Imaging optical system and imaging device including the same

Abstract

To provide a compact imaging optical system with a wide angle of view and a bright F-number.SOLUTION: An imaging optical system includes at least one concave lens, an aperture stop, a first transmissive reflective surface, a 1 / 4 wavelength plate and a second transmissive reflective surface arranged in order from the object side to the image side. Light from the object side passes through the first transmissive reflective surface and the 1 / 4 wavelength plate in sequence, is reflected toward the object side by the second transmissive reflective surface, passes through the 1 / 4 wavelength plate, is reflected toward the image side by the first transmissive reflective surface, and passes through the 1 / 4 wavelength plate and the second transmissive reflective surface in sequence toward the image side. The distance on the optical axis from the first transmissive reflective surface to the image surface, and the focal distance of the imaging optical system are appropriately set.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a wide-angle imaging optical system. [Background technology]

[0002] In recent years, the demand for miniaturized imaging optical systems is increasing against the backdrop of the spread of smartphone cameras and the shift from single-lens reflex cameras to mirrorless cameras in the high-end camera market. In addition, in the market for drones and car-mounted cameras, the demand for wide-angle imaging lenses to capture a wider field of view is also increasing, in addition to the miniaturization of cameras. Patent Document 1 discloses a compact imaging optical system using two reflective / transmissive surfaces. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-352273 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the imaging optical system of Patent Document 1 is not sufficient in terms of widening the angle, and it cannot be said that good optical performance is necessarily ensured.

[0005] An object of the present invention is to provide a compact imaging optical system that has a wide angle and a bright F-number. [Means for solving the problem]

[0006] An imaging optical system according to one aspect of the present invention has at least one concave lens, an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, arranged in this order from the object side to the image side, wherein light from the object side passes through the first transmissive-reflective surface and the quarter-wave plate in this order, is reflected by the second transmissive-reflective surface towards the object side, passes through the quarter-wave plate, is reflected by the first transmissive-reflective surface towards the image side, passes through the quarter-wave plate and the second transmissive-reflective surface in this order, and proceeds towards the image side; when the distance on the optical axis from the first transmissive-reflective surface to the image surface is zm1 and the focal length of the imaging optical system is f, 0.55≦zm1 / f≦4.80 The present invention is characterized in that the following conditional expression is satisfied:

[0007] Another aspect of the present invention is an imaging optical system having, in order from the object side to the image side, an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, wherein light from the object side passes through the first transmissive-reflective surface and the quarter-wave plate in that order, is reflected by the second transmissive-reflective surface towards the object side, passes through the quarter-wave plate, is reflected by the first transmissive-reflective surface towards the image side, passes through the quarter-wave plate and the second transmissive-reflective surface in that order, and proceeds towards the image side; when the distance on the optical axis from the first transmissive-reflective surface to the image surface is zm1 and the focal length of the imaging optical system is f, 0.80≦zm1 / f≦4.80 The present invention is characterized in that the following conditional expression is satisfied: Effect of the Invention

[0008] According to the present invention, it is possible to provide a compact imaging optical system with a wide angle and a bright F-number. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a schematic diagram showing an optical path of an optical system. [Diagram 2] FIG. 2 is a schematic diagram showing an optical path of an optical system. [Diagram 3] 1 is a cross-sectional view of an imaging optical system according to a first embodiment. [Figure 4] 4A to 4C are longitudinal aberration diagrams of the imaging optical system of Example 1 when focusing at infinity. [Diagram 5] 4A to 4C are longitudinal aberration diagrams of the imaging optical system of Example 1 at close distance focusing. [Figure 6] FIG. 11 is a cross-sectional view of an imaging optical system according to a second embodiment. [Figure 7] 11A to 11C are longitudinal aberration diagrams of the imaging optical system of Example 2 when focusing at infinity. [Figure 8] 11A to 11C are longitudinal aberration diagrams of the imaging optical system of Example 2 during close-range focusing. [Figure 9] FIG. 11 is a cross-sectional view of an imaging optical system according to a third embodiment. [Figure 10] 13A to 13C are longitudinal aberration diagrams of the imaging optical system of Example 3 when focusing at infinity. [Figure 11] 13A to 13C are longitudinal aberration diagrams of the imaging optical system of Example 3 during close-range focusing. [Figure 12] FIG. 11 is a cross-sectional view of an imaging optical system according to a fourth embodiment. [Figure 13] 13A to 13C are longitudinal aberration diagrams of the imaging optical system of Example 4 when focusing at infinity. [Figure 14] FIG. 1 is a schematic diagram of an imaging device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to refer to the same components, and duplicated descriptions will be omitted.

