Optical system and imaging device

Abstract

[Problem] To provide an optical system capable of imaging for obtaining an omnidirectional image of high image quality. [Solution] The optical system has a first negative lens Gn1. The optical system satisfies the condition 90° < θ1 ≤ 180°, where θ1 is defined as the half-open angle at the effective diameter position of the lens surface on the object side of the first negative lens.

Description

Optical system and imaging device

[0001] The present invention relates to an optical system suitable for omnidirectional imaging.

[0002] When performing omnidirectional imaging, if a single optical system cannot provide a 360° angle of view, a stitching process is performed to join together multiple captured images obtained using multiple optical systems and image sensors provided for each optical system, as disclosed in Patent Document 1.

[0003] Japanese Patent Application Laid-Open No. 2013-025255

[0004] However, in stitching, misalignment occurs at the seams of captured images due to misalignment of the nodal points (entrance pupils) of multiple optical systems, making it difficult to obtain high-quality omnidirectional images.

[0005] The present invention provides an optical system that enables imaging to obtain a high-quality omnidirectional image.

[0006] An optical system according to one aspect of the present invention includes a first negative lens, and is characterized in that, when θ1 is a half aperture angle at an effective diameter position of the object-side lens surface of the first negative lens, the following condition is satisfied: 90°<θ1≦180°. Note that an imaging device including the above optical system also constitutes another aspect of the present invention.

[0007] According to the present invention, it is possible to provide an optical system that enables imaging to obtain a high-quality omnidirectional image.

[0008] 1 is a cross-sectional view showing the configuration and optical path of an optical system of Example 1. 2 is a longitudinal aberration diagram of the optical system of Example 1. 3 is a cross-sectional view showing the configuration and optical path of an optical system of Example 2. 4 is a longitudinal aberration diagram of the optical system of Example 2. 5 is a diagram showing an imaging device including the optical system of Example 1 or 2.

[0009] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0010] 1 and 3 show cross sections of the optical systems of Examples 1 and 2, respectively. These figures also show the optical paths of the axial light beam and the light beam at the maximum half angle of view. In each figure, the left side is the object side (front side), and the right side is the image side (rear side).

[0011] The optical system of each embodiment is used as an imaging optical system for imaging devices such as digital cameras, video cameras, broadcast cameras, cinema cameras, surveillance cameras, and cameras for silver halide film.

[0012] The optical system of each embodiment forms an optical image of a subject (not shown) located on the object side. The image plane IP is provided with an imaging surface (light receiving surface) of an imaging element such as a CCD sensor or a CMOS sensor, or a film surface (photosensitive surface) of a silver halide film.

[0013] The optical system of each embodiment can also be used as a projection optical system that magnifies light from a display element such as a liquid crystal panel or a digital micromirror device located at a position corresponding to the image plane IP and projects the magnified light onto a projection surface such as a spherical screen. In this case, the object side in each diagram is the magnification conjugate side, and the image side is the reduction conjugate side.

[0014] The optical system of each embodiment is composed of a first lens unit L1 with positive refractive power, an aperture stop SP, and a second lens unit L2 with positive refractive power, arranged in this order from the object side to the image side. The first lens unit L1 includes a first negative lens Gn1, which is a negative meniscus lens arranged closest to the object side. Note that a light-transmitting member other than a lens, such as a cover glass having no or almost no refractive power, may be arranged closer to the object side than the first negative lens Gn1. Each lens unit is composed of one or more lenses.

[0015] Furthermore, the object-side lens surface of the first negative lens Gn1 has a half aperture angle exceeding 90° at its effective diameter position (which will be described later). In other words, when the half aperture angle at the effective diameter position of the object-side lens surface of the first negative lens Gn1 is θ1, the condition of the following formula (1) is satisfied.

[0016] 90°<θ1≦180° (1) The effective diameter position is the position of the intersection of the lens surface and the marginal ray. A marginal ray is a ray that passes through a lens surface and has an intersection point at the farthest position along the optical axis from the vertex of the lens surface.

