Optical system and imaging apparatus having the same

Abstract

To provide an optical system that is compact and well-corrected for various aberrations such as a chromatic aberration.SOLUTION: An optical system comprises a first lens group L1 having positive refractive power, a second lens group L2 having negative refractive power, and a rear group LR comprising one lens group and having positive refractive power, which are arranged in order from an object side to an image side. When focusing from an infinite-distance object to a short-distance object, the second lens group L2 moves toward the image side, and distances between the adjacent lens groups change. A distance D1 on an optical axis from a lens surface closest to the object side in the first lens group L1 to a lens surface closest to the image side in the first lens group L1, a distance LD on the optical axis from the lens surface closest to the object side in the optical system to an image surface, a focal length fL1 of the first lens group L1, a distance BF from the lens surface closest to the image side to the image surface in the optical system when focusing the infinite-distance object, and a focal length f of the optical system satisfy predetermined conditional expressions.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system and an imaging device having the same, and is suitable for imaging devices such as digital still cameras, video cameras, surveillance cameras, and vehicle-mounted cameras. [Background technology]

[0002] Patent Document 1 discloses an optical system in which various aberrations such as chromatic aberration are well corrected by appropriately arranging a positive lens that uses a material with a small Abbe number (high dispersion material). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-215495 Summary of the Invention [Problem to be solved by the invention]

[0004] In the optical system disclosed in Patent Document 1, a positive lens element made of a high-dispersion material is positioned relatively far from the focus group. This makes it difficult to increase the focus sensitivity of the focus group (the amount of focus movement relative to the amount of movement of the focus group). As a result, the amount of movement of the focus group during focusing increases, making it difficult to miniaturize the optical system.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an imaging optical system that is compact and in which various aberrations such as chromatic aberration are well corrected. [Means for solving the problem]

[0006] An optical system according to one aspect of the present invention is an optical system comprising, arranged in order from the object side to the image side, a first lens group having positive refractive power, a second lens group having negative refractive power, and a rear group consisting of one or more lens groups and having positive refractive power, wherein, during focusing from an object at infinity to an object at a close distance, the second lens group moves toward the image side and the spacing between adjacent lens groups changes, the rear group has a negative lens, and the Abbe number νdGP of the material of the positive lens in the first lens group arranged closest to the image, the distance D1 on the optical axis from the lens surface in the first lens group closest to the object to the lens surface in the first lens group closest to the image, the distance LD on the optical axis from the lens surface in the optical system closest to the object to the image plane, and the focal length fL1 of the first lens group satisfy predetermined conditional expressions.

[0007] Other objects and features of the present invention are illustrated in the following examples. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide an optical system and an imaging device that are small and in which various aberrations such as chromatic aberration are well corrected. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 2 is a cross-sectional view of the optical system according to the first embodiment when focused at infinity. [Figure 2] 4A to 4C are aberration diagrams of the optical system in Example 1 when focused on infinity. [Figure 3] FIG. 10 is a cross-sectional view of the optical system according to the second embodiment when focused at infinity. [Figure 4] 10A and 10B are aberration diagrams of the optical system in Example 2 when focused on infinity. [Figure 5] FIG. 10 is a cross-sectional view of the optical system according to the third embodiment when focused at infinity. [Figure 6] 10A and 10B are aberration diagrams of the optical system in Example 3 when focused at infinity. [Figure 7] FIG. 10 is a cross-sectional view of the optical system according to the fourth embodiment when focused at infinity. [Figure 8] 10A and 10B are aberration diagrams of the optical system in Example 4 when focused at infinity. [Figure 9] 1 is a schematic diagram of an imaging device including an optical system in each embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of an optical system and an imaging apparatus having the same according to the present invention will be described with reference to the accompanying drawings.

[0011] 1, 3, 5, and 9 are cross-sectional views of optical systems 1a to 1d of Examples 1 to 4, respectively, when focused at infinity. The optical systems of the examples are used in imaging devices such as digital still cameras, video cameras, surveillance cameras, and vehicle-mounted cameras. In each cross-sectional view, the left side is the object side (front) and the right side is the image side (rear).

[0012] The optical system of each embodiment is configured with multiple lens groups. In this specification, a lens group refers to a group of lenses that move or remain stationary as a unit during focusing. That is, in the optical system of each embodiment, the spacing between adjacent lens groups changes during focusing from an object at infinity to a close object. Note that a lens group may be composed of a single lens or multiple lenses. A lens group may also include an aperture stop.

