrees), θ_M (degrees), and θ_HM (degrees) are illustrated in FIG. 5. Referring to the cross-sectional view of the optical system 100 illustrated in FIG. 4, the shape of the diffraction grating section 114 is symmetric with respect to the optical axis. FIG. 5 illustrates the relations only for the upper side of the optical axis. θ_H and θ_M are positive when measured in the counterclockwise direction with respect to the optical axis, and negative when measured in the clockwise direction with respect to the optical axis. θ_HM is calculated by θ_HM=|θ_H−θ_M|.

As illustrated in FIG. 5, in the DOE 110, all of the grating wall surfaces are negatively inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface decreases as the grating wall surface comes closer from the first lens 112 to the second lens 113. The DOE 110 satisfies the formula (1). Therefore, as described above with reference to the first exemplary embodiment, the DOE 110 is shaped to facilitate the manufacturing while reducing flare generation. As illustrated in FIG. 5, the DOE 110 is shaped to satisfy the formula (5).

In the DOE 110 according to the present exemplary embodiment, the first diffraction grating is formed of resin (N_d=1.566, υ_d=19.0, and θ_gF=0.418) made of a mixture of acrylic resin and Indium Tin Oxide (ITO) fine particles. θ_gF denotes the partial dispersion ratio and is given by the following formula (6).θ_gF=(N_g−N_F)/(N_F-N_C)  (6)where N_g, N_F, N_d, and N_C are the refractive indices for the Fraunhofer g line, F line, d line, and C line, respectively.

The second diffraction grating is formed of resin (N_d=1.619, υ_d=43.2, and θ_gF=0.564) made of a mixture of acrylic resin and Zirconium Oxide (ZrO2) fine particles. The grating height d is 10.79 μm.

Forming the first and second diffraction gratings by using such materials enables obtaining high diffraction efficiency in a wide wavelength range.

FIG. 6 illustrates aberration charts of the optical system 100 in a state of focusing at infinity. Referring to FIG. 6, lines d, g, and C denote the aberration curves for the Fraunhofer d line, g line, and C line, respectively. Referring to the astigmatism chart, ΔM denotes astigmatism at the meridional image plane, ΔS denotes astigmatism at the sagittal image plane, Y denotes the image height (mm), and Fn denotes the F-number.

As illustrated in FIG. 6, in the optical system 100, aberrations including the chromatic aberration are suitably corrected.

To further improve the effect of correcting the chromatic aberration by the DOE 110, it is desirable to dispose the DOE 110 at a position where a large diameter of the luminous flux of axial ray is provided. Generally, the diameter of the luminous flux of axial ray in a telephoto lens is larger on the object side of the aperture diaphragm SP than on the image side thereof. Therefore, it is more desirable to dispose the DOE 110 on the object side of the aperture diaphragm SP.

The partial optical system composed of all of the lenses disposed on the object side of the DOE 110 has positive refractive power. When the refractive power of the partial optical system disposed on the object side of the DOE 110 is positive, the axial ray will enter the DOE 110 as convergence (convergent) light. This enables reducing the angle formed by the grating wall surface of the DOE 110 and the incident light, making it possible to effectively reduce flare. When only one lens is disposed on the object side of the DOE 110, the lens serves as a partial optical system disposed on the object side of the DOE 110.

It is desirable that the following conditional expression (7) is satisfied.0.10<L_d/L_t<0.50  (7)where L_d is the distance on the optical axis from the lens surface of the optical system on the most object side to the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed, and L_t is the total length of the optical system. L_t is the distance on the optical axis from the lens surface of the optical system on the most object side to the image plane.

Although, as described above, disposing the DOE 110 on the object side of the aperture diaphragm SP enables more effectively correcting the chromatic aberration, light that does not normally reach the image plane (unnecessary light), such as light from the outside of the imaging field angle, is likely to enter the DOE 110. If such unnecessary light that entered the DOE 110 is reflected on the grating wall surface, the unnecessary light reaches the image plane and produces flare. When L_d is small to such an extent that L_d/L_t falls below the lower limit of the formula (7), unnecessary light other than the imaging light is likely to enter the DOE 110, producing flare. On the other hand, when L_d/L_t exceeds the upper limit of the formula (7), the diameter of the luminous flux of axial ray incident on the DOE 110 decreases, making it difficult to sufficiently correct the chromatic aberration. When L_d/L_t exceeds the upper limit of the formula (7), the optical system will increase in size.

It is desirable that the range of the formula (7) is set within the range of the following formula (7a).0.20<L_d/L_t<0.45  (7a)

It is desirable that the following conditional expression (8) is satisfied.−2.0<f/(R_d×Fn)<−0.20  (8)where R_d is the curvature radius of the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed, f is the focal length of the entire optical system, and Fn is the F-number.

When the absolute value of the curvature of the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed is large to such an extent that f/(R_d×Fn) falls below the lower limit of the formula (8), the incident angle of light incident on the diffraction grating section 114 will increase. As a result, the amount of light incident on the grating wall surface will increase, making it difficult to reduce flare.

On the other hand, when the absolute value of the curvature of the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed is small to such an extent that f/(R_d×Fn) exceeds the upper limit of the formula (8), it becomes difficult to correct aberrations such as the spherical aberration.

