Optical system and imaging device

Abstract

Provided is an imaging device that is small and thin as a whole and has a wide-angle and bright optical system and an optical system mounted thereon. **Solution**: The optical system includes, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90° and a lens group composed of a plurality of lenses that converge light onto the imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging element moves away from the lens group, and the reflective optical element group does not move. Furthermore, the following conditional expression (1) is satisfied. (1) 0.50 ≤ PTI / THP ≤ 3.50 However, PTI: Distance from the reflective optical element group to the image plane THP: Thickness of the prism as the reflective optical element

Description

Technical Field

[0001] The present invention relates to an optical system and an imaging device.

Background Art

[0002] In an imaging device that requires thinning, particularly in a smartphone or the like, increasing the thickness of the product is a major issue in how to combine a large-sized image sensor and an optical system that utilizes its performance. For example, according to Patent Document 1, an optical system capable of obtaining a high-resolution image even in digital zoom has been proposed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Among the problems of achieving compatibility between a large-sized image sensor and an optical system that utilizes the performance of the image sensor, it is particularly difficult to realize a wider-angle and brighter optical system. For example, according to the above-mentioned prior art documents, the angle of view is not sufficient to achieve a sufficiently wide angle, being about 26 degrees to 32 degrees, and the F-number is about F2.8, and it cannot be said that the aperture is sufficiently large. Also, the sensor size is as small as about 1 / 3 inch, and the enlargement of the sensor size is insufficient, and high image quality has not been achieved. In addition, an imaging device has emerged in which a prism is arranged on the object side to bend the optical path, and even an optical system with a long overall length can be thinned, but many of these are telephoto lenses or zoom lenses, and they still do not achieve compatibility between a large-sized image sensor and a wide-angle and bright optical system.

[0005] The present invention has been made to solve such problems, and provides an optical system that is overall small and thin, has a wide angle, is bright, and has high performance, and an imaging device equipped with such an optical system.

Means for Solving the Problems

[0006] The optical system according to the first aspect of the present invention has, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging element moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (1). (1) 0.50 ≦ PTI / THP ≦ 3.50 However, PTI: Distance from the reflective optical element group to the image plane THP: Thickness of the prism as the reflective optical element

[0007] In the above first aspect, the reflective optical element group may be composed only of reflective optical elements such as a prism, or a plurality of lenses may be used in addition, or a prism and a lens may be joined, or a curvature may be given to the surface of the prism to provide power. PTI is the distance from the reflective optical element group to the image plane, and more precisely, it is the distance from the image-side surface of the most image-side optical element of the reflective optical element group to the image plane. The shorter this distance, the smaller the volume of the entire optical system can be.

[0008] In addition, THP is the thickness of a prism as a reflective optical element, and the thinner this thickness is, the thinner the overall thickness of the optical system can be made. The most commonly used reflective optical element placed within the reflective optical element group is a prism. Here, a prism is a general term for an optical element made of an optical material with a certain refractive index and having an incident surface, a reflective surface, and an exit surface. Even if curvature is given to each surface of the prism to provide power, the effect of refracting the image is not limited or significantly changed, and there is no particular difference in handling between a prism with each surface being a plane and other prisms. Also, when another optical element is joined and integrated with the prism, the thickness target is the portion of the prism composed of a single material excluding the joined optical element. Note that the thickness of the prism refers to the axial distance between the incident surface and the exit surface when the chief ray on the optical axis traverses from the incident surface of the prism to the reflective surface and then to the exit surface.

[0009] In the first aspect described above, the conditional expression (1) defines the ratio of PTI to THP. If this numerical value is below the lower limit of the conditional expression (1), THP becomes too large, making it difficult to reduce the thickness of the optical system. Also, if this numerical value exceeds the upper limit of the conditional expression (1), PTI becomes too large, making it difficult to reduce the volume of the optical system.

[0010] Further, the optical system in the second aspect of the present invention has, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light to an imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging element moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (2). (2) 15.00 ≦ CRA ≦ 50.00 However, CRA: Chief ray incident angle to the diagonal end of the imaging element

[0011] In the second aspect described above, the conditional expression (2) defines the principal ray incident angle on the diagonal end of the imaging device. In order to design the lens group to be thin, small, and high-performance, it is important to reduce the outer diameter of the lens on the object side to reduce the lens thickness and increase the number of lens elements. Also, for the lens on the image side, although aberration correction such as spherical aberration and field curvature is performed, an extra lens power is required to change the angle of the light beam incident on the imaging device, and furthermore, the number of lens elements increases and the lens group becomes enlarged, which is not preferable. Therefore, in the case of a wide-angle lens, it is preferable for miniaturization to keep the incident angle on the outermost periphery (diagonal end) of the imaging device to be somewhat large.