[0017] A first negative lens with a half aperture angle θ1 of 90° or less cannot capture light rays with a half angle of view of 180° or more that wrap around from the image side of the optical system. In order to capture light rays with a half angle of view of 180° or more, a first negative lens with a half angle of view larger than 90° is required, and a larger half angle is preferable in order to capture light rays with a wider angle of view.

[0018] It is more preferable that the numerical range of the formula (1) is as follows:

[0019] 92.6°≦θ1≦153.4° (1a) Furthermore, it is more preferable to set the numerical range of the formula (1) as follows.

[0020] 95.3°≦θ1≦126.7° (1b) Such a lens with a half aperture angle exceeding 90° can be manufactured, for example, by manufacturing it in a shape divided along a plane parallel to the optical axis and then bonding the divided shapes together by adhesive or the like.

[0021] By satisfying the above configuration and conditions, it is possible to obtain a high-quality omnidirectional image without the inconsistency of the joints that occurs in the past.

[0022] Next, conditions and configurations that the optical system of each example should preferably satisfy will be described. The optical system of each example should preferably satisfy at least one of the conditions and configurations of the following expressions (3) to (8).

[0023] The optical system in each example preferably includes multiple lenses closer to the image side than the first negative lens Gn1. By including multiple lenses closer to the image side than the first negative lens Gn1, it becomes possible for the first negative lens Gn1, which is closest to the object, to capture light rays with a half angle of view of 180° or more, and for the multiple lenses closer to the image side to form a good image of those light rays.

[0024] In addition, it is preferable that the optical system of each embodiment satisfies the condition of the following expression (2), where θ2 is the half aperture angle at the effective diameter position of the image-side lens surface of the first negative lens Gn1.

[0025] 90°<θ2≦180° (2) A first negative lens having a half aperture angle θ2 of 90° or less on the image-side lens surface is not preferable because it is difficult to take in light rays with a half angle of view of 180° or more.

[0026] It is more preferable to set the numerical range of the formula (2) as follows:

[0027] 95.5°≦θ2≦156.1° (2a) Furthermore, it is more preferable to set the numerical range of the formula (2) as follows.

[0028] 100.9°≦θ2≦132.1° (2b) Furthermore, it is preferable that the optical system of each embodiment satisfies the condition of the following equation (3), where FG1 is the focal length of the first negative lens Gn1 and f is the focal length of the entire optical system.

[0029] -75.0≦FG1 / |f|≦-30.0 (3) If the refractive power of the first negative lens Gn1 becomes strong so that FG1 / |f| falls below the lower limit of formula (3), off-axis aberrations such as field curvature, astigmatism, and chromatic aberration of magnification will occur significantly, making it difficult to obtain a high-quality omnidirectional image, which is undesirable. If the refractive power of the first negative lens Gn1 becomes weak so that FG1 / |f| exceeds the upper limit of formula (3), it will be impossible to capture light rays with a half angle of view exceeding 180°, which is also undesirable.

[0030] It is more preferable to set the numerical range of the formula (3) as follows:

[0031] −68.2≦FG1 / |f|≦−38.6 (3a) Furthermore, it is more preferable to set the numerical range of the formula (3) as follows.

[0032] −64.9≦FG1 / |f|≦−47.2 (3b) Furthermore, it is preferable that the optical system of each embodiment satisfies the condition of the following expression (4), where nd is the refractive index of the first negative lens Gn1 at the d line (wavelength 587.6 nm).

[0033] 1.6≦nd≦2.3 (4) If the refractive index of the first negative lens Gn1 is small so that nd is below the lower limit of formula (4), it is not possible to capture light rays with a half angle of view exceeding 180°, which is not preferable. If the refractive index of the first negative lens Gn1 is large so that nd exceeds the upper limit of formula (4), it is usually made of a high-dispersion material, which causes large chromatic aberration of magnification and makes it difficult to obtain a high-quality omnidirectional image, which is not preferable.