It is desirable that the range of the formula (8) is set within the range of the following formula (8a), more desirably, set within the range of the formula (8b).−1.9<f/(R_d×Fn)<−0.30  (8a)−1.8<f/(R_d×Fn)<−0.41  (8b)

Further, it is desirable that the following conditional expression (9) is satisfied.0.6<(E₁−E_d)/L_d+E_d×P_fE_d/R_d<2.0  (9)where E₁ is the effective diameter of the lens surface of the optical system on the most object side, E_d is the effective diameter of the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed, and P_f is the refractive power of the lens surface of the first lens 112 on the object side (the lens surface of the first lens 112 on the side where the diffraction grating section 114 is not formed).

The refractive power P_f of the lens surface of the first lens 112 on the object side is given by the following formula (10).P_f=(N_LL−1)/R_LL  (10)where R_LL is the curvature radius of the lens surface of the first lens 112 on the object side.

The angle formed by the axial marginal ray and the optical axis when the axial marginal ray that entered the optical system 100 is incident on the first lens 112 can be approximately represented by (E₁-E_d)/(2L_d). The angle formed by the travelling directions of the axial marginal ray before and after the axial marginal ray enters the lens surface of the first lens 112 on the object side can be approximately represented by E_d×P_f/2.

The angle formed by the optical axis and the surface normal of the lens surface of the first lens 112 on the side where the diffraction grating section 114 is formed at the incidence position of the axial marginal ray can be represented by E_d/(2R_d). More specifically, the upper and lower limits of the formula (9) correspond to the incident angle of the axial marginal ray incident on the diffraction grating section 114.

When (E₁−E_d)/L_d+E_d×P_f−E_d/R_d exceeds the upper limit of the formula (9), the incident angle of the axial marginal ray incident on the diffraction grating section 114 will be too large. As a result, the amount of light incident on the grating wall surface will increase, making it difficult to reduce flare.

When (E₁−E_d)/L_d+E_d×P_f−E_d/R_d falls below the lower limit of the formula (9), the refractive power of the lens surface of the first lens 112 on the object side becomes too small, or the aberration correction effect by the DOE 110 cannot sufficiently be acquired. As a result, it becomes difficult to correct aberrations of the entire optical system.

It is desirable that the range of the formula (9) is set within the range of the following formula (9a), more desirably, set within the range of the formula (9b).0.70<(E₁−E_d)/L_d+E_d×P_f−E_d/R_d<1.8  (9a)0.75<(E₁−E_d)/L_d+E_d×P_f−E_d/R_d<1.7  (9b)

It is desirable that the following formula (11) is satisfied.10 degrees<|θ_D|<57 degrees  (11)where θ_D is the incident angle of the axial marginal ray incident on the diffraction grating section 114 when focusing at infinity. The incident angle θ_D refers to the angle formed by the axial marginal ray and the surface normal of the lens surface of the first lens 112 on the image side at the incidence position where the axial marginal ray is incident on the diffraction grating section 114.

When |θ_D| exceeds the upper limit of the formula (11), the incident angle of the axial marginal ray incident on the diffraction grating section 114 will be too large. As a result, the amount of light incident on the grating wall surface will increase, making it difficult to reduce flare.

When |θ_D| falls below the lower limit of the formula (11), the refractive power of the lens surface of the first lens 112 on the object side becomes too small, or the aberration correction effect by the DOE 110 cannot sufficiently be acquired. As a result, it becomes difficult to correct aberrations of the entire optical system.

It is desirable that the range of the formula (11) is set within the range of the following formula (11a), more desirably, set within the range of the formula (11b).15 degrees<|θ_D|<51 degrees  (11a)20 degrees<|θ_D|<45 degrees  (11b)

Next, an optical system according to a third exemplary embodiment will be described. FIG. 7 is a cross-sectional view illustrating an optical system 200 according to the third exemplary embodiment in a state of focusing at infinity. The optical system 200 includes a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, an aperture diaphragm SP, and a third lens group L3 having negative refractive power, which are disposed in this order from the object side to the image side. In focusing from infinity to the shortest distance, the second lens group L2 moves to the image side. The first lens group L1 and the third lens group L3 do not move during focusing.

The first lens group L1 includes a DOE 210. As illustrated in FIG. 7, the DOE 210 includes a first lens 212 having a convex surface on the image side, and a second lens 213 having a concave surface on the object side. A diffraction grating section 214 is formed between the first lens 212 and the second lens 213. In the optical system 200, the DOE 210 is disposed in such a manner that the first lens 212 is disposed on the object side of the second lens 213.

The configuration of the diffraction grating section 214 is similar to the configuration of the DOE 10 according to the first exemplary embodiment. More specifically, the diffraction grating section 214 includes a first diffraction grating and a second diffraction grating having a larger refractive index than the first diffraction grating in this order from the first lens 212. The optical power by the diffraction of the diffraction grating section 214 is positive.

Each grating wall surface of the diffraction grating section 214 is inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface gradually decreases as the grating wall surface comes closer from the first lens 212 to the second lens 213. From another viewpoint, the grating wall surface of the diffraction grating section 214 is inclined to satisfy the formula (1).

For each grating wall surface of the diffraction grating section 214 of the DOE 210, relations between the distance from the grating wall surface to the optical axis, and respective θ_H (degrees), θ_M (degrees), and θ_HM (degrees) are illustrated in FIG. 8. Referring to the cross-sectional view of the optical system 200 illustrated in FIG. 7, the shape of the diffraction grating section 214 is symmetric with respect to the optical axis. FIG. 8 illustrates the relations only for the upper side of the optical axis.