[0013] The optical systems 1a to 1c in Figures 1, 3, and 5 (Examples 1 to 3) each consist of, arranged in order from the object side to the image side, a first lens group L1 with positive refractive power, a second lens group L2 with negative refractive power, and a rear group LR including one or more lens groups, with a positive refractive power as a whole. During focusing from an object at infinity to a close-up object, the second lens group L2 moves toward the image side, changing the spacing between adjacent lens groups. Note that during focusing, the first lens group L1 and the rear group LR (i.e., optical elements other than the second lens group L2) remain stationary.

[0014] SP is an aperture stop that determines (limits) the light flux of Fno. IP is an image plane, and when the optical system of each embodiment is used as the imaging optical system of a digital still camera or digital video camera, the imaging surface of a solid-state imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor is disposed thereon. When the optical system 1a to 1d of each embodiment is used as the imaging optical system of a silver halide film camera, a photosensitive surface equivalent to the film surface is disposed on the image plane IP. GB is an optical filter disposed on the object side of the image plane IP. In each cross-sectional view, the arrow pointing along the optical axis OA (optical axis direction) indicates the movement direction of the focus group (second lens group L2) when focusing from an object at infinity to a close-up object.

[0015] The optical system 1d of FIG. 7 (Example 4) further includes a third lens group L3 as a vibration-reduction group between the second lens group L2 and the rear group LR. That is, the optical system 1d is composed of, arranged in order from the object side to the image side, a first lens group L1 with positive refractive power, a second lens group L2 with negative refractive power, a third lens group L3 that moves in a direction including a component perpendicular to the optical axis, and a rear group LR that includes one or more lens groups and has positive refractive power as a whole. The optical system 1d is configured so that changes in image position due to vibrations of the optical system 1d are corrected by moving the vibration-reduction group in a direction including a component perpendicular to the optical axis OA. When focusing from an object at infinity to a close object, the second lens group L2 moves toward the image side. During focusing, the first lens group L1, the third lens group L3, and the rear group LR (i.e., optical elements other than the second lens group L2) remain stationary.

[0016] In each embodiment, the rear group LR is composed of only one lens group, but this is not limited to this and it may be composed of multiple lens groups. Here, a lens group is a group of lenses that move or remain stationary as a unit during focusing or zooming.

[0017] 2, 4, 6, and 8 are aberration diagrams of the optical systems 1a to 1d of Examples 1 to 4, respectively, when focused at infinity. In each aberration diagram, Fno is the F-number, and ω is the half angle of view (degrees), which is the angle of view in paraxial calculations. In the spherical aberration diagrams, the amount of spherical aberration for the d-line (wavelength 587.56 nm) and the g-line (wavelength 435.835 nm) is shown. In the astigmatism diagrams, S indicates the amount of astigmatism in the sagittal image plane for the d-line, and M indicates the amount of astigmatism in the meridional image plane for the d-line. In the distortion diagrams, the amount of distortion for the d-line is shown. In the chromatic aberration diagrams, the amount of chromatic aberration for the g-line is shown.

[0018] Next, the characteristic configurations of the optical systems 1a to 1d of Examples 1 to 4 will be described. The optical system of each Example has, arranged in order from the object side to the image side, a first lens group L1 with positive refractive power, a second lens group L2 with negative refractive power, and a rear lens group LR with positive refractive power. During focusing from an object at infinity to a close-up object, the second lens group L2 moves toward the image side. The Abbe number of the material of the positive lens GP located closest to the image in the first lens group L1 is denoted by νdGP. The distance on the optical axis from the lens surface S1 closest to the object in the first lens group L1 (the object-side lens surface of the lens G1) to the lens surface S2 closest to the image in the first lens group L1 (the image-side lens surface of the positive lens GP) is denoted by D1. The distance on the optical axis from the lens surface closest to the object in each of the optical systems 1a to 1d (the object-side lens surface S1 of the lens G1 in each Example) to the image plane IP (total lens length) is denoted by LD. The focal length of the first lens group L1 is denoted by fL1. In this case, the following conditional expressions (1) to (3) are satisfied.

[0019] 15<νdGP<24 (1) 0.20 <D1 / LD<0.45 ···(2) 1.60 <LD / fL1<2.50 ···(3) Conditional formula (1) defines the Abbe number νdGP of the material of the positive lens element GP. If the Abbe number νdGP is small enough to exceed the lower limit of conditional formula (1), excessive chromatic aberration occurs in the positive lens element GP, which is undesirable. On the other hand, if the Abbe number νdGP is large enough to exceed the upper limit of conditional formula (1), the chromatic aberration occurring in the positive lens element GP becomes insufficient, which is undesirable.