[0011] The imaging optical system in each embodiment is an optical system that forms an image of an object on an image plane, and is an optical system for acquiring an image by a solid-state imaging element or a photosensitive film arranged on the image plane.

[0012] The imaging optical system of each embodiment has a first transmissive-reflective surface, a quarter-wave plate (QWP), and a second transmissive-reflective surface, which are arranged in this order from the object side to the image side. Light from the object side passes through the first transmissive-reflective surface and the QWP in this order, and is reflected by the second transmissive-reflective surface. The light then passes through the QWP, is reflected by the first transmissive-reflective surface, passes through the QWP, and the second transmissive-reflective surface, and travels toward an imaging unit such as a solid-state imaging element or a photosensitive film.

[0013] Here, the first transmissive-reflective surface and the second transmissive-reflective surface do not necessarily have a transmittance of 50% and a reflectance of 50%. The ratio of the transmittance to the reflectance for randomly polarized light is preferably in the range of 1:3 to 3:1. Randomly polarized light is light with Stokes parameters S0=1, S1=S2=S3=0. Furthermore, the first transmissive-reflective surface and the second transmissive-reflective surface may absorb light.

[0014] Furthermore, a lens may be formed or bonded to either or both sides of each of the transmission / reflection surfaces.

[0015] As the QWP, for example, a polymer film having birefringence or a liquid crystal alignment layer can be used. Also, a laminate of such polymer films or liquid crystal alignment layers can be used as the QWP. By appropriately laminating these, a phase difference close to a quarter of the wavelength can be obtained in a wide wavelength range. In addition to the above, for example, an inorganic wave plate from Dexerials Corporation can be used as the QWP.

[0016] The QWP can be arranged, for example, by bonding with the first transmissive-reflective surface or the second transmissive-reflective surface. The QWP can also be arranged separately from these transmissive-reflective surfaces. For example, the film can be inserted directly into the optical path, or a film bonded to a glass plate can be inserted into the optical path. A lens can be formed or bonded on one or both sides of the QWP. For example, a lens can be molded on one or both sides of the inorganic waveplate using wafer-level optics technology with the inorganic waveplate as a substrate.

[0017] The characteristic configuration of the imaging optical system of each embodiment will be described below.

[0018] The imaging optical system of each embodiment has an aperture stop located closer to the object side than the first transmissive-reflective surface, which makes it possible to effectively correct coma aberration and curvature of field in particular.

[0019] Moreover, the imaging optical system of each embodiment has at least one concave lens on the object side of the aperture stop, which makes it possible to shorten the focal length of the imaging optical system and achieve a wide angle.

[0020] Furthermore, the imaging optical system of each embodiment satisfies the following conditional expression (1-1).

[0021] 0.55≦zm1 / f≦4.80 (1-1) Here, zm1 is the distance on the optical axis from the first transmissive-reflective surface to the image plane, and f is the focal length of the imaging optical system.

[0022] If the distance zm1 is short, falling below the lower limit of conditional formula (1-1), it is undesirable because it becomes difficult to ensure the back focus of the imaging optical system. Also, if the focal length f is long, falling below the lower limit of conditional formula (1-1), it is undesirable because it becomes difficult to achieve a wide angle of the imaging optical system. If the distance zm1 is long, exceeding the upper limit of conditional formula (1-1), it is undesirable because it leads to an increase in the size of the entire device and makes it difficult to correct the field curvature aberration in particular.

[0023] The imaging optical system of each embodiment does not necessarily have to have at least one concave lens on the object side of the aperture stop. In this case, the optical system of each embodiment satisfies the following conditional expression (1-2).

[0024] 0.80≦zm1 / f≦4.80 (1-2) With the above-mentioned configuration, it is possible to realize a compact imaging optical system with a wide angle and a bright F-number.

[0025] It is preferable that the numerical ranges of the conditional expressions (1-1) and (1-2) be the numerical range of the following conditional expression (1a).

[0026] 0.80≦zm1 / f≦4.50 (1a) It is further preferable that the numerical ranges of the conditional expressions (1-1) and (1-2) be the numerical range of the following conditional expression (1b).

[0027] 0.85≦zm1 / f≦4.20 (1b) Next, a description will be given of configurations that are preferably satisfied in the imaging optical system of each embodiment.

[0028] It is preferable that the second transmissive-reflective surface has a concave shape facing the object side. With such a configuration, it is possible to effectively correct the aberration, particularly the curvature of field.