As illustrated in FIG. 8, in the DOE 210, all of the grating wall surfaces are negatively inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface decreases as the grating wall surface comes closer from the first lens 212 to the second lens 213. The DOE 210 satisfies the formula (1). Therefore, as described above with reference to the first exemplary embodiment, the DOE 210 is shaped to facilitate the manufacturing while reducing flare generation. Further, as illustrated in FIG. 8, the DOE 210 is shaped to satisfy the formula (5).

In the DOE 210 according to the present exemplary embodiment, the first diffraction grating is formed of a resin material (N_d=1.528, υ_d=34.7, and θ_gF=0.605).

The second diffraction grating is formed of a resin material (N_d=1.557, υ_d=50.2, and θ_gF=0.568). The grating height d is 19.9 μm.

Forming the first and the second diffraction gratings by using such materials enables obtaining high diffraction efficiency in a wide wavelength range.

FIG. 9 illustrates aberration charts of the optical system 200 in a state of focusing at infinity. As illustrated in FIG. 9, in the optical system 200, aberrations including the chromatic aberration are suitably corrected.

Next, an optical system according to a fourth exemplary embodiment will be described. FIG. 10 is a cross-sectional view illustrating an optical system 300 according to the fourth exemplary embodiment in a state of focusing at infinity. The optical system 300 includes a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, an aperture diaphragm SP, and a third lens group L3 having positive refractive power, which are disposed in this order from the object side to the image side. In focusing from infinity to the shortest distance, the second lens group L2 moves to the image side. The first lens group L1 and the third lens group L3 do not move during focusing.

The first lens group L1 includes a DOE 310, As illustrated in FIG. 10, the DOE 310 includes a first lens 312 having a convex surface on the image side, and a second lens 313 having a concave surface on the object side. A diffraction grating section 314 is formed between the first lens 312 and the second lens 313. In the optical system 300, the DOE 310 is disposed in such a manner that the first lens 312 is disposed on the object side of the second lens 313.

The configuration of the diffraction grating section 314 is similar to the configuration of the DOE 10 according to the first exemplary embodiment. More specifically, the diffraction grating section 314 includes a first diffraction grating and a second diffraction grating having a larger refractive index than the first diffraction grating in this order from the first lens 312. The optical power by the diffraction of the diffraction grating section 314 is positive.

Each grating wall surface of the diffraction grating section 314 is inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface gradually decreases as the grating wall surface comes closer from the first lens 312 to the second lens 313. From another viewpoint, the grating wall surface of the diffraction grating section 314 is inclined to satisfy the formula (1).

For each grating wall surface of the diffraction grating section 314 of the DOE 310, relations between the distance from the grating wall surface to the optical axis, and respective θ_H (degrees), θ_M (degrees), and θ_HM (degrees) are illustrated in FIG. 11. Referring to the cross-sectional view of the optical system 300 illustrated in FIG. 10, the shape of the diffraction grating section 314 is symmetric with respect to the optical axis. FIG. 11 illustrates the relations only for the upper side of the optical axis.

As illustrated in FIG. 11, in the DOE 310, all of the grating wall surfaces are negatively inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface decreases as the grating wall surface comes closer from the first lens 312 to the second lens 313. From another viewpoint, the DOE 310 satisfies the formula (1). Therefore, as described above with reference to the first exemplary embodiment, the DOE 310 is shaped to facilitate the manufacturing while reducing flare generation. Further, as illustrated in FIG. 11, the DOE 310 is shaped to satisfy the formula (5).

In the DOE 310 according to the present exemplary embodiment, the first diffraction grating is formed of a resin material (N_d=1.615, υ_d=26.5, and θ_gF=0.612).

The second diffraction grating is formed of a resin material (N_d=1.643, υ_d=38.8, and θ_gF=0.578). The grating height d is 21.5 μm.

Forming the first and the second diffraction gratings by using such materials enables obtaining high diffraction efficiency in a wide wavelength range.

FIG. 12 illustrates aberration charts of the optical system 300 in a state of being focused at infinity. As illustrated in FIG. 12, in the optical system 300, aberrations including the chromatic aberration are suitably corrected.

Next, an optical system according to a fifth exemplary embodiment will be described. FIG. 13 is a cross-sectional view illustrating an optical system 400 according to the fifth exemplary embodiment in a state of being focused at infinity. The optical system 400 includes a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, an aperture diaphragm SP, and a third lens group L3 having positive refractive power, which are disposed in this order from the object side to the image side. In focusing from infinity to the shortest distance, the second lens group L2 moves to the image side. The first lens group L1 and the third lens group L3 do not move during focusing.

The first lens group L1 includes a DOE 410. As illustrated in FIG. 13, the DOE 410 includes a first lens 412 having a convex surface on the image side, and a second lens 413 having a concave surface on the object side. A diffraction grating section 414 is formed between the first lens 412 and the second lens 413. In the optical system 400, the DOE 410 is disposed in such a manner that the first lens 412 is disposed on the object side of the second lens 413.

The configuration of the diffraction grating section 414 is similar to the configuration of the DOE 10 according to the first exemplary embodiment. More specifically, the diffraction grating section 414 includes a first diffraction grating and a second diffraction grating having a larger refractive index than the first diffraction grating in this order from the first lens 412. The optical power by the diffraction of the diffraction grating section 414 is positive.

Each grating wall surface of the diffraction grating section 414 is inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface gradually decreases as the grating wall surface comes closer from the first lens 412 to the second lens 413. From another viewpoint, the grating wall surface of the diffraction grating section 414 is inclined to satisfy the formula (1).

