Optical system and image capturing device having the same

Abstract

To provide an optical system which is compact and yet offers a deep depth of field.SOLUTION: An optical system is provided, comprising at least one lens unit and a diffractive optical element, each of the at least one lens unit being configured to be stationary while focusing. A focal length of the optical system at 850-nm wavelength and a focal length of the optical system at 950-nm wavelength are each set appropriately.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system used in an imaging device, and is suitable for, for example, an endoscope or a head-mounted display. [Background technology]

[0002] Patent Documents 1 and 2 describe methods for manufacturing compact optical systems used in endoscopes and head-mounted displays (HMDs). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 9,798,115 [Patent Document 2] Patent No. 5434457 Summary of the Invention [Problem to be solved by the invention]

[0004] In optical systems where miniaturization is important, it is difficult to provide a focus mechanism, and therefore, if the subject deviates from the designed object distance, the imaging performance is likely to deteriorate due to defocusing.

[0005] An object of the present invention is to provide an optical system that is compact yet has a deep depth of field. [Means for solving the problem]

[0006] An optical system according to one aspect of the present invention is an optical system having at least one lens unit and a diffractive optical element, wherein each of the at least one lens unit is stationary during focusing, and when the focal length of the optical system at a wavelength of 850 nm is f85 and the focal length of the optical system at a wavelength of 950 nm is f95, 0.004<|(f85-f95) / (f85+f95)|<0.200 The present invention is characterized in that the following conditional expression is satisfied: [Effects of the Invention]

[0007] According to the present invention, it is possible to provide an optical system that is compact yet has a deep depth of field. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a cross-sectional view of an optical system according to a first embodiment. [Figure 2] 3A to 3C are aberration diagrams of the optical system of Example 1. [Figure 3] 3A to 3C are diagrams showing lateral aberrations of the optical system of Example 1. [Figure 4] FIG. 3 is a diagram showing the diffraction efficiency of the diffractive optical element of Example 1. [Figure 5] FIG. 10 is a cross-sectional view of an optical system according to a second embodiment. [Figure 6] 10A to 10C are aberration diagrams of the optical system of Example 2. [Figure 7] FIG. 10 is a cross-sectional view of an optical system according to a third embodiment. [Figure 8] 10A to 10C are aberration diagrams of the optical system of Example 3. [Figure 9] FIG. 10 is a cross-sectional view of an optical system according to a fourth embodiment. [Figure 10] 10A to 10C are aberration diagrams of the optical system of Example 4. [Figure 11] FIG. 10 is a diagram showing the diffraction efficiency of the diffractive optical element of Example 4. [Figure 12] FIG. 10 is a cross-sectional view of an optical system according to a fifth embodiment. [Figure 13] 10A to 10C are aberration diagrams of the optical system of Example 5. [Figure 14] FIG. 1 is a schematic diagram illustrating a main part of an electronic device. [Figure 15] FIG. 1 is a schematic diagram of a main part of an imaging device. DETAILED DESCRIPTION OF THE INVENTION

[0009] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to designate the same components, and redundant explanations will be omitted.

[0010] In each embodiment, an optical system obtained using a technology called wafer-level optics will be described. Wafer-level optics is a technology (wafer process) that uses semiconductor manufacturing technology to create multiple lenses on a wafer, stack and bond multiple wafers (planar substrates), and then cut them to manufacture multiple optical systems at once. An optical system manufactured using wafer-level optics is called a wafer-level lens, and an imaging device that uses a wafer-level lens as an imaging optical system is called a wafer-level camera. The optical system of each embodiment is used in optical systems for gaze detection, etc., used together with observation optical systems such as viewfinders and HMDs. It is also used in cameras built into electronic devices such as mobile phones, smartphones, and wearable devices, as well as the objective optical system of endoscopes. Note that while the optical system of each embodiment is a wafer-level lens, the present invention is not limited thereto. FIGS. 1, 5, 7, 9, and 12 are cross-sectional views of the optical systems (wafer-level lenses) of Examples 1 to 5. In each cross-sectional view, the left side is the object side (front) and the right side is the image side (rear). SP is the aperture stop, and IP is the image plane. On the image plane IP, an imaging surface of an imaging element (image sensor) such as a CCD sensor or CMOS sensor in an imaging device, or a photosensitive surface equivalent to the film surface of a silver halide film camera, is arranged.

[0011] The optical system of each embodiment has a first unit L1, a second unit L2, and a third unit L3 arranged in this order from the object side to the image side. Although the optical system of each embodiment has three units, it may have two units or four or more units. Furthermore, the optical system of each embodiment does not have a focus mechanism, and each unit does not move during focusing. It is preferable that each unit also remains stationary during zooming, vibration reduction, and the like.

[0012] The first unit L1, the second unit L2, and the third unit L3 are manufactured using a wafer process. Specifically, they are manufactured by forming a lens layer made of a curable resin material on a wafer made of a glass material. In the second unit L2, an aperture stop SP is formed on a substrate by the wafer process. The first unit L1, the second unit L2, the third unit L3, and the image sensor are arranged at desired intervals, bonded outside the effective light beam, and then cut to manufacture a large number of wafer-level lenses.

[0013] The material for forming the lens layer may be a thermoplastic resin or an ultraviolet curable resin as long as it is a curable resin material, such as an acrylic resin, a silicone resin, or a cycloolefin polymer.

[0014] The aperture stop SP can be formed by depositing a light-shielding film such as chromium using a mask, or by forming an opening by etching after deposition. In this case, it is preferable from the viewpoint of manufacturing to form the aperture stop SP on a flat surface such as a substrate, since this makes it easier to control the mask placement in the thickness direction.

[0015] 2, 6, 8, 10, and 13 are aberration diagrams of the optical systems of Examples 1 to 5. In the spherical aberration diagrams, the amount of spherical aberration is shown for wavelengths of 850 nm, 900 nm, and 950 nm. In the astigmatism diagrams, ΔS indicates the amount of astigmatism on the sagittal image plane for the design wavelength, and ΔM indicates the amount of astigmatism on the meridional image plane for the design wavelength. In the distortion diagrams, the amount of distortion is shown for the design wavelength. In the lateral chromatic aberration diagrams, the amount of chromatic aberration at wavelengths of 900 nm and 950 nm for the design wavelength is shown. Fno is the F-number, and Y is the image height.

[0016] The characteristic configuration of the optical system of each embodiment will be described below.

[0017] The optical system of each embodiment is an optical system designed with a near-infrared wavelength as its design wavelength, and is used, for example, as an optical system for detecting the line of sight. The design wavelength is specifically 850 nm or 950 nm.