[0012] When this numerical value is below the lower limit of the conditional expression (2), in order to reduce the angle of the light beam incident on the lens group at a large angle due to the wide-angle lens with the lens on the image side, an extra lens power is required, and the number of lens elements increases, making it difficult to miniaturize the lens group. Also, when this numerical value exceeds the upper limit of the conditional expression (2), due to the pupil mismatch (mismatch between the incident angle and the capture angle) between the incident angle of the light ray and the on-chip microlens of the imaging device, it becomes difficult to achieve high image quality because it causes a decrease in peripheral light quantity and a decrease in peripheral image quality.

[0013] Further, the optical system in the third aspect of the present invention has, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto the imaging device. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging device moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (3). (3) 0.20 ≦ EPD / THP ≦ 0.94 However, EPD: Entrance pupil diameter of the lens group THP: Thickness of the prism as the reflective optical element

[0014] In the third aspect described above, the EPD is the entrance pupil diameter of the lens group. The larger the entrance pupil diameter of the lens group, the smaller the F-number and the brighter the optical system can be configured. However, in this optical system, since there is a reflective optical element group on the object side, the entrance pupil diameter cannot be increased beyond the prism thickness, and the peripheral light beam must not be cut too much to ensure the peripheral light quantity. Therefore, it is necessary to set an arbitrary ratio between the entrance pupil and the prism thickness. Thus, the conditional expression (3) defines the ratio between the entrance pupil and the prism thickness. If this value is below the lower limit, the entrance pupil diameter becomes too small, making it difficult to configure a bright optical system. If this value exceeds the upper limit, the prism thickness relative to the entrance pupil becomes too small, making it difficult to pass the peripheral light beam and reducing the peripheral light quantity, thus making it difficult to achieve high image quality.

[0015] Further, the optical system according to the fourth aspect of the present invention has, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light to the imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging element moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (4). (4) 0.26 ≦ EPD / PTI ≦ 0.80 However, EPD: Entrance pupil diameter of the lens group PTI: Distance from the reflective optical element group to the image plane

[0016] In the fourth aspect described above, the EPD is the entrance pupil diameter of the lens group. The larger the entrance pupil diameter of the lens group, the smaller the F-number and the brighter the optical system can be configured. However, when the entrance pupil diameter is large, it becomes difficult to correct aberrations in the lens group, resulting in an increase in the number of lens elements, a longer lens, and difficulty in miniaturization. The PTI is the distance from the image-side surface of the rightmost image-side optical element of the reflective optical element group to the image plane, which can also be regarded as the range where the lens group can exist. If this distance is small, the volume of the optical system can be reduced. However, if the distance is made too short, the number of lens elements becomes too small, making it difficult to improve the performance of a bright optical system. Therefore, it is necessary to appropriately set the ratio between the entrance pupil diameter and the distance from the image-side surface of the rightmost image-side optical element of the reflective optical element group to the image plane. Thus, the conditional expression (4) defines the ratio of the EPD to the PTI. If this numerical value falls below the lower limit, the EPD becomes too small, making it difficult to configure a bright optical system. Also, if this numerical value exceeds the upper limit, the PTI becomes too small, the number of lens elements becomes too small, making it difficult to correct aberrations in a bright optical system, and making it difficult to improve the performance of the optical system.

[0017] Further, the optical system according to the fifth aspect of the present invention has, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the imaging element moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (5). (5) 0.20 ≦ IH / PTI ≦ 0.94 However, IH: Maximum image height on one side PTI: Distance from the reflective optical element group to the image plane

[0018] In the above-described aspect 5, IH is the maximum image height on one side. The larger the maximum image height on one side, the higher the resolution can be increased, enabling higher performance. Also, PTI represents the distance from the group of reflective optical elements to the image plane. The longer this distance, the more lens elements can be used for aberration correction, enabling higher performance. Therefore, conditional expression (5) defines the ratio of IH to PTI. If this numerical value falls below the lower limit, IH becomes too small and the sensor size cannot be increased, making it difficult to achieve higher performance. Also, if this numerical value exceeds the upper limit, PTI becomes too small and the number of lens elements becomes too small, making it difficult to correct the aberration of a bright optical system and difficult to achieve higher performance of the optical system.