[0034] It is more preferable to set the numerical range of the formula (4) as follows:

[0035] 1.7≦nd≦2.2 (4a) Furthermore, it is more preferable to set the numerical range of the formula (4) as follows.

[0036] 1.9≦nd≦2.1 (4b) Furthermore, when the radius of curvature of the object-side lens surface of the first negative lens Gn1 is R1, the radius of curvature of the image-side lens surface is R2, and the shape factor S is defined as S=(R1+R2) / (R1-R2), it is preferable that the optical system of each embodiment satisfies the condition of the following equation (5).

[0037] 2.0≦S≦5.5 (5) If S is below the lower limit of formula (5), the refractive power of the first negative lens Gn1 becomes strong, which causes significant off-axis aberrations such as field curvature, astigmatism, and lateral chromatic aberration, making it difficult to obtain a high-quality omnidirectional image, which is undesirable. If S is above the upper limit of formula (5), the refractive power of the first negative lens Gn1 becomes weak, making it difficult to capture light rays with a half angle of view of 180° or more, which is undesirable.

[0038] It is more preferable to set the numerical range of the formula (5) as follows:

[0039] 2.6≦S≦5.0 (5a) Furthermore, it is more preferable to set the numerical range of the formula (5) as follows.

[0040] 3.0≦S≦4.4 (5b) Furthermore, it is preferable that the optical system of each embodiment satisfies the condition of the following expression (6) when the maximum half angle of view is ω.

[0041] 180.0°≦ω (6) If ω is below the lower limit of equation (6), image loss occurs in part of the omnidirectional image, which is not preferable.

[0042] It is more preferable to set the numerical range of the formula (6) as follows:

[0043] 181.7°≦ω (6a) Furthermore, it is more preferable to set the numerical range of the formula (6) as follows.

[0044] 183.3°≦ω (6b) Furthermore, when the length (distance) on the optical axis from the object-side surface of the first negative lens Gn1 to the aperture stop SP is D1 and the length on the optical axis from the aperture stop SP to the image plane IP is D2, it is preferable that the optical system of each embodiment satisfies the condition of the following equation (7).

[0045] It is preferable that the following relationship is satisfied: 3.0≦D1 / D2≦12.0 (7).

[0046] If D1 / D2 falls below the lower limit of formula (7), D1 becomes too short, and the refractive power of each lens, including the first negative lens Gn1 located closer to the object than the aperture stop SP, becomes too strong. As a result, the refractive power of the lens through which off-axis light rays pass at a high position becomes strong, which undesirably causes significant off-axis aberrations such as field curvature, astigmatism, and chromatic aberration of magnification, making it impossible to obtain a high-quality omnidirectional image. If D1 / D2 exceeds the upper limit of formula (7), D1 becomes too long, and the diameter of the lens G1 closest to the object becomes too large, which is also undesirable.

[0047] It is more preferable to set the numerical range of the formula (7) as follows:

[0048] 3.6≦D1 / D2≦10.3 (7a) Furthermore, it is more preferable to set the numerical range of the formula (7) as follows.

[0049] 4.1≦D1 / D2≦8.5 (7b) Furthermore, it is preferable that the optical system of each embodiment satisfies the condition of the following equation (8), where f1 is the focal length of the first lens unit L1 and f is the focal length of the entire optical system.

[0050] 0.5≦f1 / f≦2.2 (8) The first lens unit L1 is composed of an object-side negative group and an image-side positive group to capture light rays at a wide angle of view. In this case, if f1 / f is below the lower limit of equation (8), the refractive power of the object-side negative group of the first lens unit L1 becomes weak, making it difficult to capture light rays at a wide angle of view, which is undesirable. If f1 / f is above the upper limit of equation (8), the refractive power of the object-side negative group of the first lens unit L1 becomes strong, causing significant off-axis aberrations such as field curvature, astigmatism, and chromatic aberration of magnification, making it difficult to obtain a high-quality omnidirectional image, which is undesirable.