[0018] An optical system for gaze detection irradiates the eye with near-infrared light, which is invisible to the human eye, and observes the image reflected from the eye. If an optical system without a focusing function is used, the focal point will change from the imaging plane when the eye position changes, making it difficult to obtain high-definition (high-resolution) images.

[0019] Therefore, in the optical system of each embodiment, a diffractive optical element (DO) is provided in the optical system to generate chromatic aberration, thereby achieving a wide depth of field that allows high-resolution images to be acquired even if the object distance to the subject changes.

[0020] The effect of increasing the depth of field by generating chromatic aberration using a diffractive optical element will be described below.

[0021] As shown in the aberration diagrams for each example, axial chromatic aberration is intentionally generated in the optical system of each example. Furthermore, in the optical system of each example, astigmatism, distortion, and lateral chromatic aberration other than axial chromatic aberration are well corrected.

[0022] Here, the transverse aberration of the optical system of Example 1 will be described as an example of transverse aberration. Fig. 3 is a transverse aberration diagram of the optical system of Example 1. The light source that illuminates the eye for gaze detection is an LED light source or the like, and LED light sources generally have spectral sensitivity in a wavelength range of about ±50 nm centered around a reference wavelength. In the optical system of Example 1, the design wavelength is 850 nm, and the light source used is also assumed to have a central wavelength of 850 nm.

[0023] Fig. 3(a) is a diagram of axial lateral aberration at an object distance of 25 mm for the optical system of Example 1. Fig. 3(a) shows the amount of lateral aberration for wavelengths of 800 nm, 850 nm, and 900 nm. As shown in Fig. 3(a), the lateral aberration at wavelengths of 800 nm, 850 nm, and 900 nm is well corrected, so a high-resolution image can be obtained at an object distance of 25 mm.

[0024] Here, FIG. 3(b) shows the lateral aberration when the object distance is changed to 50 mm, and FIG. 3(c) shows the lateral aberration when the object distance is changed to 16 mm.

[0025] As shown in Figure 3(b), even when the object distance is changed to 50 mm, the transverse aberration at a wavelength of 800 nm is well corrected, so degradation of the captured image is suppressed despite the change in object distance.

[0026] Similarly, as shown in Figure 3(c), even when the object distance is changed to 16 mm, the transverse aberration at a wavelength of 900 nm is well corrected, so degradation of the captured image is suppressed despite the change in object distance.

[0027] The optical system of each embodiment may further include a means for widening the depth of field.

[0028] Fig. 3(d) shows the lateral aberration diagram on the optical axis at an object distance of 10 mm for the optical system of Example 1. Fig. 3(d) shows the amount of lateral aberration for wavelengths of 800 nm, 850 nm, and 900 nm. As shown in Fig. 3(d), when the object distance changes significantly to 10 mm, the amount of lateral aberration at wavelengths of 800 nm to 900 nm also increases, making it difficult to obtain good imaging performance.

[0029] Here, consider a configuration in which two types of LED light sources are used for illumination, with different central wavelengths. For example, when capturing an image of a subject at an object distance of 10 mm using an LED light source with a central wavelength of 950 nm, the lateral aberration of the optical system of Example 1 is as shown in Figure 3(e). Figure 3(e) shows the amount of lateral aberration for wavelengths of 900 nm, 950 nm, and 1000 nm. As shown in Figure 3(e), the lateral aberration for wavelengths of 900 nm, 950 nm, and 1000 nm is well corrected, so a high-resolution image can be obtained even at an object distance of 10 mm by using a light source with a central wavelength of 950 nm.

[0030] As described above, by generating axial chromatic aberration using a diffractive optical element, it is possible to obtain a good image even when the object distance changes, that is, to obtain an optical system with a wide depth of field.

[0031] Moreover, the optical system of each embodiment satisfies the following conditional expression (1).

[0032] 0.004<|(f85-f95) / (f85+f95)|<0.100 (1) Here, f85 is the focal length of the optical system at a wavelength of 850 nm, and f95 is the focal length of the optical system at a wavelength of 950 nm.

[0033] By satisfying conditional expression (1), it is possible to obtain a wide depth of field while correcting various aberrations. If the lower limit of conditional expression (1) is not met, the amount of chromatic aberration generated is reduced, making it difficult to obtain a wide depth of field. If the upper limit of conditional expression (1) is exceeded, the amount of chromatic aberration generated increases, and although the depth of field is widened, this is undesirable because the degradation of imaging performance due to chromatic aberration increases.

[0034] It is preferable that the numerical range of conditional expression (1) be the numerical range of conditional expression (1a) below.

[0035] 0.006<|(f85-f95) / (f85+f95)|<0.050 (1a) It is more preferable that the numerical range of conditional expression (1) be the numerical range of the following conditional expression (1b).

[0036] 0.009<|(f85-f95) / (f85+f95)|<0.030 (1b) In the following, configurations that are preferably satisfied in the optical systems of the respective embodiments will be described.

[0037] The optical system of each embodiment preferably includes a lens unit LP that includes a substrate and a lens (positive lens) with positive power (refractive power) disposed on the substrate.

[0038] Moreover, the optical system of each embodiment preferably includes a lens unit LN including a substrate and a lens (negative lens) LN1 with negative power arranged on the substrate.

[0039] The substrate included in the lens unit LP and the substrate included in the lens unit LN may be the same.

[0040] The above configuration is preferable because it makes it easier to realize an optical system with a small size and simple configuration that produces appropriate axial chromatic aberration while satisfactorily correcting astigmatism, distortion, and lateral chromatic aberration.

[0041] Because the optical systems in each embodiment are manufactured using a wafer process, the diffraction pitch of the diffraction grating of the diffractive optical element is small, sometimes on the order of a few micrometers. When using a diffractive optical element in an optical system used over a wide wavelength range, it is preferable that the diffractive optical element have high diffraction efficiency over that wide wavelength range. Forming the diffraction grating using two or more materials can achieve the desired phase difference over a wide wavelength range, but this results in a large grating height. Furthermore, as the wavelength used in the optical system increases, such as in the near-infrared wavelength range, the grating height tends to increase. When the grating height is large and the diffraction pitch is small, the influence of unwanted diffracted light generated at the grating wall surface cannot be ignored. Therefore, in the optical systems in each embodiment, it is preferable to form at least a portion of the diffraction grating surface of the diffractive optical element on a surface facing air, thereby increasing the refractive index difference before and after the diffraction grating surface and reducing the grating height.

[0042] The conditions that the optical system of each embodiment should preferably satisfy are described below: The optical system of each embodiment should preferably satisfy one or more of the following conditional expressions (2) to (10).