[0019] Further, the optical system in the sixth aspect of the present invention is the optical system of any one of the above-described first to fifth aspects, and furthermore, the lens closest to the imaging element is composed of a lens surface having an inflection point on at least one of the object side surface and the image side surface, the outer shape of the lens has a length corresponding to the short side of the imaging element smaller than the length corresponding to the long side of the imaging element, and further satisfies the following conditional expression (6). (6) CAV / CAD ≦ 0.90 However, CAV: Effective diameter at the short side end CAD: Effective diameter at the diagonal end

[0020] In the sixth aspect described above, the lens closest to the imaging device is composed of a lens surface having at least one surface of the object side or the image side with an inflection point. The inflection point mentioned here refers to a point where, in the case of an aspherical surface shape, as it goes from the axis to the periphery, the curvature of the surface switches from a convex surface to a concave surface, or vice versa. In order to reduce the thickness of the lens group while improving performance, a lens with a relatively large outer diameter close to the imaging device corrects the field curvature and astigmatism. However, if it is composed of lenses without an inflection point, it is difficult to reduce the thickness of the lens group. Also, in this optical system, in order to reduce the prism thickness, it is bent and reflected in the short side direction of the imaging device. If the effective diameter in the short side direction of the lens close to the imaging device can be made smaller and the outer shape of the lens can also be made smaller, the volume of the lens group can be made smaller and thinner. Therefore, conditional expression (6) defines the ratio between the effective diameter at the short side end and the effective diameter at the diagonal end. If this numerical value exceeds the upper limit, since the effective diameter of the short side end with respect to the diagonal end cannot be made small, it becomes difficult to reduce the volume of the lens group and it becomes difficult to make it thinner.

[0021] Further, the optical system in the seventh aspect of the present invention is the optical system of any one of the first to fifth aspects described above, and further satisfies the following conditional expression (7). (7) 50.00 ≦ EFL / m1m ≦ 250.00 However, EFL: Focal length of the lens group m1m: Focus movement amount of the lens group or the imaging device when the object distance is from infinity to 1 m

[0022] In the seventh aspect described above, EFL represents the focal length of the lens group, and m1m represents the focus movement amount of the lens group or the imaging device when the object distance is from infinity to 1 m. Also, conditional expression (7) defines the ratio between EFL and m1m. If this numerical value is below the lower limit, the focus movement amount of the lens group or the imaging device becomes too large and it becomes difficult to miniaturize the optical system. Also, if this numerical value exceeds the upper limit, the focal length becomes large and it becomes difficult to widen the angle of view of the optical system.

[0023] Moreover, the optical system in the eighth aspect of the present invention is the optical system according to any one of the first to fifth aspects, and further satisfies the following conditional expression (8). (8) 0.01 ≦ STO / PTI ≦ 0.35 However,[[]] STO: Distance from the group of reflective optical elements to the aperture

[0024] In the eighth aspect described above, STO represents the distance from the group of reflective optical elements to the aperture. Also, PTI represents the distance from the group of reflective optical elements to the image plane. The conditional expression (8) defines the ratio between STO and PTI. If this numerical value falls below the lower limit, the aperture of the lens group gets too close to the reflective optical element, making it difficult to secure the focus movement amount of the lens group and difficult to enhance the performance of the optical system. Also, if this numerical value exceeds the upper limit, the aperture of the lens group gets too far from the reflective optical element, making it difficult to make the optical system thinner due to the increase in size of the reflective optical element.

[0025] Moreover, the optical system in the ninth aspect of the present invention is the optical system according to any one of the first to fifth aspects, and further satisfies the following conditional expression (9). (9) 1.70 ≦ ndP However,[[]] ndP: Refractive index of the prism as the reflective optical element based on the d-line

[0026] In the above aspect 9, ndP represents the refractive index of the prism as a reflective optical element based on the d-line. In this optical system, the reflecting surface of the prism as a reflective optical element is configured to cause total reflection so that the light quantity is not reduced. Also, since it is a wide-angle lens, it is important to be a high-refractive-index material in order to satisfy the total reflection condition. On the other hand, the higher the refractive index of the prism, the smaller the ray angle in the prism can be made, so that the effective diameter and thickness of the prism can be reduced. Therefore, the conditional expression (9) defines the range of the refractive index of the prism as a reflective optical element based on the d-line. If this numerical value falls below the lower limit, it becomes difficult to satisfy the total reflection condition when used as a wide-angle lens. As a result, not only does the light quantity decrease at the reflecting surface of the prism, but also when the refractive index of the prism is small, it is insufficient to reduce the ray angle in the prism, so that the effective diameter and thickness of the prism cannot be reduced, making it difficult to make the optical system thinner.