[0029] Next, conditions that the imaging optical system of each embodiment should preferably satisfy will be described. The imaging optical system of each embodiment should preferably satisfy one or more of the following conditional expressions (2) to (9).

[0030] 1.2≦zp / f≦6.5 (2) 1.0≦Dp / f≦6.5 (3) 0.005≦Dm / L≦0.060 (4) 0.50≦zm2 / f≦4.50 (5) 0.07≦Dm1 / L≦0.40 (6) 0.09≦Dm2 / L≦0.42 (7) 2.5≦L / f≦11.5 (8) 0.55≦Fno≦2.50 (9) Here, zp is the distance on the optical axis from the aperture stop to the image plane. Dp is the distance on the optical axis from the lens surface closest to the object in the imaging optical system to the aperture stop. L is the total length of the imaging optical system (the distance on the optical axis from the lens surface closest to the object in the imaging optical system to the image plane). Dm is the distance on the optical axis from the first transmissive reflective surface to the second transmissive reflective surface. zm2 is the distance on the optical axis from the second transmissive reflective surface to the image plane. Dm1 is the distance on the optical axis from the aperture stop to the first transmissive reflective surface. Dm2 is the distance on the optical axis from the aperture stop to the second transmissive reflective surface. Fno is the F-number of the imaging optical system.

[0031] If the distance zp is shorter than the lower limit of conditional expression (2), it is difficult to correct the field curvature aberration, which is not preferable. Also, if the focal length f is longer than the lower limit of conditional expression (2), it is difficult to achieve a wide angle as an imaging optical system, which is not preferable. On the other hand, if the distance zp is longer than the upper limit of conditional expression (2), it is unpreferable because it leads to an increase in the size of the entire device.

[0032] If the distance Dp is short and falls below the lower limit of conditional formula (3), it is undesirable because it becomes difficult to correct distortion aberration in particular. Also, if the focal length f is long and falls below the lower limit of conditional formula (3), it is undesirable because it becomes difficult to achieve a wide angle as an imaging optical system. On the other hand, if the distance Dp is long and exceeds the upper limit of conditional formula (3), it is undesirable because it leads to an increase in the size of the entire device.

[0033] If the distance Dm becomes shorter than the lower limit of condition (4), interference between the transmissive and reflective surfaces occurs, which is not preferable, whereas if the distance Dm becomes longer than the upper limit of condition (4), it becomes difficult to correct coma aberration in particular, which is not preferable.

[0034] If the distance zm2 is short, below the lower limit of condition (5), it is difficult to ensure the back focus of the imaging optical system, which is not preferable. Also, if the focal length f is long, below the lower limit of condition (5), it is difficult to achieve a wide angle of the imaging optical system, which is not preferable. On the other hand, if the distance zm2 is long, above the upper limit of condition (5), it is unpreferable because it increases the size of the entire device and makes it difficult to correct the field curvature aberration in particular.

[0035] If the distance Dm1 is short and falls below the lower limit of condition (6), it becomes difficult to correct the field curvature aberration, which is not preferable, whereas if the distance Dm1 is long and exceeds the upper limit of condition (6), it leads to an increase in the size of the entire apparatus, which is not preferable.

[0036] If the distance Dm2 is short and falls below the lower limit of condition (7), it becomes difficult to correct the field curvature aberration, which is not preferable, whereas if the distance Dm2 is long and exceeds the upper limit of condition (7), it leads to an increase in the size of the entire apparatus, which is not preferable.

[0037] If the focal length f is longer than the lower limit of condition (8), it becomes difficult to achieve a wide angle as an imaging optical system, which is not preferable. On the other hand, if the distance L is longer than the upper limit of condition (8), it becomes unpreferable because it leads to an increase in the size of the entire device.

[0038] If the F-number Fno falls below the lower limit of condition (9), it is undesirable because it becomes difficult to correct spherical aberration and coma aberration in particular, whereas if the F-number Fno exceeds the upper limit of condition (9), it is undesirable because the brightness of the imaging optical system becomes insufficient.

[0039] It is preferable that the numerical ranges of the conditional expressions (2) to (9) be the numerical ranges of the following conditional expressions (2a) to (9a).

[0040] 1.4≦zp / f≦6.0 (2a) 1.2≦Dp / f≦6.0 (3a) 0.008≦Dm / L≦0.055 (4a) 0.60≦zm2 / f≦4.30 (5a) 0.08≦Dm1 / L≦0.35 (6a) 0.10≦Dm2 / L≦0.37 (7a) 2.8≦L / f≦10.5 (8a) 0.60≦Fno≦2.30 (9a) It is further preferable that the numerical ranges of the conditional expressions (2) to (9) be the numerical ranges of the following conditional expressions (2a) to (9a).