[0043] 0.05 <Ld / f<0.80 (2) 1.0<|fdo / f|<15.0 (3) 0.0010<(f85―f95) / fdo<0.0400 (4) 1.0 <Lds / f<4.0 (5) -3.0 <fn / f<-0.2 (6) 0.7<(Nm-1)×d / λm<1.3 (7) 0.65<(Nm-1)×d<1.15 (8) 1.0 <fdo×λm / (Ddo×d)<20.0 (9) 10 <Lobj / f<100 (10) Here, Ld is the distance on the optical axis from the diffractive optical element to the aperture stop SP (the distance between the diffractive optical element and the aperture stop SP). f is the focal length at the design wavelength of the optical system. fdo is the focal length due to diffraction of the diffractive optical element. Lds is the distance on the optical axis from the diffractive optical element to the image plane. fn is the focal length of the negative power lens LN1 included in the lens unit LN. d (μm) is the grating height of the diffraction grating of the diffractive optical element. λm (μm) is the median value of the wavelength range used by the optical system. Nm is the refractive index at wavelength λm of the material that forms the diffraction grating of the diffractive optical element. Ddo is the effective diameter of the diffractive optical element. Lobj is the focused object distance at the design wavelength of the optical system.

[0044] The effective diameter (effective ray diameter) is twice the distance from the farthest position from the optical axis to the optical axis within the area of ​​the optical surface through which the effective imaging light beam can pass. The effective imaging light beam refers to the light beam excluding stray light and light rays that form an image outside the area on the imaging surface where the image is recorded. For the surface closest to the object in each example of the optical system, the effective diameter is twice the longer of the distance to the optical axis and the position where the lower or upper line of the most off-axis light beam passes through the optical surface. The optical surface refers to a lens surface, both sides of a flat plate, or the cemented surface thereof. The value obtained by dividing the effective diameter by 2 is called the effective radius.

[0045] Positioning the diffractive optical element near the aperture stop so as to satisfy conditional expression (2) is preferable because it generates only axial chromatic aberration appropriately, making it easier to effectively correct other aberrations. Below the lower limit of conditional expression (2), the distance between the diffractive optical element and the aperture stop becomes too close, which is undesirable because it reduces the lens thickness and makes manufacturing difficult, and because the aperture stop must be formed on the lens surface, which increases the manufacturing difficulty. Above the upper limit of conditional expression (2), the chromatic aberration of magnification generated by the diffractive optical element increases, resulting in poor imaging performance at the periphery of the image.

[0046] If the lower limit of conditional expression (3) is exceeded, the amount of chromatic aberration increases, and the depth of field becomes wider, but this is undesirable because the degradation of imaging performance due to chromatic aberration becomes significant.If the upper limit of conditional expression (3) is exceeded, the amount of chromatic aberration decreases, and it becomes difficult to obtain a wide depth of field.

[0047] In the optical systems of the respective embodiments, it is preferable that aberrations other than longitudinal chromatic aberration are well corrected. To achieve this, it is important to balance the power of the diffractive optical element with the longitudinal chromatic aberration generated by the optical system. Satisfying conditional expression (4) is preferable because it makes it easier to achieve this balance. If the lower limit of conditional expression (4) is not met, the amount of chromatic aberration generated by the diffractive optical element will be reduced, making it difficult to obtain a wide depth of field. If the upper limit of conditional expression (4) is exceeded, chromatic aberration generated by elements other than the diffractive optical element will increase, which is undesirable because it will worsen various aberrations other than longitudinal chromatic aberration.

[0048] It is preferable to increase the distance on the optical axis from the diffractive optical element to the image plane to a certain extent so as to satisfy conditional expression (5), as this makes it easier to generate axial chromatic aberration. If the lower limit of conditional expression (5) is exceeded, the amount of chromatic aberration generated decreases, making it difficult to obtain a wide depth of field. If the upper limit of conditional expression (5) is exceeded, the overall length of the optical system increases, making it difficult to reduce the size, which is undesirable.

[0049] Satisfying conditional expression (6) is preferable because it makes it easier to obtain an optical system in which various aberrations are well corrected. If the lower limit of conditional expression (6) is not met, the power of the negative lens LN1 becomes weak, making it difficult to reduce the size of the optical system. If the upper limit of conditional expression (6) is exceeded, the power of the negative lens LN1 becomes strong, making it difficult to well correct various aberrations such as astigmatism.

[0050] Satisfying conditional expression (7) is preferable because it allows the grating height to be reduced while still achieving high diffraction efficiency over a wide wavelength range.

[0051] If the lower limit of conditional expression (7) is exceeded or the upper limit is exceeded, the diffraction efficiency will decrease at the minimum or maximum wavelength in the wavelength range used, which is not preferable.

[0052] Satisfying conditional expression (8) is preferable because it allows for a diffractive optical element with optimized diffraction efficiency in the near-infrared wavelength region. If the lower limit of conditional expression (8) is exceeded or the upper limit of conditional expression (8) is exceeded, the diffraction efficiency will decrease in wavelength regions away from the near-infrared wavelength of 0.9 μm, which is undesirable.

[0053] Satisfying conditional expression (9) is preferable because the minimum diffraction pitch is sufficiently large relative to the grating height of the diffraction grating of the diffractive optical element, reducing the influence of unwanted diffracted light generated at the grating wall surfaces and achieving high diffraction efficiency. Below the lower limit of conditional expression (9), the ratio of the grating height to the diffraction pitch becomes high, which increases the amount of unwanted diffracted light generated at the grating wall surfaces, which is undesirable. Above the upper limit of conditional expression (9), the power of the diffractive optical element weakens, reducing the amount of axial chromatic aberration generated, making it difficult to achieve a wide depth of field, which is undesirable.

[0054] In order for the optical systems of the embodiments to obtain a wide depth of field, it is preferable that the in-focus object distance at the design wavelength of the optical system is a finite distance rather than infinity. If the lower limit of conditional expression (10) is exceeded, it becomes difficult to obtain a high-resolution image when the object distance is long. If the upper limit of conditional expression (10) is exceeded, it becomes difficult to obtain a high-resolution image when the object distance is short.

[0055] It is preferable that the numerical ranges of the conditional expressions (2) to (10) be the numerical ranges of the following conditional expressions (2a) to (10a).