[0051] It is more preferable to set the numerical range of the formula (8) as follows:

[0052] 0.7≦f1 / f≦1.9 (8a) Furthermore, it is more preferable to set the numerical range of the formula (8) as follows.

[0053] 0.8≦f1 / f≦1.5 (8b) Furthermore, it is preferable that the optical system (first lens unit L1) in each example includes a second negative lens Gn2 adjacent to the first negative lens Gn1 on the image side. When the half aperture angles at the effective diameter positions of the object-side and image-side lens surfaces of the second negative lens Gn2 are θ3 and θ4, respectively, it is preferable that the following conditions of formulas (a) and (b) are satisfied:

[0054] 40°<θ3≦100° (9) 65°<θ4≦110° (10) 48.2°<θ3≦90.7° (9a) 70.4°<θ4≦101.1° (10a) 56.3°<θ3≦81.3° (9b) 75.8°<θ4≦92.2° (10b) The optical system of Example 1 includes a first negative lens Gn1, a second negative lens Gn2, and two negative lenses as negative single lenses arranged successively in order from the object side in the first lens unit L1. The optical system of Example 2 includes a first negative lens Gn1, a second negative lens Gn2, and three negative lenses as negative single lenses arranged successively in order from the object side in the first lens unit L1.

[0055] Numerical Examples 1 and 2 corresponding to Examples 1 and 2, respectively, are shown below. In each numerical example, surface number i indicates the order of the surface when counted from the object side. r indicates the radius of curvature (mm) of the i-th surface from the object side, and d indicates the lens thickness or air gap (mm) on the optical axis between the i-th and (i+1)-th surfaces. nd is the refractive index at the d-line of the optical material between the i-th and (i+1)-th surfaces. νd is the Abbe number based on the d-line of the optical material between the i-th and (i+1)-th surfaces.

[0056] The Abbe number vd based on the d-line is expressed as vd=(Nd-1) / (NF-NC), where Nd, NF, and NC are 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.

[0057] The distance d, focal length (mm), F-number, and half angle of view (°) are all values ​​when the optical system in each numerical example is focused on an object at infinity. BF represents back focus (mm). Back focus is the length on the optical axis from the lens surface closest to the image side of the optical system to the paraxial image plane, expressed as an air-equivalent length. The total lens length is the length on the optical axis from the lens surface closest to the object side to the lens surface closest to the image side of the optical system, plus the back focus.

[0058] An "*" attached to a surface number means that the surface has an aspherical shape. The aspherical shape is expressed by the following formula, where x is the displacement from the vertex of the surface in the optical axis direction, h is the height from the optical axis in a direction perpendicular to the optical axis, the direction of light travel is positive, R is the paraxial radius of curvature, K is the conic constant, and A4, A6, A8, and A10 are aspherical coefficients. The conic constant and the aspherical coefficient "e±Z" are multiplied by 10. ±Ｚ means.

[0059] x = (h ２ / R) / [1+{1-(1+K)(h / R) ２ ] １／２　 +A4 x h ４ + A6 x h ６ +A8 x h ８ + A10 x h １０ The values ​​of formulas (1) to (8) in Numerical Examples 1 and 2 are summarized in Table 1. In each of the numerical examples, the d-line is used as the reference wavelength, and the values ​​shown in Table 1 are for this reference wavelength. The optical systems in each of the numerical examples satisfy all of the conditions in formulas (1) to (8).