For each grating wall surface of the diffraction grating section 414 of the DOE 410, relations between the distance from the grating wall surface to the optical axis, and respective θ_H (degrees), θ_M (degrees), and θ_HM (degrees) are illustrated in FIG. 14. Referring to the cross-sectional view of the optical system 400 illustrated in FIG. 13, the shape of the diffraction grating section 414 is symmetric with respect to the optical axis. FIG. 14 illustrates the relations only for the upper side of the optical axis.

As illustrated in FIG. 14, in the DOE 410, all of the grating wall surfaces are negatively inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface decreases as the grating wall surface comes closer from the first lens 412 to the second lens 413. The DOE 410 satisfies the formula (1). Therefore, as described above with reference to the first exemplary embodiment, the DOE 410 is shaped to facilitate the manufacturing while reducing flare generation. Further, as illustrated in FIG. 14, the DOE 410 is shaped to satisfy the formula (5).

In the DOE 410 according to the present exemplary embodiment, the first diffraction grating is formed of resin (N_d=1.566, υ_d=19.0, and θ_gF=0.418) made of a mixture of acrylic resin and ITO fine particles.

The second diffraction grating is formed of resin (N_d=1.619, υ_d=43.2, and θ_gF=0.564) made of a mixture of acrylic resin and ZrO2 fine particles. The grating height d is 10.79 μm.

Forming the first and the second diffraction gratings by using such materials enables obtaining high diffraction efficiency in a wide wavelength range.

FIG. 15 illustrates aberration charts of the optical system 400 in a state of being focused at infinity. As illustrated in FIG. 15, in the optical system 400, aberrations including the chromatic aberration are suitably corrected.

Next, an optical system according to a sixth exemplary embodiment will be described. FIG. 16 is a cross-sectional view illustrating an optical system 500 according to the sixth exemplary embodiment in a state of being focused at infinity. The optical system 500 includes a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, an aperture diaphragm SP, and a third lens group L3 having positive refractive power, which are disposed in this order from the object side to the image side. In focusing from infinity to the shortest distance, the second lens group L2 moves to the image side. The first lens group L1 and the third lens group L3 do not move during focusing.

The first lens group L1 includes a DOE 510. As illustrated in FIG. 16, the DOE 510 includes a first lens 512 having a convex surface on the image side, and a second lens 513 having a concave surface on the object side. A diffraction grating section 514 is formed between the first lens 512 and the second lens 513. In the optical system 500, the DOE 510 is disposed in such a manner that the first lens 512 is disposed on the object side of the second lens 513.

The configuration of the diffraction grating section 514 is similar to the configuration of the DOE 10 according to the first exemplary embodiment. More specifically, the diffraction grating section 514 includes a first diffraction grating and a second diffraction grating having a larger refractive index than the first diffraction grating in this order from the first lens 512. The optical power by the diffraction of the diffraction grating section 514 is positive.

Each grating wall surface of the diffraction grating section 514 is inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface gradually decreases as the grating wall surface comes closer from the first lens 512 to the second lens 513. From another viewpoint, the grating wall surface of the diffraction grating section 514 is inclined to satisfy the formula (1).

For each grating wall surface of the diffraction grating section 514 of the DOE 510, relations between the distance from the grating wall surface to the optical axis, and respective θ_H (degrees), θ_M (degrees), and θ_HM (degrees) are illustrated in FIG. 17. Referring to the cross-sectional view of the optical system 500 illustrated in FIG. 16, the shape of the diffraction grating section 514 is symmetric with respect to the optical axis. FIG. 17 illustrates the relations only for the upper side of the optical axis.

As illustrated in FIG. 17, in the DOE 510, all of the grating wall surfaces are negatively inclined relative to the optical axis in such a manner that the inner diameter of the grating wall surface decreases as the grating wall surface comes closer from the first lens 512 to the second lens 513. The DOE 510 satisfies the formula (1). Therefore, as described above with reference to the first exemplary embodiment, the DOE 510 is shaped to facilitate the manufacturing while reducing flare generation. Further, as illustrated in FIG. 17, the DOE 510 is shaped to satisfy the formula (5).

In the DOE 510 according to the present exemplary embodiment, the first diffraction grating is formed of resin (N_d=1.480, υ_d=21.7, and θ_gF=0.383) made of a mixture of fluororesin and ITO fine particles.

The second diffraction grating is formed of a resin material (N_d=1.524, υ_d=51.6, and θ_gF=0.562). The grating height d is 12.95 μm.

Forming the first and the second diffraction gratings by using such materials enables obtaining high diffraction efficiency in a wide wavelength range.

FIG. 18 illustrates aberration charts of the optical system 500 in a state of being focused at infinity. As illustrated in FIG. 18, in the optical system 500, aberrations including the chromatic aberration are suitably corrected.

The first to the fifth numerical examples corresponding to the optical systems 100 to 500 according to the second to the sixth exemplary embodiments, are shown below.

In the surface data according to each numerical example, r denotes the curvature radius of each optical surface, and d (mm) denotes the on-axis interval (distance on the optical axis) between the m-th and the (m+1)-th surfaces, where m is the surface number of the optical system counted from the light incident side, nd is the refractive index of the d line of each optical member, and υ_d is the Abbe number for the d line of the optical member.

“e±B” in aspherical surface data and diffractive surface data means “10^±B”. The aspherical surface shape of the optical surface is represented by the following formula (12) where X is the amount of displacement from the surface vertex in the optical axis direction, H is the height from the optical axis in the direction perpendicular to the optical axis direction, R is the paraxial curvature radius, k is the conic constant, and A₄, A₆, A₈, A₁₀, A₁₂, and A₁₄ are aspheric surface coefficients.