[0041] 1.6≦zp / f≦5.5 (2b) 1.4≦Dp / f≦5.5 (3b) 0.011≦Dm / L≦0.050 (4b) 0.70≦zm2 / f≦4.10 (5b) 0.09≦Dm1 / L≦0.25 (6b) 0.11≦Dm2 / L≦0.29 (7b) 3.1≦L / f≦10.0 (8b) 0.65≦Fno≦2.10 (9b) [Polarized Configuration 1] A configuration using polarized light will be described with reference to FIG. 1. The imaging optical system of this configuration has two transmissive and reflective surfaces. Here, the transmissive and reflective surface disposed on the object side of the imaging optical system of this configuration is configured by disposing a polarization-selective transmissive and reflective element (PBS):A. The transmissive and reflective surface disposed on the image plane side of the imaging optical system of this configuration is configured by disposing a half mirror (HM):C. In addition, a first quarter-wave plate (QWP1):B is disposed between the polarization-selective transmissive and reflective element PBS and the half mirror HM. Furthermore, a second quarter-wave plate (QWP2):D and a linear polarizer (POL):E are disposed between the half mirror HM and the imaging plane IM, in that order from the object side to the image side.

[0042] Here, the polarization selective transmission reflection element A is an element configured to reflect linearly polarized light polarized in the same direction as when it was transmitted through the linear polarizing plate E, and transmit linearly polarized light perpendicular to the direction. The polarization selective transmission reflection element A is, for example, a wire grid polarizer or a reflective polarizer having a retardation film laminated structure. In this case, the wire grid forming surface or retardation film surface of the polarization selective transmission reflection element A functions as a transmission reflection surface. Note that the wire grid polarizer does not necessarily have to be one in which metal wires are aligned, and it is sufficient that it has thin metal or dielectric layers at a predetermined interval and functions as a polarization selective transmission reflection element. For example, an element in which metal or dielectric layers are aligned by vapor deposition can be used.

[0043] The first quarter-wave plate B and the second quarter-wave plate D are arranged with their slow axes tilted at 45° with respect to the polarization transmission axis of the linear polarizer E. Here, the first quarter-wave plate B and the second quarter-wave plate D are preferably arranged with their slow axes tilted at 90°. With this arrangement, when a light beam passes through the first quarter-wave plate B and the second quarter-wave plate D, the wavelength dispersion characteristics of the wave plates are offset.

[0044] The half mirror C is a half mirror formed by, for example, a dielectric multilayer film or metal deposition, and the mirror surface of the half mirror C functions as a transmission / reflection surface. The linear polarizing plate E is, for example, an absorptive linear polarizer.

[0045] Next, the optical path selection and operation in the polarization utilization configuration will be described.

[0046] Light entering the imaging optical system from the object side becomes linearly polarized light by the polarization-selective transflective element A, becomes circularly polarized light by the first quarter-wave plate B, and enters the half mirror C. A part of the light that reaches the half mirror C is reflected and becomes circularly polarized in the reverse direction, returning to the first quarter-wave plate B.

[0047] The counter-circularly polarized light that has returned to the first quarter-wave plate B returns to the polarization-selective transflector A as linearly polarized light polarized in a direction perpendicular to the direction when the light first passed through the polarization-selective transflector A. The light that has returned to the polarization-selective transflector A is reflected by the polarization-selective transflector A. Due to the polarization selectivity of the polarization-selective transflector A, the linearly polarized light polarized in a direction perpendicular to the direction when the light first passed through the polarization-selective transflector A is reflected.

[0048] On the other hand, a portion of the light that reaches the half mirror C is transmitted through and becomes linearly polarized light polarized in the same direction as when it passed through the polarization-selective transmission / reflection element A by the second quarter-wave plate D, and is incident on the linear polarizer E and absorbed by the linear polarizer E.

[0049] The light reflected by the polarization-selective transflector A is converted to circularly polarized light by the first quarter-wave plate B and enters the half mirror C. A portion of the light that reaches the half mirror C is transmitted and enters the second quarter-wave plate D. The second quarter-wave plate D converts the incident light into linearly polarized light that is oriented parallel to the linearly polarized light reflected by the polarization-selective transflector A. The light that passes through the second quarter-wave plate D enters the linear polarizer E. Here, the polarization of the light and the transmission axis of the linear polarizer E are aligned, so most of the light is transmitted and directed to the imaging plane IM.