[0027] Moreover, the imaging device according to the 10th aspect of the present invention includes any one of the optical systems of the above-described 1st aspect to 9th aspect, and an imaging element that converts the formed optical image into an electrical signal.

Advantages of the Invention

[0028] According to the present invention, it is possible to provide an optical system that is small and thin as a whole, has a wide angle, is bright, and has high performance, and an imaging device equipped with such an optical system.

Brief Description of the Drawings

[0029]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Embodiments for Carrying Out the Invention

[0030] Hereinafter, the present invention will be described through embodiments of the invention, but the invention according to the claims is not limited to the following embodiments. Also, not all of the configurations described in the embodiments are essential as means for solving the problems.

[0031] Hereinafter, the zoom lens according to the present invention and the imaging device including the zoom lens will be described with reference to tables and drawings. In the following tables, r represents the radius of curvature, d represents the lens thickness or the lens interval, nd represents the refractive index of the d-line, and vd represents the Abbe number based on the d-line (wavelength 587.5620 nm). The surface number represents the order of the optical surfaces from the object side along the direction in which the light travels. Also, "∞" for the radius of curvature indicates a plane or an aperture, and the refractive index of air "1.000000" is omitted. When the optical surface is an aspherical surface, an asterisk (*) is attached to the surface number, and the paraxial radius of curvature is shown in the column of the radius of curvature r. Note that the length is "mm" unless otherwise specified, but the optical system can obtain the same optical performance even if it is enlarged or reduced, and thus is not limited thereto.

[0032] Also, when c is the curvature, k is the conic coefficient, A4, A6, A8, A10... are the aspherical coefficients of each order, and z is the displacement in the optical axis direction at the position of the height h from the optical axis with respect to the vertex of the surface, the aspherical shape is defined by the following formula. z = ch 2 / [1 + {1 - (1 + k)c 2 h 2} 1 / 2 + A4h 4 + A6h 6 + A8h 8 + A10h 10 …

[0033] Also, each longitudinal aberration diagram shows, in order from the left, spherical aberration (SA (mm)), astigmatism (AST (mm)), and distortion (DIS (%)). In the spherical aberration diagram, the vertical axis represents the F-number (indicated by FNO in the figure), the solid line represents the characteristics of the d-line, the short dashed line represents the F-line, and the long dashed line represents the C-line. In the astigmatism diagram, the vertical axis represents the angle of view (indicated by W in the figure), the solid line represents the characteristics of the sagittal plane (indicated by S in the figure), and the dashed line represents the characteristics of the meridional plane (indicated by M in the figure). In the distortion diagram, the vertical axis represents the angle of view (indicated by W in the figure).

[0034] (Example 1) FIG. 1 is a lens configuration diagram of an optical system according to an embodiment (Embodiment 1) of the present embodiment in (A) an infinite focus state, (B) a 1 m focus state, and (C) a closest focus state, and (D) a diagram showing the state change of the reflective optical element group (not moving) and the lens group from the infinite focus state to the closest focus state. FIG. 2 is a longitudinal aberration diagram of the infinite focus state in Embodiment 1.

[0035] The optical system according to Embodiment 1 includes a reflective optical element group GR and a lens group GL, and an aperture stop S for adjusting the amount of light is disposed between the lens groups GL. Further, an optical filter CG composed of an infrared cut filter or the like is disposed between the lens group GL and the imaging position I.

[0036] The values of various parameters in Embodiment 1 are shown below. Surface data [Table 1]

[0037] Aspherical data (the aspherical coefficients not shown are 0.00.) [Table 2]

[0038] Various data [Table 3]

[0039] Object distance data [Table 4]

[0040] (Embodiment 2) FIG. 3 is a lens configuration diagram of an optical system (Example 2) according to this embodiment in (A) an infinity focus state, (B) a 1 m focus state, and (C) a closest focus state, and (D) a diagram showing the state change of the reflective optical element group (not moving) and the lens group from the infinity focus state to the closest focus state. FIG. 4 is a longitudinal aberration diagram of the infinity focus state in Example 2.