$\begin{array}{cc}x\left(h\right)=\frac{\left(1/r\right)h^2}{1+\sqrt{\left\{1-\left(1+k\right){\left(h/r\right)}^2\right\}}}+A_4{h}^4+A_6{h}^6+A_8{h}^8+A_{10}{h}^{10}+A_{12}{h}^{12}+A_{14}{h}^{14}& \left(12\right)\end{array}$

In each numerical example, the values of d, focal length (mm), F-number, and half-field angle (degrees) are values when the optical system according to each exemplary embodiment is focused on an object at infinity. A back focus BF is the distance from the last lens surface to the image plane. The total lens length is the sum of the distance from the first lens surface to the last lens surface and the value of the back focus BF.

A phase shape ψ of the diffractive surface of the diffractive optical element according to each numerical example is represented by the following formula (13).ψ(h,m)=(2π/mλ0)(C₂h²+C₄h⁴+C₆h⁶ . . . )  (13)

Referring to the formula (13), h is the height from the optical axis, λ0 is the design wavelength, m is the diffraction order, and C_i (i=2, 4, 6 . . . ) is the phase coefficient.

In this case, the power φ_D of the diffraction grating for an arbitrary wavelengths λ and an arbitrary diffraction order m can be represented by the following formula (14), where C₂ is the lowest order phase coefficient.φ_D(λ,m)=−2C₂mλ/λ0  (14)

According to each numerical example, for each diffraction grating constituting the diffractive optical element, the diffraction order m is 1, and the design wavelength λ0 is the wavelength of the d line (587.56 nm).

First Numerical Example

Unit (mm)
Surface data
					Effective
Surface No.	r	d	nd	νd	diameter
1 (Aspherical	139.485	16.40	1.4875	70.2	134.48
surface)
2	600.728	144.94			133.46
3 (Aspherical	85.670	13.09	1.4970	81.5	72.94
surface)
4	−320.403	0.002	1.5660	19.0	71.19
5 (Diffractive	−320.403	0.05	1.6199	43.2	71.18
surface)
6	−320.403	3.50	2.0033	28.3	71.16
7	648.053	2.88			68.88
8	679.319	2.34	1.8081	22.8	67.15
9	584.762	3.20	1.8348	42.7	66.22
10 (Aspherical	134.826	86.48			63.93
surface)
11 (Diaphragm)	∞	34.82			40.14
12	76.077	4.06	1.8081	22.8	29.69
13	−67.737	1.50	1.8348	42.7	29.36
14	69.304	9.24			27.89
15	142.465	3.69	1.8467	23.9	27.97
16	−94.529	2.50	1.6056	43.7	27.80
17	64.379	2.50			27.11
18	−132.204	2.00	1.8040	46.6	27.12
19	78.220	5.70			27.56
20 (Aspherical	44.962	6.66	1.7380	32.3	31.33
surface)
21	−65.436	0.40			31.24
22	−57.768	3.00	1.8929	20.4	31.17
23	40.092	7.40	1.6134	44.3	31.43
24	−80.410	9.15			31.92
25	62.000	11.57	1.5673	42.8	32.68
26	−22.748	2.00	1.5952	67.7	32.26
27	61.925	25.00			30.99
28	∞	4.40	1.5163	64.1	33.73
29	∞	77.59			34.07
30	∞
Aspherical surface data
	First surface
	k = −1.42907E−02
	A4 = −2.35660E−09
	A6 = −2.78502E−13
	A8 = −3.73974E−18
	A10 = −1.71054E−21
	A12 = −1.71054E−21
	Third surface
	k = −1.88438E+00
	A4 = 3.08070E−07
	A6 = −9.77306E−13
	A8 = −3.56770E−15
	A10 = 1.58909E−18
	A12 = −4.86078E−22
	Tenth surface
	k = −2.05982E+00
	A4 = 8.09083E−08
	A6 = 5.54689E−12
	A8 = −3.18893E−15
	A10 = 4.15591E−19
	Twentieth surface
	k = 1.19638E+00
	A4 = −1.76591E−06
	A6 = −8.99919E−10
	A8 = 7.80397E−13
	A10 = 8.35620E−17
	Diffractive surface data
	Fifth surface
	C2 = −4.03065E−05
	C4 = 2.26824E−09
	C6 = −9.56622E−13
Various data
	Focal length	780.0
	F-number	5.80
	Half-field angle (degrees)	1.6
	Image height	21.6
	Total lens length	486.06
	BF	77.59
Lens group data
Group	Starting surface	Focal length
1	1	224.5
2	8	−202.7
3	12	−200.0