[0020] Conditional expression (2) defines the ratio of the total lens length LD to the axial distance D1 from the lens surface S1 closest to the object to the lens surface S2 closest to the image in each of optical systems 1a-1d of the first lens group L1. If distance D1 is so small that it falls below the lower limit of conditional expression (2), it becomes easier to reduce the size of optical systems 1a-1d, but it becomes more difficult to correct spherical aberration and axial chromatic aberration. On the other hand, if distance D1 is so large that it falls below the upper limit of conditional expression (2), it is advantageous from the perspective of aberration correction, but it becomes more difficult to reduce the size of optical systems 1a-1d.

[0021] Conditional expression (3) defines the ratio between the total lens length LD and the focal length fL1 of the first lens unit L1. If the focal length fL1 of the first lens unit L1 becomes so large that it exceeds the lower limit of conditional expression (3), the refractive power of the first lens unit L1 becomes too small, resulting in an increase in the total lens length LD. On the other hand, if the focal length fL1 of the first lens unit L1 becomes so small that it exceeds the upper limit of conditional expression (3), the refractive power of the first lens unit L1 becomes too large, making it difficult to correct spherical aberration and axial chromatic aberration.

[0022] In this way, the optical systems of the examples are configured so that the elements are appropriately set to satisfy the above-mentioned conditional expressions (1) to (3), thereby making it possible to obtain compact optical systems in which various aberrations, such as chromatic aberration, are well corrected.

[0023] In each embodiment, it is preferable to set the numerical ranges of conditional expressions (1) to (3) as shown in the following conditional expressions (1a) to (3a), respectively.

[0024] 15<νdGP<23 (1a) 0.23 <D1 / LD<0.43 ···(2a) 1.63 <LD / fL1<2.30 ···(3a) In each embodiment, it is more preferable to set the numerical ranges of conditional expressions (1a) to (3a) as shown in the following conditional expressions (1b) to (3b), respectively.

[0025] 16<νdGP<21 (1b) 0.27 <D1 / LD<0.40 ···(2b) 1.66 <LD / fL1<2.00 ···(3b) In each embodiment, it is more preferable to satisfy at least one of the following conditional expressions (4) to (7).

[0026] 1.00 <LD / f ···(4) 2.00<|LD / fL2|<3.00 (5) 1.00 <LD / fLR<3.00 ···(6) BF / f<0.50 (7) Here, the focal length of the entire optical system 1a-1d is f, the focal length of the second lens group L2 is fL2, and the focal length of the rear group LR is fLR. Also, the distance from the lens surface closest to the image side of each of the optical systems 1a-1d (the lens surface S3 closest to the image side of the rear group LR in each embodiment) to the image plane IP when focused on an object at infinity (the back focus in air) is BF.

[0027] Conditional formula (4) defines an appropriate range for the total lens length LD and the focal length f of each of the optical systems 1a to 1d. The optical systems of the examples are preferably so-called medium telephoto lenses with a focal length of the entire system of about 75 mm to 135 mm, and therefore satisfy conditional formula (4).

[0028] Conditional expression (5) defines the ratio between the total lens length LD and the focal length fL2 of the second lens group L2. If the focal length fL2 of the second lens group L2 becomes so large that it exceeds the lower limit of conditional expression (5), the focus sensitivity of the second lens group L2 becomes too small, resulting in an increase in the total lens length LD. On the other hand, if the focal length fL2 of the second lens group L2 becomes so small that it exceeds the upper limit of conditional expression (5), the focus sensitivity of the second lens group L2 becomes too large, making it difficult to satisfy the optical performance during focusing.

[0029] Conditional expression (6) defines the ratio between the total lens length LD and the focal length fLR of the rear group LR. If the focal length fLR of the rear group LR becomes so large that it exceeds the lower limit of conditional expression (6), the refractive power of the rear group LR becomes too small, resulting in an increase in the total lens length LD. On the other hand, if the focal length fLR of the rear group LR becomes so small that it exceeds the upper limit of conditional expression (6), the refractive power of the rear group LR becomes too large, making it difficult to correct chromatic aberration of magnification.

[0030] Conditional expression (7) defines the ratio between the back focus BF and the focal length f of the entire system of each of the optical systems 1a to 1d. If the back focus BF becomes large enough to exceed the upper limit of conditional expression (7), the total lens length LD will increase.

[0031] In each embodiment, it is preferable to set the numerical ranges of conditional expressions (5) to (7) as shown in the following conditional expressions (5a) to (7a), respectively.