[0056] 0.08 <Ld / f<0.60 (2a) 2.0<|fdo / f|<10.0 (3a) 0.0020<(f85―f95) / fdo<0.0200 (4a) 1.2 <Lds / f<3.0 (5a) -2.0 <fn / f<-0.3 (6a) 0.9<(Nm-1)×d / λm<1.1 (7a) 0.8<(Nm-1)×d<1.0 (8a) 1.5 <fdo×λm / (Ddo×d)<15.0 (9a) 12 <Lobj / f<50 (10a) It is more preferable that the numerical ranges of the conditional expressions (2) to (10) be the numerical ranges of the following conditional expressions (2b) to (10b).

[0057] 0.10 <Ld / f<0.50 (2b) 3.0<|fdo / f|<8.5 (3b) 0.0023<(f85−f95) / fdo<0.0150 (4b) 1.3 <Lds / f<2.5 (5b) -1.6 <fn / f<-0.4 (6b) 0.95<(Nm-1)×d / λm<1.05 (7b) 0.85<(Nm-1)×d<0.95 (8b) 2.0 <fdo×λm / (Ddo×d)<11.0 (9b) 13 <Lobj / f<30 (10b) The optical system of each embodiment will be described in detail below.

[0058] In the optical system of Example 1, the design wavelength is 850 nm, and the operating wavelength is 800 nm to 1000 nm. The optical system of Example 1 is designed to focus on an object located 25 mm from the surface of the first unit L1 closest to the object. The first unit L1 includes a first substrate 11, a first lens 12 with positive power arranged on the object side of the first substrate 11, and a second lens 13 with negative power arranged on the image side of the first substrate 11. The first substrate 11 is a flat substrate, the first lens 12 is a convex lens, and the second lens 13 is a concave lens. The first lens 12 is formed on the object side surface of the first substrate 11 using a wafer process. The second lens 13 is formed on the image side surface of the first substrate 11 using a wafer process. The object side surface of the first lens 12 and the image side surface of the second lens 13 are aspherical. The second unit L2 includes a second substrate 21, an aperture stop SP arranged on the image side of the second substrate 21, and a third lens 22 arranged on the image side of the second substrate 21. The second substrate 21 is a flat substrate, and the second lens 22 is a convex lens. The second lens 22 is formed on the image side surface of the second substrate 21 using a wafer process. The image side surface of the second lens 22 is a diffractive surface with a diffraction grating formed on an aspherical surface. The third unit L3 includes a third substrate 31. The third substrate 31 is a flat substrate and is provided to protect the image sensor. In other words, the third substrate 31 also serves as a cover glass for the image sensor.

[0059] In the optical system of Example 1, the third lens 22 is a diffractive optical element having a diffraction grating formed on the image-side surface. In the optical system of Example 1, a diffractive optical element with positive power is used to generate chromatic aberration, thereby shortening the focal length of the optical system at a wavelength of 950 nm, which is longer than the design wavelength of 850 nm.

[0060] In the optical system of Example 1, the wavelength range used is 800 nm to 1000 nm as described above, the intermediate wavelength is 900 nm, the refractive index of the material forming the diffraction grating at a wavelength of 900 nm is 1.5135, and the grating height is 1.734 μm.

[0061] Furthermore, in the optical system of Example 1, the maximum diffraction pitch is 111 μm and the minimum diffraction pitch is 21 μm, and since the minimum diffraction pitch is sufficiently large relative to the grating height, the influence of unnecessary diffracted light at the grating wall surface can be reduced, and high diffraction efficiency can be obtained.

[0062] Fig. 4 is a diagram showing the diffraction efficiency in the wavelength range of 750 nm to 1050 nm of the diffractive optical element of Example 1. As shown in Fig. 4, a high diffraction efficiency of 95% or more is obtained in the wide wavelength range of 800 nm to 1000 nm.

[0063] In the optical system of Example 1, the first substrate 11, the second substrate 21, and the third substrate 31 are each made of glass, and the first lens 12, the second lens 13, and the third lens 22 are each made of resin, but the present invention is not limited to this. As long as the first substrate 11 and the first lens 12 have different refractive indices, both the first substrate 11 and the first lens 12 may be made of resin. This also applies to the second lens 13 and the second unit L2.

[0064] In the optical system of Example 1, the first lens 12 and the second lens 13 are formed on the object-side surface and the image-side surface, respectively, of one first substrate 11 of the first unit L1, but the present invention is not limited to this. For example, a configuration with improved manufacturing stability can be achieved by dividing the first substrate 11 into two, forming the first lens 12 on the object-side surface of one substrate, and forming the second lens 13 on the image-side surface of the other substrate, and then bonding the substrates together.

[0065] In addition, in Example 1, the back cover glass (third substrate 31) is formed from a single substrate and also serves as a sensor cover glass, but the present invention is not limited to this. For example, after forming the first unit L1, the second unit L2, and the third unit L3, a sensor cover glass (not shown) and the third substrate 31 can be bonded to each other, thereby achieving a configuration with improved manufacturing stability.

[0066] In the optical system of Example 2, the design wavelength is 850 nm, and the operating wavelength is 800 nm to 1000 nm. The optical system of Example 2 is designed to focus on an object located 25 mm away from the surface of the first unit L1 closest to the object. The first unit L1 includes a first substrate 11, a first lens 12 with negative power arranged on the object side of the first substrate 11, and a second lens 13 with negative power arranged on the image side of the first substrate 11. The second unit L2 includes a second substrate 21, an aperture stop SP arranged on the image side of the second substrate 21, and a third lens 22 with positive power arranged on the image side of the second substrate 21. The third unit L3 includes a third substrate 31 and a fourth lens 32 arranged on the object side of the third substrate 31. The third substrate 31 also serves as a cover glass for the image sensor.

[0067] In the optical system of Example 2, the fourth lens 32 is a diffractive optical element having a diffraction grating formed on the object-side surface. In the optical system of Example 2, a diffractive optical element with positive power is used to generate chromatic aberration, thereby shortening the focal length of the optical system at a wavelength of 950 nm, which is longer than the design wavelength of 850 nm.

[0068] In the optical system of Example 2, the material forming the diffraction grating is the same as that of Example 1, and the grating height is 1.734 μm.

[0069] Furthermore, in the optical system of Example 2, the maximum diffraction pitch is 101 μm and the minimum diffraction pitch is 13 μm. Since the minimum diffraction pitch is sufficiently large relative to the grating height, the influence of unnecessary diffracted light on the grating wall surface can be reduced, and high diffraction efficiency can be obtained.

[0070] The diffractive optical element of Example 2 also provides a high diffraction efficiency of 95% or more in a wide wavelength range of 800 nm to 1000 nm, similar to Example 1.