[0060] 2 and 4 show longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the optical systems of Numerical Examples 1 and 2, respectively, when focused on an object at infinity. In the spherical aberration diagrams, Fno indicates the F-number, the solid line indicates spherical aberration for the d-line (wavelength 587.6 nm), and the dashed-dotted line indicates spherical aberration for the g-line (wavelength 435.8 nm). In the astigmatism diagrams, the solid line ΔS indicates astigmatism on the sagittal image plane, and the dashed line ΔM indicates astigmatism on the meridional image plane. The distortion diagrams show distortion at the d-line. Note that distortion is calculated based on equidistant projection. The chromatic aberration diagrams show chromatic aberration of magnification at the g-line. ω is the half angle of view (°). [Numerical example 1] Unit: mm Surface data Surface number rd nd νd 1 51.037 2.50 2.00502 28.9 2 30.339 18.85 3 30.875 2.00 1.90984 36.4 4 17.114 10.08 5 31.645 1.50 1.96763 25.9 6 8.583 11.13 7 -14.184 0.80 1.87280 40.5 8 12.756 1.72 9 26.631 6.23 1.87141 23.6 10 -15.148 3.46 11 -13.598 0.80 1.92845 34.6 12 -17.977 1.04 13 31.410 6.38 1.71016 30.9 14 -15.500 0.48 15 -9.748 0.50 1.49802 79.3 16 -79.400 1.00 17 17.151 1.19 1.91277 36.1 18 -25.037 0.53 19 (Aperture) ∞ 0.41 20 -13.139 0.50 1.82135 22.9 21 10.101 0.30 22 9.772 2.02 1.78043 48.7 23 145.789 0.30 24 150.450 1.03 1.76147 50.7 25 -12.785 0.40 26 8.793 1.35 1.50466 80.3 27 -9.126 0.32 28 -8.598 1.00 1.80815 22.8 29 -9.445 0.36 30 -7.249 0.80 1.81135 22.8 31 12.536 0.39 32 23.338 1.34 1.72923 54.7 33 -14.200 4.34 Image plane ∞ Various data Focal length 1.42 F-number 2.79 Half angle of view (°) 185.0 Image height 4.00 Total lens length 85.02 BF 4.34 Lens group data Group Initial surface Focal length 1 1 1.64 2 19 27.57 [Numerical example 2] Unit: mm Surface data Surface number rd nd νd 1 71.897 5.00 2.00100 29.1 2 39.099 22.20 3 37.580 2.50 2.00100 29.1 4 23.676 12.00 5* 31.554 2.00 1.88668 38.8 6 12.410 8.37 7 78.963 1.00 1.59522 67.7 8 12.638 5.16 9 -15.698 0.80 1.59522 67.7 10 11.710 2.30 11* 40.794 2.95 1.96568 24.9 12 -509.283 0.30 13 1062.045 4.21 1.93160 33.8 14* -17.841 7.92 15* -12.309 0.80 1.81466 22.8 16 -16.614 0.75 17 20.369 2.64 1.59513 67.5 18 341.584 1.20 19 273.339 2.16 1.72922 54.7 20 -16.554 0.41 21 -9.991 0.50 1.49700 81.5 22 -37.488 0.85 23 15.501 1.48 1.80725 46.0 24 -23.981 0.48 25(Aperture) ∞ 0.39 26* -12.961 0.50 1.69301 29.3 27 10.172 0.47 28 11.950 1.09 1.73908 53.4 29 189.429 0.30 30 299.903 0.91 1.65588 61.0 31 -14.519 0.31 32 10.859 1.11 1.49700 81.5 33 -11.802 0.75 34 -10.865 0.80 1.80810 22.8 35 12.142 0.44 36 25.444 1.20 1.77250 49.6 37* -141.365 0.30 38 209.095 1.21 1.59522 67.7 39 -13.727 3.10 Image plane ∞ Aspheric surface data Surface 5 K = 0.00000e+00 A 4=-8.10166e-06 A 6= 1.38874e-08 A 8=-3.09318e-11 A10= 3.96310e-14 Surface 11 K = 0.00000e+00 A 4=-2.38743e-05 A 6=-8.27601e-07 A 8=-7.32387e-09 A10=-2.64958e-11 Surface 14 K = 0.00000e+00 A 4=-2.21199e-06 A 6=-3.65149e-07 A 8=-4.42720e-10 A10= 2.22297e-11 Surface 15 K = 0.00000e+00 A 4= 8.11020e-05 A 6=-2.08816e-08 A 8= 1.94718e-07 A10=-4.55195e-09 Surface 26 K = 0.00000e+00 A 4=-2.50906e-04 A 6=-1.88517e-04 A 8= 1.20885e-04 A10=-2.18158e-05 Surface 37 K = 0.00000e+00 A 4=-5.47859e-05 A 6=-2.97190e-05 A 8= 4.25749e-07 A10=-1.66408e-07 Data Focal length 1.55 F-number 2.79 Half angle of view (°) 185.0 Image height 4.00 Lens length 100.86 BF 3.10 Lens Group Data Group Initial Surface Focal Length 1 1 1.57 2 25 27.93 .