[0032] 2.20<|LD / fL2|<2.95 (5a) 1.70 <LD / fLR<2.75 ···(6a) BF / f<0.35 (7a) It is more preferable to set the numerical ranges of the conditions (5a) to (7a) as follows:

[0033] 2.40<|LD / fL2|<2.90 (5b) 1.40 <LD / fLR<2.50 ···(6b) BF / f<0.22 (7b) Numerical Examples 1 to 4 corresponding to Examples 1 to 4, respectively, are shown below. In the surface data of each numerical example, the surface number is the surface number counted from the object side (light incident side), r is the radius of curvature of each optical surface, and d (mm) is the axial distance (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the object side. Also, nd is the refractive index of each optical element with respect to the d-line, and vd is the Abbe number of the optical element. Where the distance is variable, this is the value when the object distance (photographing magnification) is changed. The Abbe number vd of a certain material is given by, 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:

[0034] In each numerical example, d, focal length (mm), F-number, and half angle of view (degrees) 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.

[0035] Table 1 also shows the values ​​of the respective numerical values ​​and conditional expressions in each of Numerical Examples 1 to 4.

[0036] (Numerical Example 1) Unit: mm Surface Data Surface number rd nd νd 1 83.952 5.47 1.75500 52.3 2 232.198 0.20 3 66.102 8.39 1.49700 81.5 4 616.793 0.20 5 41.449 11.19 1.43875 94.7 6 1294.215 2.26 1.59551 39.2 7 30.596 4.27 8 49.083 8.22 1.52841 76.5 9 -102.078 1.74 1.78472 25.7 10 72.702 7.35 11 (Aperture) ∞ 0.52 12 93.021 2.54 1.98612 16.5 13 1235.996 (variable) 14 -1586.300 2.15 1.95906 17.5 15 -110.796 1.33 1.69680 55.5 16 31.310 (variable) 17 102.081 8.33 1.83481 42.7 18 -56.202 6.05 19 -56.514 1.54 1.95906 17.5 20 -318.002 23.88 21 ∞ 1.50 1.51633 64.1 22 ∞ (variable) Image plane ∞ Focal length 130.77 F-number 2.06 Half angle of view 9.39 Image height 21.64 Lens length 132.51 BF 25.26 d13 1.98 d16 33.00 d22 0.40 Lens group data Group starting plane focal length 1 1 77.26 2 14 -48.84 3 17 91.57 (Numerical Example 2) Unit: mm Surface Data Surface number rd nd νd 1 103.676 5.18 1.74100 52.6 2 433.772 0.20 3 66.853 7.76 1.53775 74.7 4 388.775 0.20 5 41.981 11.07 1.49700 81.5 6 762.916 3.00 1.62004 36.3 7 31.374 4.34 8 52.272 7.62 1.59522 67.7 9 -117.495 1.73 1.85896 22.7 10 69.495 4.44 11 (Aperture) ∞ 1.05 12 158.457 1.73 1.64769 33.8 13 70.677 3.38 1.98612 16.5 14 -1358.025 (variable) 15 -8807.532 2.42 1.95906 17.5 16 -96.227 1.33 1.75500 52.3 17 31.995 (variable) 18 100.382 8.00 1.77250 49.6 19 -53.373 5.38 20 -55.873 1.56 1.95906 17.5 21 -184.770 26.29 22 ∞ 1.50 1.51633 64.1 23 ∞ (variable) Image plane ∞ Focal length 130.52 F-number 2.06 Half angle of view 9.41 Image height 21.64 Lens length 132.58 BF 27.68 d14 1.98 d17 32.00 d23 0.40 Lens group data Group starting plane focal length 1 1 75.24 2 15 -46.40 3 18 87.15 (Numerical Example 3) Unit: mm Surface Data Surface number rd nd νd 1 96.717 5.36 1.72916 54.1 2 361.994 0.20 3 71.559 7.59 1.43875 94.7 4 539.696 0.20 5 42.055 12.00 1.49700 81.5 6 -2951.988 2.49 1.58144 40.8 7 31.238 3.88 8 45.862 7.51 1.53775 74.7 9 -252.663 1.74 1.85478 24.8 10 59.408 8.02 11 82.307 2.79 1.95906 17.5 12 649.522 2.21 13 (Aperture) ∞ (Variable) 14 -687.828 2.18 1.95906 17.5 15 -97.721 1.33 1.67790 55.3 16 31.153 (variable) 17 92.898 5.66 1.80400 46.5 18 -59.094 6.09 19 -64.940 1.54 1.95906 17.5 20 -400.559 25.87 21 ∞ 1.50 1.51633 64.1 22 ∞ (variable) Image plane ∞ Focal length 130.70 F-number 2.06 Half angle of view 9.40 Image height 21.64 Lens length 132.53 BF 27.26 d13 1.98 d16 32.00 d22 0.40 Lens group data Group starting plane focal length 1 1 79.47 2 14 -49.31 3 17 87.07 (Numerical Example 4) Unit: mm Surface Data Surface number rd nd νd 1 82.948 4.58 1.98612 16.5 2 160.683 0.24 3 84.024 7.09 1.49700 81.5 4 1473.206 0.20 5 45.404 8.46 1.49700 81.5 6 162.104 2.35 1.60342 38.0 7 32.247 4.07 8 46.543 10.84 1.49700 81.5 9 -73.771 1.87 1.95906 17.5 10 84.334 5.22 11 (Aperture) ∞ 4.70 12 104.201 4.31 1.92286 20.9 13 -144.035 (variable) 14 639.220 1.54 1.59522 67.7 15 33.673 (variable) 16 -444.458 3.20 1.92286 20.9 17 -48.623 1.21 1.78880 28.4 18 81.570 (variable) 19 32.814 5.68 1.63854 55.4 20 -228.874 1.43 1.92286 20.9 21 49.521 5.61 22 58.638 8.36 1.85896 22.7 23 -112.475 21.03 24 -24.376 1.53 1.56384 60.7 25 -45.947 16.54 26 ∞ 1.50 1.51633 64.1 27 ∞ (variable) Image plane ∞ Focal length 130.46 F-number 2.06 Half angle of view 9.42 Image height 21.64 Lens length 146.16 BF 17.93 d13 1.00 d15 19.59 d18 3.60 d27 0.40 Lens group data Group starting plane focal length 1 1 76.46 2 14 -59.78 3 16 -111.00 4 19 60.72