[0071] In the optical system of Example 3, the design wavelength is 950 nm and the operating wavelength is 800 nm to 1000 nm. The optical system of Example 3 is designed to focus on an object located 25 mm away from the surface of the first unit L1 closest to the object. The first unit L1 includes a first substrate 11, a first lens 12 with positive power arranged on the object side of the first substrate 11, and a second lens 13 with negative power arranged on the image side of the first substrate 11. The second unit L2 includes a second substrate 21, an aperture stop SP arranged on the image side of the second substrate 21, and a third lens 22 with positive power arranged on the image side of the second substrate 21. The third unit L3 includes a third substrate 31. The third substrate 31 also serves as a cover glass for the image sensor.

[0072] In the optical system of Example 3, the third lens 22 is a diffractive optical element having a diffraction grating formed on its image-side surface. In the optical system of Example 3, a diffractive optical element with negative power is used to generate chromatic aberration, thereby shortening the focal length of the optical system at a wavelength of 850 nm, which is shorter than the design wavelength of 950 nm. For example, when the object distance from the surface of the first unit L1 closest to the object is shorter than 25 mm, a high-resolution image can be acquired by illuminating the subject with a light source having a dominant wavelength shorter than the design wavelength of 950 nm, for example, a wavelength of 850 nm.

[0073] In the optical system of Example 3, the material forming the diffraction grating is the same as that of Example 1, and the grating height is 1.734 μm.

[0074] Furthermore, in the optical system of Example 3, the maximum diffraction pitch is 151 μm and the minimum diffraction pitch is 40 μm. Since the minimum diffraction pitch is sufficiently large relative to the grating height, the influence of unnecessary diffracted light on the grating wall surface can be reduced, and high diffraction efficiency can be obtained.

[0075] The diffractive optical element of Example 3 also provides a high diffraction efficiency of 95% or more in a wide wavelength range of 800 nm to 1000 nm, similar to Example 1.

[0076] In the optical system of Example 4, the design wavelength is 850 nm, and the operating wavelength is 800 nm to 1000 nm. The optical system of Example 4 is designed to focus on an object located 15 mm away from the surface of the first unit L1 closest to the object. The first unit L1 includes a first substrate 11 and a first lens 12 with negative power that is arranged on the image side of the first substrate 11. The second unit L2 includes a second substrate 21, an aperture stop SP that is arranged on the image side of the second substrate 21, and a second lens 22 with positive power that is arranged on the object side of the second substrate 21. The third unit L3 includes a third substrate 31 and a third lens 32 with positive power that is arranged on the object side of the third substrate 31. The third substrate 31 also serves as a cover glass for the image sensor.

[0077] In the optical system of Example 4, the second lens 22 is a diffractive optical element having a diffraction grating formed on the object-side surface. In the optical system of Example 4, a diffractive optical element with positive power is used to generate chromatic aberration, thereby shortening the focal length of the optical system at a wavelength of 950 nm, which is longer than the design wavelength of 850 nm.

[0078] In the optical system of Example 4, the wavelength range used is 800 nm to 1000 nm as described above, the intermediate wavelength is 900 nm, the refractive index of the material forming the diffraction grating at a wavelength of 900 nm is 1.6084, and the grating height is 1.464 μm.

[0079] Furthermore, in the optical system of Example 4, the maximum diffraction pitch is 80 μm and the minimum diffraction pitch is 12 μm, and since the minimum diffraction pitch is sufficiently large relative to the grating height, the influence of unnecessary diffracted light on the grating wall surface can be reduced, and high diffraction efficiency can be obtained.

[0080] Fig. 11 is a diagram showing the diffraction efficiency in the wavelength range of 750 nm to 1050 nm of the diffractive optical element of Example 4. As shown in Fig. 11, a high diffraction efficiency of 95% or more is obtained in the wide wavelength range of 800 nm to 1000 nm.

[0081] In the optical system of Example 5, the design wavelength is 850 nm and the operating wavelength is 800 nm to 1000 nm. The optical system of Example 5 is designed to focus on an object located 25 mm from the surface of the first unit L1 closest to the object. The first unit L1 includes a first substrate 11 and a first lens 12 with negative power arranged on the image side of the first substrate 11. The second unit L2 includes a second substrate 21, a second lens 22 with positive power arranged on the object side of the second substrate 21, an aperture stop SP, a third substrate 23 arranged on the image side of the second substrate 21, and a third lens 24 with positive power arranged on the image side of the third substrate 23. The second substrate 21 and the third substrate 23 are bonded together with the aperture stop SP sandwiched therebetween. The third unit L3 includes a fourth substrate 31. The fourth substrate 31 also serves as a cover glass for the image sensor.

[0082] In the optical system of Example 5, a diffractive optical element having a diffraction grating pattern is formed on the image-side surface of the third lens 24. In the optical system of Example 5, a diffractive optical element with positive power is used to generate chromatic aberration, thereby shortening the focal length of the entire optical system at a wavelength of 950 nm, which is longer than the design wavelength of 850 nm.

[0083] In the optical system of Example 5, the material forming the diffraction grating is the same as that of Example 1, and the grating height is 1.734 μm.

[0084] Furthermore, in the optical system of Example 5, the maximum diffraction pitch is 78 μm and the minimum diffraction pitch is 7.6 μm. Since the minimum diffraction pitch is sufficiently large relative to the grating height, the influence of unnecessary diffracted light on the grating wall surface can be reduced, and high diffraction efficiency can be obtained.

[0085] The diffractive optical element of Example 5 also provides a high diffraction efficiency of 95% or more in a wide wavelength range of 800 nm to 1000 nm, similar to Example 1.

[0086] Numerical examples 1 to 5 corresponding to the first to fifth embodiments, respectively, are shown below.

[0087] In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the axial spacing (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the light incident side. Also, nd represents the refractive index of each optical element at the d-line, and vd represents the Abbe number of the optical element. Furthermore, N850 and N950 are the refractive index of each optical element at wavelengths of 850 nm and 950 nm, respectively. The Abbe number vd of a certain material is given by the following when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively: νd=(Nd-1) / (NF-NC) It is expressed as:

[0088] In each numerical example, d, focal length (mm), F-number, and half angle of view (°) are all values ​​when the optical system of each example is focused on an object at infinity. "Back focus" is the distance on the optical axis from the final lens surface (the lens surface closest to the image) to the paraxial image plane, expressed as an air-equivalent length. "Total lens length" is the distance on the optical axis from the forefront lens surface (the lens surface closest to the object) to the final lens surface plus the back focus. "Lens group" is not limited to cases where it is composed of multiple lenses, but also includes cases where it is composed of a single lens.