Second Numerical Example

Unit (mm)
					Effective
Surface No.	r	d	nd	νd	diameter
1 (Aspherical	125.499	16.25	1.4875	70.2	134.48
surface)
2	371.022	143.96			133.34
3 (Aspherical	87.614	14.70	1.497	81.5	74.01
surface)
4	−190.068	0.05	1.5276	34.7	72.22
5 (Diffractive	−190.068	0.05	1.5569	50.2	72.19
surface)
6	−190.068	3.50	1.9108	35.3	72.17
7	427.093	11.90			69.30
8	344.680	8.00	1.8830	40.8	63.16
9 (Aspherical	140.097	84.85			59.69
surface)
10 (Diaphragm)	∞	31.74			36.31
11	98.207	3.53	1.8081	22.8	28.10
12	−64.390	1.50	1.8348	42.7	27.96
13	76.509	9.24			27.39
14	3660.189	3.67	1.8467	23.9	27.88
15	−59.545	2.50	1.6056	43.7	27.94
16	140.542	1.77			27.75
17	−133.850	2.00	1.8040	46.6	27.77
18	78.158	5.70			28.27
19 (Aspherical	47.721	6.64	1.7380	32.3	32.25
surface)
20	−69.436	0.40			32.23
21	−58.607	3.00	1.8929	20.4	32.21
22	48.545	7.61	1.6134	44.3	32.80
23	−65.327	10.00			33.35
24	64.850	11.75	1.5673	42.8	33.7
25	−23.631	2.00	1.5952	67.7	33.23
26	57.876	25.00			31.71
27	∞	4.40	1.5163	64.1	34.46
28	∞	70.34			34.81
29 (Image surface)	∞
Aspherical surface data
	First surface
	k = −1.42907E−02
	A4 = −2.35660E−09
	A6 = −2.78502E−13
	A8 = −3.73974E−18
	A10 = −1.71054E−21
	A12 = −1.71054E−21
	Third surface
	k = −1.88438E+00
	A4 = 3.08070E−07
	A6 = −9.77306E−13
	A8 = −3.56770E−15
	A10 = 1.58909E−18
	A12 = −4.86078E−22
	Ninth surface
	k = −2.05982E+00
	A4 = 8.09083E−08
	A6 = 5.54689E−12
	A8 = −3.18893E−15
	A10 = 4.15591E−19
	Nineteenth surface
	k = 1.19638E+00
	A4 = −1.76591E−06
	A6 = −8.99919E−10
	A8 = 7.80397E−13
	A10 = 8.35620E−17
	Diffractive surface data
	Fifth surface
	C2 = −4.03065E−05
	C4 = 2.26824E−09
	C6 = −9.56622E−13
Various data
	Focal length	780.0
	F-number	5.80
	Half-field angle (degrees)	1.6
	Image height	21.6
	Total lens length	486.05
	BF	70.34
Lens group data
Group	Starting surface	Focal length
1	1	268.4
2	8	−272.3
3	11	−204.5

Third Numerical Example

Unit (mm)
					Effective
Surface No.	r	d	nd	νd	diameter
1 (Aspherical surface)	119.357	22.64	1.4875	70.2	134.48
2	763.342	83.12			132.44
3 (Aspherical surface)	92.232	17.81	1.4970	81.5	82.94
4	−179.273	0.05	1.6151	26.5	80.36
5 (Diffractive surface)	−179.273	0.05	1.6431	38.8	80.35
6	−179.273	2.95	1.8503	32.3	80.32
7	−2248.283	11.49			77.26
8	515.704	5.13	1.8081	22.8	65.99
9	−509.658	3.20	1.8830	40.8	64.40
10 (Aspherical surface)	78.038	69.22			59.43
11 (Diaphragm)	∞	26.48			46.26
12	461.982	1.80	1.8081	22.8	40.66
13	57.130	7.10	1.7570	47.8	40.40
14	−148.092	4.62			40.54
15	84.730	6.11	1.8467	23.9	39.79
16	−105.171	1.70	1.6056	43.7	39.28
17	39.636	7.85			36.33
18	−90.783	1.80	1.8040	46.6	36.34
19	69.793	2.85			37.50
20 (Aspherical surface)	72.730	7.03	1.7380	32.3	40.31
21	−225.107	0.20			41.03
22	55.025	3.00	1.8467	23.9	42.35
23	40.048	5.86	1.6134	44.3	41.24
24	115.630	15.00			41.01
25	∞	2.20	1.5163	64.1	41.41
26	∞	59.79			41.45
27 (Image surface)	∞
Aspherical surface data
	First surface
	k = −3.23271E−01
	A4 = 2.49708E−08
	A6 = 1.09496E−12
	A8 = 3.71896E−17
	A10 = 7.60080E−21
	Third surface
	k = −3.13925E+00
	A4 = 3.75224E−07
	A6 = −5.06005E−11
	A8 = 4.11091E−15
	A10 = −8.01472E−19
	A12 = −1.03023E−22
	Tenth surface
	k = −9.30267E−02
	A4 = 4.40544E−08
	A6 = 1.08005E−11
	A8 = 3.85360E−15
	A10 = −1.69391E−18
	Twentieth surface
	k = 8.04214E−01
	A4 = −5.78950E−07
	A6 = −1.51948E−10
	A8 = −9.75920E−14
	A10 = 5.87507E−17
	Diffractive surface data
	Fifth surface
	C2 = −4.81608E−05
	C4 = 2.48823E−09
	C6 = −7.84815E−13
Various data
	Focal length	390.0
	F-number	2.90
	Half-field angle (degrees)	3.2
	Image height	21.6
	Total lens length	369.07
	BF	59.79
Lens group data
Group	Starting surface	Focal length
1	1	163.2
2	8	−101.9
3	12	345.2