[0061]

[0062] [Imaging Device] Figure 5 shows an omnidirectional digital camera as an imaging device that uses the optical system of each of the above-described embodiments as its imaging optical system. Reference numeral 20 denotes a camera body, and 21 denotes an imaging optical system configured with the optical system of either embodiment 1 or 2. Reference numeral 22 denotes a solid-state imaging element such as a CCD sensor or CMOS sensor that is built into the camera body 20 and photoelectrically converts the optical image (subject image) formed by the imaging optical system 21, i.e., captures the subject image through the imaging optical system 21. Reference numeral 23 denotes a recording unit that records image data generated by processing the imaging signal from the imaging element 22.

[0063] By using the optical system of each embodiment, it is possible to configure an omnidirectional digital camera capable of obtaining omnidirectional images as high-quality captured images. Note that the imaging optical system may be detachable (replaceable) from the camera body.

[0064] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention.

Claims

1. An optical system having a first negative lens, wherein when the half aperture angle at the effective diameter position of the object-side lens surface of the first negative lens is θ1, the optical system satisfies the condition 90°<θ1≦180°.

2. The optical system according to claim 1, wherein the condition 90°<θ2≦180° is satisfied, where θ2 is the half aperture angle at the effective diameter position of the image-side lens surface of said first negative lens.

3. The optical system according to claim 1 or 2, characterized in that the following condition is satisfied: -75.0≦FG1 / |f|≦-30.0, where FG1 is the focal length of the first negative lens and f is the focal length of the optical system.

4. The optical system according to any one of claims 1 to 3, characterized in that the following condition is satisfied: 1.6≦nd≦2.3, where nd is the refractive index of the first negative lens at the d-line.

5. An optical system according to any one of claims 1 to 4, characterized in that when the radius of curvature of the object-side lens surface of the first negative lens is R1, the radius of curvature of the image-side lens surface of the first negative lens is R2, and S = (R1 + R2) / (R1 - R2), the condition 2.0 ≦ S ≦ 5.5 is satisfied.

6. An optical system according to any one of claims 1 to 5, characterized in that the following condition is satisfied: 180.0°≦ω, where ω is the maximum half angle of view of the optical system.

7. The optical system according to any one of claims 1 to 6, characterized in that it includes an aperture stop, and satisfies the condition 3.0≦D1 / D2≦12.0, where D1 is the length on the optical axis from the object-side lens surface of the first negative lens to the aperture stop, and D2 is the length on the optical axis from the aperture stop to the image plane.

8. The optical system according to any one of claims 1 to 7, characterized in that the optical system is composed of a first lens group with positive refractive power, an aperture stop, and a second lens group with positive refractive power, arranged in that order from the object side to the image side, and satisfies the condition 0.5≦f1 / f≦2.2, where f1 is the focal length of the first lens group and f is the focal length of the optical system.

9. The optical system according to any one of claims 1 to 8, characterized in that it has a second negative lens adjacent to the first negative lens on the image side, and when half aperture angles at the effective diameter positions of the object side lens surface of the second negative lens and the image side lens surface of the second negative lens are θ3 and θ4, respectively, it satisfies the following conditions: 90°<θ3≦180° 90°<θ4≦180° 10. The optical system according to claim 9, characterized in that the optical system includes the first negative lens, the second negative lens, and at least two negative lenses, which are single negative lenses arranged consecutively from the object side.

11. An imaging device comprising: an optical system according to any one of claims 1 to 10; and an imaging element for capturing an image of a subject through said optical system.

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