[0089] If the optical surface is aspherical, an asterisk (*) is added to the right of the surface number. The aspherical shape is expressed as follows: X is the displacement from the vertex of the surface in the optical axis direction, h is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, k is the conic constant, and A4, A6, A8, A10, and A12 are the aspherical coefficients of each order. x=(h 2 / R) / [1+{1-(1+k)(h / R) 2} 1 / 2 ]+A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 +A12×h 12 +A14×h 14 In addition, "e±XX" in each aspherical coefficient is "×10± XX " means.

[0090] Furthermore, the phase shape Ψ of each diffractive surface of the diffractive optical element in each numerical example is expressed as follows, where h is the height in the direction perpendicular to the optical axis, m is the diffraction order of the diffracted light, λ is the design wavelength, and Ci is a phase coefficient (i=2, 4, 6, . . . ): Ψ(h,m)=(2π / mλ0)(C2h 2 +C4h 4 +C6h 6 …) It is expressed as:

[0091] Furthermore, the focal length fdo produced by the diffraction grating for an arbitrary wavelength λ and an arbitrary diffraction order m is expressed by the following equation using the lowest-order phase coefficient C2.

[0092] fdo(λ,m)=-1 / (2C2mλ / λ0) In each numerical example, the diffraction order m of each diffraction grating constituting the diffractive optical element is 1, and the design wavelength λ0 of the diffraction grating is the same as the design wavelength λ of each optical system.

[0093] In each numerical example, the stop is an aperture stop. The effective diameter indicates the maximum diameter of a light beam that contributes to image formation when it passes through each surface.

[0094] (Numerical Example 1) Unit: mm Surface Data Surface number rd nd νd Effective diameter N850 N950 1* 4.2540 0.306 1.5230 50.3 1.88 1.5144 1.5126 2 ∞ 0.500 1.5168 64.2 1.67 1.5098 1.5082 3 ∞ 0.152 1.5900 31.0 0.81 1.5754 1.5725 4* 0.6624 0.108 0.48 5 ∞ 0.400 1.516 64.2 2.00 1.5098 1.5082 6 (Aperture) ∞ 0.300 1.5230 50.3 0.55 1.5144 1.5126 7 (diffraction) -0.5188 0.712 0.61 8 ∞ 1.353 1.5168 64.2 2.00 1.5098 1.5082 9 ∞ 0.100 2.87 Image plane ∞ Aspheric data Front page K = 1.62718e+01 ,A4= 1.22529e-01 ,A6= 1.27776e-02 ,A8=-1.70085e-01 ,A10= 2.15205e-01 ,A12=-9.80892e-02 Side 4 K =-6.46783e+00 ,A4= 6.90928e+00 ,A6=-6.74034e+01 ,A8= 1.47901e+03 ,A10=-1.50941e+04 ,A12= 8.68784e+04 Side 7 K =-8.29507e+00 ,A4=-6.74374e+00 ,A6= 5.41472e+01 ,A8=-3.60460e+02 ,A10= 1.14941e+03 ,A12=-8.36337e+02 Surface 7 (diffractive surface) C2=-7.72514e-02 C4=-1.92323e-02 Focal length 1.247 F-number 2.90 Half angle of view [degrees] 39.80 Image height 1.000 Lens length 3.931 BF 0.100 (Numerical Example 2) Unit: mm Surface Data Surface number rd nd νd Effective diameter N850 N950 1* -53.2849 0.117 1.5230 50.3 1.93 1.5144 1.5126 2 ∞ 0.500 1.5168 64.2 1.90 1.5098 1.5082 3 ∞ 0.152 1.5900 31.0 1.24 1.5754 1.5725 4* 1.0092 0.321 0.83 5 ∞ 0.400 1.5168 64.2 0.79 1.5098 1.5082 6 (Aperture) ∞ 0.144 1.5230 50.3 0.70 1.5144 1.5126 7* -0.6496 0.450 0.69 8(Diffraction) ∞ 0.100 1.5230 50.3 0.80 1.5144 1.5126 9 ∞ 2.256 1.5168 64.2 0.86 1.5098 1.5082 10 ∞ 0.100 2.11 Image plane ∞ Aspheric data Front page K =-3.13204e+26 ,A4= 1.00659e-01 ,A6= 1.93424e-01 ,A8=-5.75284e-01 ,A10= 6.68865e-01 ,A12=-2.90972e-01 Side 4 K =-3.81608e+01 ,A4= 5.25697e+00 ,A6=-4.39875e+01 ,A8= 4.37915e+02 ,A10=-2.32783e+03 ,A12= 6.04677e+03 Side 7 K =-7.28411e+00 ,A4=-3.21823e+00 ,A6= 1.96026e+01 ,A8=-1.37200e+02 ,A10= 6.19336e+02 ,A12=-1.28109e+03 Surface 8 (diffractive surface) C2=-8.25145e-02 Focal length 1.390 F-number 2.91 Half angle of view [degrees] 40.74 Image height 1.000 Lens length 4.540 BF 0.100 (Numerical Example 3) Unit: mm Surface Data Surface number rd nd νd Effective diameter N850 N950 1* 1.7015 0.600 1.5900 31.0 2.21 1.5754 1.5725 2 ∞ 0.500 1.5168 64.2 2.01 1.5098 1.5082 3 ∞ 0.152 1.5230 50.3 1.05 1.5144 1.5126 4* 0.4624 0.235 0.56 5 ∞ 0.400 1.5168 64.2 0.47 1.5098 1.5082 6 (Aperture) ∞ 0.300 1.5230 50.3 0.59 1.5144 1.5126 7 (diffraction) -0.4597 0.712 0.64 8 ∞ 1.500 1.5168 64.2 1.32 1.5098 1.5082 9 ∞ 0.100 2.11 Image plane ∞ Aspheric data Front page K =-6.63982e+00 ,A4= 2.48534e-01 ,A6=-1.77081e-01 ,A8= 2.50116e-01 ,A10=-1.81651e-01 ,A12= 6.50439e-02 Side 4 K =-1.04150e+00 ,A4= 4.57478e+00 ,A6=-3.76110e+01 ,A8= 1.49398e+03 ,A10=-1.88029e+04 ,A12= 1.17210e+05 Side 7 K =-6.13536e+00 ,A4=-6.76189e+00 ,A6= 5.13580e+01 ,A8=-3.23063e+02 ,A10= 9.82174e+02 ,A12=-7.03634e+02 Surface 7 (diffractive surface) C2= 4.16559e-02 Focal length 1.617 F-number 2.905 Half angle of view [degrees] 28.89 Image height 1.000 Lens total length 4.499 BF 0.100 (Numerical Example 4) Unit: mm Surface Data Surface number rd nd νd Effective diameter N850 N950 1 ∞ 0.300 1.5168 64.2 1.60 1.5098 1.5082 2 ∞ 0.135 1.5230 50.3 1.21 1.5144 1.5126 3* 0.5662 0.450 0.81 4 (diffraction) 0.5528 0.098 1.6300 24.0 0.49 1.6104 1.6066 5 ∞ 0.200 1.5168 64.2 0.44 1.5098 1.5082 6 (Aperture) ∞ 0.150 0.36 7* 2.1251 0.114 1.5230 50.3 0.52 1.5144 1.5126 8 ∞ 1.200 1.5168 64.2 0.62 1.5098 1.5082 9 ∞ 0.060 1.74 Image plane ∞ Aspheric data 3rd page K =-2.56362e+01 ,A4= 1.28895e+01 ,A6=-2.00570e+02 ,A8= 2.47706e+03 ,A10=-1.87882e+04 ,A12= 7.77472e+04 ,A14=-1.32962e+05 Side 4 K =-3.92921e+01 ,A4= 2.21121e+01 ,A6=-9.30226e+02 ,A8= 2.80131e+04 ,A10=-4.96246e+05 ,A12= 4.84496e+06 ,A14=-1.97585e+07 Surface 4 (diffractive surface) C 2=-1.17510e-01 C 4=-2.66451e+00 C6= 3.35343e+01 Side 7 K =-2.66363e+03 ,A4= 6.83347e+00 ,A6=-5.17977e+02 ,A8= 1.64227e+04 ,A10=-2.93248e+05 ,A12= 2.64754e+06 ,A14=-9.55528e+06 Focal length 0.808 F-number 2.78 Half angle of view [degrees] 54.01 Image height 0.840 Lens length 2.707 BF 0.060 (Numerical Example 5) Unit: mm Surface Data Surface number rd nd νd Effective diameter N850 N950 1 ∞ 0.500 1.5168 64.2 2.37 1.5098 1.5082 2 ∞ 0.050 1.5230 50.3 1.60 1.5144 1.5126 3* 0.4778 0.363 1.02 4* 0.9206 0.150 1.5900 31.0 0.88 1.5754 1.5725 5 ∞ 0.546 1.5168 64.2 0.84 1.5098 1.5082 6 (Aperture) ∞ 0.300 1.5168 64.2 0.46 1.5098 1.5082 7 ∞ 0.128 1.5230 50.3 0.77 1.5144 1.5126 8 (diffraction) -0.9743 0.450 0.82 9 ∞ 1.500 1.5168 64.2 1.30 1.5098 1.5082 10 ∞ 0.050 2.26 Image plane ∞ Aspheric data 3rd page K =-2.37981e+00 ,A4= 2.73118e+00 ,A6=-1.03706e+01 ,A8= 6.14452e+01 ,A10=-1.87706e+02 ,A12= 2.12113e+02 Side 4 K =-3.80283e+00 ,A4= 7.37065e-01 ,A6=-7.36296e+00 ,A8= 7.04397e+01 ,A10=-3.77744e+02 ,A12= 7.05013e+02 Side 8 K = 2.43405e+00 ,A4=-1.30930e-01 ,A6= 2.01808e+01 ,A8=-2.52414e+02 ,A10= 1.71540e+03 ,A12=-5.43637e+03 ,A14= 6.47049e+03 Surface 8 (diffractive surface) C2=-1.39653e-01 Focal length 0.962 F-number 3.19 Half angle of view [degrees] 60.00 Image height 1.000 Lens length 4.037 BF 0.050 The various values ​​in each numerical example are summarized in Table 1 below.