Fourth Numerical Example

Unit (mm)
					Effective
Surface No.	r	d	nd	νd	diameter
1 (Aspherical surface)	79.068	20.71	1.4875	70.2	102.86
2	643.847	37.00			100.65
3 (Aspherical surface)	64.836	16.86	1.4875	70.2	63.99
4	−106.833	0.002	1.5660	19.0	60.48
5 (Diffractive	−106.833	0.05	1.6190	43.2	60.47
surface)
6	−106.833	3.95	1.6727	32.1	60.43
7	267.129	7.51			54.81
8	3209.381	5.34	1.8081	22.8	49.50
9	−251.490	3.00	1.7292	54.7	47.63
10	52.944	25.10			43.36
11 (Diaphragm)	∞	4.44			39.22
12 (Aspherical	100.674	2.10	1.8081	22.8	37.93
surface)
13	47.692	8.03	1.7725	49.6	36.74
14	−577.611	2.89			35.51
15	−160.477	1.75	1.6968	55.5	34.15
16	50.108	2.99			32.80
17	195.498	4.88	1.8467	23.8	32.84
18	−122.150	1.80	1.5407	47.2	32.69
19	71.436	3.12			32.16
20	81.909	4.69	1.7200	43.7	34.92
21	−246.705	2.00	1.8081	22.8	35.50
22	188.446	0.20			36.42
23	70.535	5.76	1.8830	40.8	37.67
24	−1243.51	3.40			37.86
25	∞	2.20	1.5163	64.1	38.18
26	∞	56.05			38.30
27 (Image surface)	∞
Aspherical surface data
	First surface
	k = 2.94671E−01
	A4 = −2.65433E−08
	A6 = −1.12941E−11
	A8 = −1.23216E−15
	A10 = −9.03682E−19
	A12 = 2.64633E−22
	A14 = −8.68831E−26
	Third surface
	k = 0.00000E+00
	A4 = −4.13485E−07
	A6 = −1.01773E−10
	A8 = −4.09820E−14
	A10 = −1.35393E−16
	A12 = 1.46462E−19
	Twelfth surface
	k = 2.40879E+00
	A4 = 8.23477E−07
	A6 = −1.07278E−09
	A8 = 7.33823E−12
	A10 = −1.88191E−14
	A12 = 1.75521E−17
	Diffractive surface data
	Fifth surface
	C2 = −1.13342E−04
	C4 = 3.23399E−08
	C6 = −1.50255E−11
Various data
	Focal length	195.9
	F-number	2.05
	Half-field angle (degrees)	6.3
	Image height	21.6
	Total lens length	225.84
	BF	56.05
Lens group data
Group	Starting surface	Focal length
1	1	117.5
2	8	−75.9
3	12	122.8

Fifth Numerical Example

Unit (mm)
					Effective
Surface No.	r	d	nd	νd	diameter
1 (Aspherical surface)	76.590	22.07	1.4875	70.2	103.11
2	577.345	37.00			100.30
3 (Aspherical surface)	68.307	16.10	1.5378	74.7	63.32
4	−104.952	0.005	1.4799	21.7	59.94
5 (Diffractive surface)	−104.952	0.05	1.5242	51.6	59.93
6	−104.952	3.95	1.7380	32.3	59.88
7	261.364	7.51			54.37
8	4389.915	3.76	1.8929	20.4	49.07
9	−357.754	3.00	1.7292	54.7	47.81
10	52.588	26.13			43.63
11 (Diaphragm)	∞	4.44			39.33
12 (Aspherical surface)	81.722	2.10	1.8081	22.8	37.97
13	47.021	7.57	1.7725	49.6	36.76
14	1282.624	2.89			35.41
15	−386.649	1.75	1.6968	55.5	33.98
16	47.511	2.97			32.47
17	230.898	4.72	1.8467	23.8	32.46
18	−120.061	1.80	1.5407	47.2	32.24
19	62.398	3.12			31.54
20	71.396	5.78	1.7200	43.7	34.36
21	−260.885	2.00	1.8081	22.8	35.11
22	187.059	0.20			35.93
23	67.544	5.42	1.8830	40.8	37.13
24	745.89	3.40			37.22
25	∞	2.20	1.5163	64.1	37.55
26	∞	54.00			37.70
27 (Image surface)	∞
Aspherical surface data
	First surface
	k = 2.62030E−01
	A4 = −3.96383E−08
	A6 = −1.15385E−11
	A8 = −1.45058E−15
	A10 = −1.53259E−18
	A12 = 5.53728E−22
	A14 = −1.45712E−25
	Third surface
	k = 0.00000E+00
	A4 = −3.77246E−07
	A6 = −1.31037E−10
	A8 = 5.75632E−14
	A10 = −2.71228E−16
	A12 = 2.43543E−19
	A14 = −8.48835E−23
	Twelfth surface
	k = 7.32909E−01
	A4 = 9.05629E−07
	A6 = −1.17485E−09
	A8 = 7.56204E−12
	A10 = −1.88342E−14
	A12 = 1.72746E−17
	Diffractive surface data
	Fifth surface
	C2 = −8.84624E−05
	C4 = 3.09506E−08
	C6 = −1.63650E−11
Various data
	Focal length	196.4
	F-number	2.05
	Half-field angle (degrees)	6.3
	Image height	21.6
	Total lens length	223.96
	BF	54.00
Lens group data
Group	Starting surface	Focal length
1	1	117.5
2	8	−75.9
3	12	125.6

Various numerical values in the optical systems according to the second to the sixth exemplary embodiments are summarized in Table 1.