[0041] The optical system according to Example 2 includes a reflective optical element group GR and a lens group GL, and an aperture stop S for adjusting the amount of light is disposed between the lens groups GL. Further, an optical filter CG composed of an infrared cut filter or the like is disposed between the lens group GL and the imaging position I.

[0042] The values of various specifications in Example 2 are shown below. Surface data [Table 5]

[0043] Aspherical data (the aspherical coefficients not shown are 0.00.) [Table 6]

[0044] Various data [Table 7]

[0045] Object distance data [Table 8]

[0046] (Example 3) FIG. 5 is a lens configuration diagram of an optical system (Example 3) according to the present embodiment in (A) an infinity focus state, (B) a 1 m focus state, and (C) a closest focus state, and (D) a diagram showing the state change of the reflective optical element group (not moving) and the lens group from the infinity focus state to the closest focus state. FIG. 6 is a longitudinal aberration diagram of the infinity focus state in Example 3.

[0047] The optical system according to Example 3 includes a reflective optical element group GR and a lens group GL, and an aperture stop S for adjusting the amount of light is disposed between the lens groups GL. Further, an optical filter CG composed of an infrared cut filter or the like is disposed between the lens group GL and the imaging position I.

[0048] The values of various specifications in Example 3 are shown below. Surface data

Table 9

[0049] Aspherical data (the aspherical coefficients not shown are 0.00.)

Table 10

[0050] Various data

Table 11

[0051] Object distance data

Table 12

[0052] (Example 4) FIG. 7 is a lens configuration diagram of an optical system (Example 4) according to the present embodiment in (A) an infinite focus state, (B) a 1 m focus state, and (C) a closest focus state, and (D) a diagram showing the state change of the reflective optical element group (not moving) and the lens group from the infinite focus state to the closest focus state. FIG. 8 is a longitudinal aberration diagram of the infinite focus state in Example 4.

[0053] The optical system according to Example 4 includes a reflective optical element group GR and a lens group GL, and an aperture stop S for adjusting the amount of light is disposed between the lens groups GL. Further, an optical filter CG composed of an infrared cut filter or the like is disposed between the lens group GL and the imaging position I.

[0054] The values of various specifications in Example 4 are shown below. Surface data

Table 13

[0055] Aspherical data (the aspherical coefficients not shown are 0.00.)

Table 14

[0056] Various data

Table 15

[0057] Object distance data

Table 16

[0058] In Examples 1 to 4 described above, an example in which a block-shaped glass material is arranged as the reflection optical element group has been described. However, in any of the examples, instead of this, a reflection optical element group capable of bending the optical path by approximately 90° can be arranged. As such an optical path bending element, a prism or a reflection mirror can be employed. FIG. 9 is a diagram showing a lens configuration in which the glass material of Example 4 is replaced with a prism that bends the optical path by approximately 90°. (A) is a lens configuration diagram in an infinite focus state, (B) is a lens configuration diagram in a 1 m focus state, (C) is a lens configuration diagram in a closest focus state, and (D) is a diagram showing the state change of the reflection optical element group (not moving) and the lens group from the infinite focus state to the closest focus state. In the example of FIG. 9, as the reflection optical element group capable of bending the optical path by approximately 90°, a prism in which all of the incident surface, the reflection surface, and the exit surface are configured as planes is arranged. However, such a reflection optical element group is not limited to a prism having such a configuration. For example, curvature can be given to each surface of the prism, or a lens other than the prism can be used as this reflection optical element group.