[0095] [Table 1]

[0096] [Electronic equipment] 14 is a schematic diagram of the main parts of the eyepiece of the electronic device (HMD). The user views a virtual image of an image displayed on a display (not shown) through an eyepiece OL. A light source LS illuminates the user's eye EY, and an image of the eye EY and an image of the light source LS reflected by the eye EY are acquired by an imaging system WL. Here, the imaging system WL can be, for example, an imaging system equipped with an optical system OBL, which is one of the optical systems of Examples 1 to 5.

[0097] The optical systems of the first to fifth embodiments have a wide depth of field, so that high image quality can be obtained even if the distance between the user's eye EY and the imaging system WL changes.

[0098] With this configuration, it is possible to acquire high-resolution images of the eyes EY and the light source LS reflected by the eyes EY, which enables more accurate gaze detection and biometric authentication such as iris recognition.

[0099] It is preferable that the light from the light source LS be invisible, as visible light would be too bright for the user. Specifically, the light emitted from the light source LS should preferably be between 700 nm and 1100 nm. If the wavelength exceeds 1100 nm, the sensitivity of the light receiving element made of inexpensive silicon semiconductors will decrease, increasing noise in the imaging system WL.

[0100] Furthermore, the present invention is not limited to HMDs, but can also be applied to other electronic devices such as the finder section of a digital camera or film camera. [Imaging device] 15 is a schematic diagram of a main part of an imaging device 100. The imaging device 100 is used in a small endoscope and has a camera head 120 and an electric cable 150. The camera head 120 includes a lens housing 121 that mounts the optical system according to any one of Examples 1 to 5, an image sensor (imaging element) 122, and a ceramic substrate 123. The wiring of the electric cable 150 is connected to the image sensor 122 via the ceramic substrate 123.

[0101] In this way, by applying any one of the optical systems of Examples 1 to 5 to an endoscopic imaging device, it is possible to realize an imaging device that is small in size but has a wide depth of field and high optical performance.