TABLE 1
Conditional	Exemplary embodiments
expression	Second	Third	Fourth	Fifth	Sixth
νLL	81.5	81.5	81.5	70.2	74.7
νLR	28.3	35.3	32.3	32.1	32.3
Formula (3)	53.3	46.3	49.3	38.1	42.4
NL	1.5660	1.5276	1.6151	1.5660	1.4799
NLL	1.4970	1.4970	1.4970	1.4875	1.5378
Formula (4)	1.046	1.020	1.079	1.053	0.962
Formula (5)	18.9	23.3	26.2	33.0	36.3
Ld	174.44	174.90	123.57	74.57	75.17
Lt	486.06	486.05	369.07	225.84	223.96
Formula (7)	0.36	0.36	0.33	0.33	0.34
f	780.00	780.00	390.00	195.90	196.40
Fn	5.80	5.80	2.90	2.05	2.05
Rd	−320.403	−190.068	−179.273	−106.833	−104.952
Formula (8)	−0.420	−0.708	−0.750	−0.894	−0.913
Formula (9)	0.998	1.146	1.319	1.589	1.617
|θD|	22.17	26.69	31.30	37.01	37.09

[Optical Apparatuses]

FIG. 19 is a schematic view illustrating an imaging apparatus (digital still camera) 600 according to an exemplary embodiment of the present invention. The imaging apparatus 600 according to the present exemplary embodiment includes a camera main body 601, an optical system 602 similar to the optical system according to any one of the second to the sixth exemplary embodiments, and a light receiving element (image sensor) 603 for performing photoelectric conversion on an image formed by the optical system 602.

Since the imaging apparatus 600 according to the present exemplary embodiment includes the optical system 602 similar to the optical system according to any one of the second to the sixth exemplary embodiments, a high-definition image in which flare caused by a grating wall surface of a DOE is reduced. As the light receiving element 603, such an image sensor as a CCD sensor and CMOS sensor can be used.

The above-described optical system according to each exemplary embodiment is applicable not only to a digital still camera illustrated in FIG. 19 but also to various types of optical apparatuses such as silver-halide film cameras, video cameras, and telescopes.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2016-213545, filed Oct. 31, 2016, which is hereby incorporated by reference herein in its entirety.

Claims

1. A An optical system comprising a diffractive optical element, wherein the diffractive optical element comprises, in an order from an object side to an image side: a first lens having a convex surface;a diffraction grating section having positive optical power through diffraction; anda second lens having a concave surface facing the convex surface,wherein the diffraction grating section includes a first diffraction grating and a second diffraction grating disposed in this order from the object side to the image side, the second diffraction grating having a larger refractive index than the first diffraction grating,wherein the diffraction grating section has a plurality of grating surfaces that contributes to the diffraction and a plurality of grating wall surfaces that is adjacent to the plurality of grating surfaces, andwherein an inner diameter of a first grating wall surface of the first diffraction grating among the plurality of grating wall surfaces decreases from the object side to the image side.

2. The optical system according to claim 1, wherein the first lens is a positive lens.

3. The optical system according to claim 1, wherein the second lens is a negative lens.

4. The optical system according to claim 1, wherein the following conditional expression is satisfied: 20<νLL−νLR<60where νLL is an Abbe number of the first lens, and νLR is an Abbe number of the second lens.

5. The optical system according to claim 1, wherein the following conditional expression is satisfied: 0.8<NL/NLL<1.2where NLL is a refractive index of the first lens, and NL is a refractive index of the first diffraction grating.

6. The optical system according to claim 1, wherein the following conditional expression is satisfied: 5 degrees<ΔθHM<45 degreeswhere ΔθHM is a difference between maximum and minimum values of an angle θHM in the diffraction grating section, θHM being defined as an absolute value of an angle formed by each of the grating wall surfaces and a surface normal to an enveloping surface of the diffraction grating section, formed by connecting apical portions of the first diffraction grating, at positions where each of the plurality of grating wall surfaces contacts the enveloping surface.

7. The optical system according to claim 1, wherein at least one of the first and the second diffraction gratings is formed of resin.

8. The optical system according to claim 1, further comprising: an aperture diaphragm disposed on an image side of the optical element.

9. The optical system according to claim 1, further comprising: a partial optical system including lenses disposed on the object side of the diffractive optical element,wherein the partial optical system has positive refractive power.

10. The optical system according to claim 1, wherein the following conditional expression is satisfied: 0.10<Ld/Lt<0.50where Ld is a distance on the optical axis from a lens surface of the first lens on a side where the diffraction grating section is formed to a lens surface of the optical system on the most object side, and Lt is a total length of the optical system.

11. The optical system according to claim 1, wherein the following conditional expression is satisfied: −2.0<f/(Rd×Fn)<−0.20where f is a focal length of the optical system, Fn is an open F-number of the optical system, and Rd is a curvature radius of a lens surface of the first lens on a side where the diffraction grating section is formed.

12. The optical system according to claim 1, wherein the following conditional expression is satisfied: 0.6<(E1−Ed)/Ld+Ed×Pf−Ed/Rd<2.0where E1 is an effective diameter of a lens surface of the optical system on the most object side, Ed is an effective diameter of a lens surface of the first lens on a side where the diffraction grating section is formed, Ld is a distance on the optical axis from the lens surface of the first lens on the side where the diffraction grating section is formed to the lens surface of the optical system on the most object side, Rd is a curvature radius of the lens surface of the first lens on the side where the diffraction grating section is formed, and Pf is refracting power of a lens surface of the first lens on a side where the diffraction grating section is not formed.

13. The optical system according to claim 1, wherein the following conditional expression is satisfied: 10 degrees<|θD|<57 degreeswhere θD is an incident angle of an axial marginal ray incident on the diffraction grating section in a state of being focused at infinity.

14. An imaging apparatus comprising: the optical system according to claim 1; andan image sensor configured to receive light of an image formed by the optical system.