[0059] The optical systems of Examples 1 to 4 have been described above as the present embodiment. Here, the correspondence between each conditional expression according to the present embodiment and each example will be described. First, in the optical system of any of the examples, in order from the object side, it has a reflection optical element group GR capable of bending the optical path by approximately 90° and a lens group GL composed of a plurality of lenses that converge light onto the imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group GL approaches the reflection optical element group GR or the imaging element moves away from the lens group GL, and the reflection optical element group GR does not move, and further satisfies the following conditional expression (1). (1) 0.50 ≦ PTI / THP ≦ 3.50 However, PTI: Distance from the reflection optical element group to the image plane THP: Thickness of the prism as the reflection optical element

[0060] Note that the above conditional expression (1) can define the following conditional expression (1a) as a more preferable condition and the following conditional expression (1b) as an even more preferable condition. (1a) 0.65 ≦ PTI / THP ≦ 2.50 (1b) 0.80 ≦ PTI / THP ≦ 2.00

[0061] In addition, the optical system in any of the embodiments satisfies the following conditional expression (2). (2) 15.00 ≦ CRA ≦ 50.00 However,[[]] CRA: Angle of incidence of the chief ray to the diagonal end of the imaging device

[0062] Note that the above conditional expression (2) may define the following conditional expression (2a) as a more preferable condition and the following conditional expression (2b) as an even more preferable condition.[[]] (2a) 17.00 ≦ CRA ≦ 45.00 (2b) 20.00 ≦ CRA ≦ 40.00

[0063] In addition, the optical system in any of the embodiments satisfies the following conditional expression (3). (3) 0.20 ≦ EPD / THP ≦ 0.94 However,[[]] EPD: Entrance pupil diameter of the lens group THP: Thickness of the prism as a reflective optical element

[0064] Note that the above conditional expression (3) may define the following conditional expression (3a) as a more preferable condition and the following conditional expression (3b) as an even more preferable condition.[[]] (3a) 0.25 ≦ EPD / THP ≦ 0.92 (3b) 0.30 ≦ EPD / THP ≦ 0.90

[0065] In addition, the optical system in any of the embodiments satisfies the following conditional expression (4). (4) 0.26 ≦ EPD / PTI ≦ 0.80 However,[[]] EPD: Entrance pupil diameter of the lens group PTI: Distance from the reflective optical element group to the image plane

[0066] Incidentally, the above conditional expression (4) can define the following conditional expression (4a) as a more preferable condition and the following conditional expression (4b) as an even more preferable condition. (4a) 0.32 ≦ EPD / PTI ≦ 0.70 (4b) 0.38 ≦ EPD / PTI ≦ 0.60

[0067] In addition, the optical system in any of the examples satisfies the following conditional expression (5). (5) 0.20 ≦ IH / PTI ≦ 0.94 However, IH: Maximum image height on one side PTI: Distance from the reflective optical element group to the image plane

[0068] Incidentally, the above conditional expression (5) can define the following conditional expression (5a) as a more preferable condition and the following conditional expression (5b) as an even more preferable condition. (5a) 0.30 ≦ IH / PTI ≦ 0.92 (5b) 0.40 ≦ IH / PTI ≦ 0.90

[0069] In addition, the optical system in any of the examples satisfies the following conditional expression (6). (6) CAV / CAD ≦ 0.90 However, CAV: Effective diameter at the short side end CAD: Effective diameter at the diagonal end

[0070] Incidentally, the above conditional expression (6) can define the following conditional expression (6a) as a more preferable condition and the following conditional expression (6b) as an even more preferable condition. (6a) CAV / CAD ≦ 0.80 (6b) CAV / CAD ≦ 0.70

[0071] In addition, the optical system in any of the examples satisfies the following conditional expression (7). (7) 50.00 ≦ EFL / m1m ≦ 250.00 However, EFL: Focal length of the lens group m1m: Focus movement amount of the lens group or imaging device when the object distance is from infinity to 1 m

[0072] Note that the above conditional expression (7) can define the following conditional expression (7a) as a more preferable condition and the following conditional expression (7b) as an even more preferable condition. (7a) 65.00 ≦ EFL / m1m ≦ 200.00 (7b) 75.00 ≦ EFL / m1m ≦ 150.00

[0073] Also, the optical systems in all the embodiments satisfy the following conditional expression (8). (8) 0.01 ≦ STO / PTI ≦ 0.35 However,[[]] STO: Distance from the reflective optical element group to the aperture

[0074] Note that the above conditional expression (8) can define the following conditional expression (8a) as a more preferable condition and the following conditional expression (8b) as an even more preferable condition. (8a) 0.03 ≦ STO / PTI ≦ 0.30 (8b) 0.05 ≦ STO / PTI ≦ 0.25

[0075] Also, the optical systems in all the embodiments satisfy the following conditional expression (9). (9) 1.70 ≦ ndP However,[[]] ndP: Refractive index of the prism as a reflective optical element based on the d-line