[0102] The disclosure of this embodiment includes the following configuration. (Configuration 1) An optical system having at least one lens unit and a diffractive optical element, each of the at least one lens unit is stationary during focusing; When the focal length of the optical system at a wavelength of 850 nm is f85 and the focal length of the optical system at a wavelength of 950 nm is f95, 0.004<|(f85-f95) / (f85+f95)|<0.200 An optical system characterized by satisfying the following conditional expression: (Configuration 2) further comprising an aperture stop; When the distance on the optical axis from the diffractive optical element to the aperture stop is Ld and the focal length at the design wavelength of the optical system is f, 0.05 <Ld / f<0.80 The optical system according to configuration 1, characterized in that the following condition is satisfied: (Configuration 3) When the focal length due to diffraction of the diffractive optical element is fdo and the focal length of the optical system is f, 1.0<|fdo / f|<15.0 3. The optical system according to configuration 1 or 2, wherein the following condition is satisfied: (Configuration 4) When the focal length due to diffraction of the diffractive optical element is fdo, 0.0010<(f85-f95) / fdo<0.0400 4. The optical system according to any one of the configurations 1 to 3, wherein the following condition is satisfied: (Configuration 5) When the distance on the optical axis from the diffractive optical element to the image plane is Lds and the focal length of the optical system is f, 1.0 <Lds / f<4.0 5. The optical system according to any one of configurations 1 to 4, wherein the following condition is satisfied: (Configuration 6) 6. The optical system of any one of configurations 1 to 5, wherein the at least one lens unit includes a lens unit having a substrate and a lens disposed on the substrate. (Configuration 7) 7. The optical system according to configuration 6, wherein the lens is a negative lens. (Configuration 8) When the focal length of the negative lens is fn and the focal length of the optical system is f, -3.0 <fn / f<-0.2 8. The optical system according to configuration 7, wherein the following condition is satisfied: (Configuration 9) 9. The optical system according to any one of configurations 1 to 8, wherein the diffractive surface of the diffractive optical element is formed on a surface that faces air, at least a portion of the diffractive surface being at least partially formed on the surface that faces air. (Configuration 10) When the grating height of the diffraction grating of the diffractive optical element is d (μm), the median value of the wavelength range used in the optical system is λm (μm), and the refractive index of the material forming the diffraction grating at wavelength λm is Nm, 0.7<(Nm-1)×d / λm<1.3 10. The optical system according to any one of configurations 1 to 9, wherein the following condition is satisfied: (Configuration 11) When the grating height of the diffraction grating of the diffractive optical element is d (μm), the median value of the wavelength range used in the optical system is λm (μm), and the refractive index of the material forming the diffraction grating at wavelength λm is Nm, 0.65<(Nm-1)×d<1.15 11. The optical system according to any one of configurations 1 to 10, wherein the following condition is satisfied: (Configuration 12) When the focal length due to diffraction of the diffractive optical element is fdo, the effective diameter of the diffractive optical element is Ddo, the median value of the wavelength range used in the optical system is λm (μm), and the grating height of the diffraction grating of the diffractive optical element is d (μm), 1.0 <fdo×λm / (Ddo×d)<20.0 12. The optical system according to any one of configurations 1 to 11, wherein the following condition is satisfied: (Configuration 13) When the in-focus object distance at the design wavelength of the optical system is Lobj and the focal length of the optical system is f, 10 <Lobj / f <100 13. The optical system according to any one of configurations 1 to 12, wherein the following condition is satisfied: (Configuration 14) 14. The optical system according to any one of configurations 1 to 13, wherein the at least one lens unit includes a unit having an aperture stop and the diffractive optical element. (Configuration 15) 15. The optical system described in any one of configurations 1 to 14, wherein the at least one lens unit includes a first unit having an aperture stop, and a second unit arranged adjacent to the image side of the first unit and having the diffractive optical element provided closest to the object. (Configuration 16) 16. An imaging device comprising the optical system according to any one of configurations 1 to 15, and an imaging element that receives an image formed by the optical system.

[0103] Although the preferred embodiments of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications and changes are possible within the scope of the gist of the present invention. [Explanation of symbols]

[0104] D0 diffractive optical element L1 1st unit L2 2nd unit L3 3rd Unit

Claims

1. An optical system having at least one lens unit and a diffractive optical element, each of the at least one lens unit is stationary during focusing; When the focal length of the optical system at a wavelength of 850 nm is f85 and the focal length of the optical system at a wavelength of 950 nm is f95, 0.004<|(f85-f95) / (f85+f95)|<0.200 An optical system characterized by satisfying the following conditional expression:

2. further comprising an aperture stop; When the distance on the optical axis from the diffractive optical element to the aperture stop is Ld and the focal length at the design wavelength of the optical system is f, 0.05<Ld / f<0.80 2. The optical system according to claim 1, wherein the following condition is satisfied:

3. When the focal length due to diffraction of the diffractive optical element is fdo and the focal length of the optical system is f, 1.0<|fdo / f|<15.0 3. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the focal length due to diffraction of the diffractive optical element is fdo, 0.0010<(f85-f95) / fdo<0.0400 3. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the distance on the optical axis from the diffractive optical element to the image plane is Lds and the focal length of the optical system is f, 1.0<Lds / f<4.0 3. The optical system according to claim 1, wherein the following condition is satisfied:

6. 3. The optical system according to claim 1, wherein the at least one lens unit includes a lens unit comprising a substrate and a lens disposed on the substrate.

7. 7. The optical system according to claim 6, wherein the lens is a negative lens.

8. When the focal length of the negative lens is fn and the focal length of the optical system is f, -3.0<fn / f<-0.2 8. The optical system according to claim 7, wherein the following condition is satisfied:

9. 3. The optical system according to claim 1, wherein the diffractive surface of the diffractive optical element is formed on a surface that faces air, at least a portion of the surface being at least partially diffractive.

10. When the grating height of the diffraction grating of the diffractive optical element is d (μm), the median value of the wavelength range used by the optical system is λm (μm), and the refractive index of the material forming the diffraction grating at wavelength λm is Nm, 0.7<(Nm-1)×d / λm<1.3 3. The optical system according to claim 1, wherein the following condition is satisfied:

11. When the grating height of the diffraction grating of the diffractive optical element is d (μm), the median value of the wavelength range used by the optical system is λm (μm), and the refractive index of the material forming the diffraction grating at wavelength λm is Nm, 0.65<(Nm-1)×d<1.15 3. The optical system according to claim 1, wherein the following condition is satisfied:

12. When the focal length due to diffraction of the diffractive optical element is fdo, the effective diameter of the diffractive optical element is Ddo, the median value of the wavelength range used in the optical system is λm (μm), and the grating height of the diffraction grating of the diffractive optical element is d (μm), 1.0<fdo×λm / (Ddo×d)<20.0 3. The optical system according to claim 1, wherein the following condition is satisfied:

13. When the in-focus object distance at the design wavelength of the optical system is Lobj and the focal length of the optical system is f, 10<Lobj / f<100 3. The optical system according to claim 1, wherein the following condition is satisfied:

14. 3. The optical system according to claim 1, wherein the at least one lens unit includes a unit including an aperture stop and the diffractive optical element.

15. 3. The optical system according to claim 1, wherein the at least one lens unit includes a first unit having an aperture stop, and a second unit arranged adjacent to the image side of the first unit and including the diffractive optical element closest to the object.

16. 3. An imaging device comprising: the optical system according to claim 1; and an imaging element that receives an image formed by the optical system.

Images