[0037] [Table 1]

[0038] Next, an example of a digital still camera (image capture device 100) using the optical system of each embodiment as an image capture optical system will be described with reference to Fig. 9. Fig. 9 is a schematic diagram of the image capture device 100 equipped with the optical system of each embodiment.

[0039] In Fig. 9, 10 denotes a camera body, and 11 denotes an imaging optical system constituted by any one of the optical systems 1a to 1d of Examples 1 to 4. 12 denotes a solid-state imaging element (photoelectric conversion element) such as a CCD sensor or CMOS sensor that is built into the camera body 10 and receives and photoelectrically converts an image (subject image, optical image) formed by the imaging optical system 11. The camera body 10 may be a so-called single-lens reflex camera that has a quick-turn mirror, or a so-called mirrorless camera that does not have a quick-turn mirror. Note that the optical systems 1a to 1d of each Example are not limited to imaging optical systems, and may also be used as other optical systems such as projection optical systems for projection devices (projectors).

[0040] According to each embodiment, it is possible to provide an optical system and an imaging device that are small in size and in which various aberrations such as chromatic aberration are well corrected.

[0041] 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]

[0042] 1a~1d Optical system L1 First lens group L2 Second lens group LR rear group

Claims

1. An optical system comprising, arranged in order from the object side to the image side, a first lens group having positive refractive power, a second lens group having negative refractive power, and a rear group consisting of one or more lens groups and having positive refractive power as a whole, During focusing from an object at infinity to an object at a close distance, the second lens group moves toward the image side, and the interval between the adjacent lens groups changes; the rear group has a negative lens, Let νdGP be the Abbe number of the material of the positive lens arranged closest to the image in the first lens group, D1 be the distance on the optical axis from the lens surface closest to the object in the first lens group to the lens surface closest to the image in the first lens group, LD be the distance on the optical axis from the lens surface closest to the object in the optical system to the image plane, and fL1 be the focal length of the first lens group. 15<νdGP<24 0.20<D1 / LD<0.45 1.60<LD / fL1<2.50 An optical system characterized by satisfying the following conditional expression:

2. When the focal length of the optical system is f, 1.00<LD / f 2. The optical system according to claim 1, wherein the following condition is satisfied:

3. When the focal length of the second lens group is fL2, 2.00<|LD / fL2|<3.00 3. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the focal length of the rear group is fLR, 1.00<LD / fLR<3.00 4. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the optical system is focused on an object at infinity, the distance from the lens surface closest to the image side to the image plane is BF, and the focal length of the optical system is f, BF / f<0.50 5. The optical system according to claim 1, wherein the following condition is satisfied:

6. 6. The optical system according to claim 1, wherein the first lens group and the rear lens group are stationary during focusing.

7. 7. 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