[0076] Note that the above conditional expression (9) can define the following conditional expression (9a) as a more preferable condition and the following conditional expression (9b) as an even more preferable condition. (9a) 1.80 ≦ ndP (9b) 1.90 ≦ ndP

[0077] Table 17 shows the values of the above respective conditional expressions and various values according to Examples 1 to 4. Conditional expression corresponding values

Table 17

[0078] Figure 10 is a diagram showing an outline of the configuration of an imaging device. The imaging device mainly includes a lens barrel 1 that supports an optical system, an imaging element 2 that converts an optical image formed by the optical system into an electrical signal, an arithmetic processing unit 3 that processes the photoelectrically converted image signal, and a liquid crystal panel 4 that displays the processed image data as an image. As shown in the figure, by arranging a reflective optical element group that bends the optical path by 90°, the thickness of the imaging device with respect to the incident direction of the light beam can be reduced. Note that examples of the imaging device including the optical system according to the present embodiment include a digital still camera, a digital video camera, a surveillance camera, a smartphone, a PC, a tablet terminal, a drive recorder, etc., and it is suitable for an imaging device that is particularly expected to be thin.

Explanation of Reference Numerals

[0079] GR... Reflective optical element group GL... Lens group S... Aperture stop CG... Optical filter I... Imaging position 1... Lens barrel 2... Imaging element 3... Arithmetic processing unit 4... Liquid crystal panel

Claims

1. An optical system having, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an image sensor. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the image sensor moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (1). (1) 0.50 ≤ PTI / THP ≤ 3.50 However, PTI: Distance from the reflective optical element group to the image plane THP: Thickness of the prism as the reflective optical element

2. An optical system having, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an image sensor. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the image sensor moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (2). (2) 15.00 ≤ CRA ≤ 50.00 However, CRA: Incident angle of the chief ray to the diagonal end of the image sensor

3. An optical system having, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an image sensor. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the image sensor moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (3). (3) 0.20 ≤ EPD / THP ≤ 0.94 However, EPD: Entrance pupil diameter of the lens group THP: Thickness of the prism as the reflective optical element

4. An optical system having, in order from the object side, a reflective optical element group capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an image sensor. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the reflective optical element group or the image sensor moves away from the lens group, and the reflective optical element group does not move, and further satisfies the following conditional expression (4). (4) 0.26 ≤ EPD / PTI ≤ 0.80 However, EPD: Entrance pupil diameter of the lens group PTI: Distance from the reflective optical element group to the image plane

5. An optical system having, in order from the object side, a group of reflective optical elements capable of bending the optical path by approximately 90°, and a lens group composed of a plurality of lenses that converge light onto an imaging element. When adjusting the focus from an object at infinity to an object at a short distance, the lens group approaches the group of reflective optical elements or the imaging element moves away from the lens group, and the group of reflective optical elements does not move, and further satisfies the following conditional expression (5). (5) 0.20 ≤ IH / PTI ≤ 0.94 However,[[]] IH: Maximum image height on one side PTI: Distance from the group of reflective optical elements to the image plane

6. The lens closest to the imaging element is composed of a lens surface having an inflection point on at least one of the object side surface and the image side surface, and the outer shape of the lens has a length corresponding to the short side of the imaging element smaller than the length corresponding to the long side of the imaging element, and further satisfies the following conditional expression (6). The optical system according to any one of claims 1 to 5 (6) CAV / CAD ≤ 0.90 However,[[]] CAV: Effective diameter at the short side end CAD: Effective diameter at the diagonal end

7. The optical system according to any one of claims 1 to 5 that satisfies the following conditional expression (7). (7) 50.00 ≤ EFL / m1m ≤ 250.00 However,[[]] EFL: Focal length of the lens group m1m: Focus movement amount of the lens group or the imaging element when the object distance is from infinity to 1 m

8. The optical system according to any one of claims 1 to 5 that satisfies the following conditional expression (8). (8) 0.01 ≤ STO / PTI ≤ 0.35 However,[[]] STO: Distance from the group of reflective optical elements to the aperture

9. The optical system according to any one of claims 1 to 5 that satisfies the following conditional expression (9). (9) 1.70 ≤ ndP However,[[]] ndP: Refractive index of the prism as a reflective optical element based on the d line

10. An imaging device comprising the optical system according to any one of claims 1 to 5 and the imaging element that converts the formed optical image into an electrical signal.

Images