[0050] Due to the above-mentioned action, only the light that has been transmitted through the polarization-selective transmission / reflection element PBS, reflected by the half mirror C, reflected by the polarization-selective transmission / reflection element PBS, and transmitted through the half mirror C is guided to the imaging plane IM.

[0051] When using a cholesteric liquid crystal instead of the half mirror C, it is preferable to set the cholesteric liquid crystal so that it reflects a large amount of circularly polarized light in the direction of the incident light during the first reflection of the cholesteric liquid crystal. In this way, it is possible to increase the amount of light on the normal optical path while reducing ghost light.

[0052] In addition, solid-state imaging elements and CCDs (Charge Coupled Devices) that can be used as the imaging surface IM generally have high surface reflectance. In this configuration, the light reflected by the imaging surface IM passes through the linear polarizer E again and is converted into circularly polarized light by the second quarter-wave plate D. After that, the light that leaves the second quarter-wave plate D is reflected by the half mirror C to become circularly polarized light in the opposite direction, and passes through the second quarter-wave plate D again. At this time, the circularly polarized light is converted by the second quarter-wave plate D into linearly polarized light in a direction perpendicular to the direction immediately after passing through the linear polarizer E immediately before. Since the direction of this linearly polarized light is orthogonal to the transmission axis of the linear polarizer E, most of the light is absorbed by the linear polarizer E. In this manner, in this configuration, most of the light reflected by the imaging surface IM and the half mirror C in this order is cut, so ghosts and flares related to the imaging surface IM are less noticeable. In order to obtain such a reflection reduction effect, it is preferable that an optical low-pass filter using birefringence does not exist between the imaging surface IM and the linear polarizer E. This is because the optical low pass filter causes the polarization state to deviate from the desired polarization state.

[0053] In this configuration, a quarter-wave plate may be disposed between the polarization-selective transmission / reflection element A and the object so that the fast axis or slow axis of the quarter-wave plate forms an angle of 45° with the transmission axis of the polarization-selective transmission / reflection element A. By doing this, even if the light incident from the object side is linearly polarized, it is possible to capture an image regardless of its polarization direction. Also, a depolarizing element may be placed instead of the quarter-wave plate. For example, "Cosmoshine SRF" by Toyobo Co., Ltd. can be used as the depolarizing element. [Polarized Configuration 2] A configuration using polarized light will be described with reference to FIG. 2. The imaging optical system of this configuration has two transmissive and reflective surfaces. Here, the transmissive and reflective surface disposed on the object side of the imaging optical system of this configuration is configured by disposing a half mirror (HM):C. The transmissive and reflective surface disposed on the image surface side of the imaging optical system of this configuration is configured by disposing a polarization selective transmissive and reflective element (PBS):A. In addition, a first quarter-wave plate (QWP1):B is disposed between the polarization selective transmissive and reflective element PBS and the half mirror HM. Furthermore, a linear polarizer (POL):E and a second quarter-wave plate (QWP2):D are disposed between the half mirror HM and the object surface, in that order from the object side to the image side.

[0054] Here, the configuration of each polarizing element and the suitable arrangement of the optical axis orientation are similar to those in the polarized light utilizing configuration 1.

[0055] Next, the optical path selection and operation in the polarization utilization configuration will be described.

[0056] Light entering the imaging optical system from the object side becomes linearly polarized light by linear polarizer E, becomes circularly polarized light by second quarter-wave plate D, and enters half mirror C. Part of the light that reaches half mirror C is reflected and becomes circularly polarized in the reverse direction, returning to the second quarter-wave plate D.

[0057] The light that reaches and is reflected by the half mirror C is circularly polarized in the opposite direction to the incident direction. This light is then polarized by the second quarter-wave plate D as linearly polarized light in a direction perpendicular to the direction that it was when it passed through the linear polarizer E, and is then incident on and absorbed by the linear polarizer E.

[0058] Meanwhile, the light that passes through half mirror C becomes linearly polarized light by first quarter-wave plate B, polarized in the same direction as the light immediately after passing through linear polarizer E. This linearly polarized light is reflected by polarization-selective transmission / reflection element A and returns to first quarter-wave plate B. The light is then converted into circularly polarized light by first quarter-wave plate B, and part of it is reflected by half mirror C. The light reflected by half mirror C enters first quarter-wave plate B again and is converted into linearly polarized light whose polarization direction is orthogonal to that when it was reflected by polarization-selective transmission / reflection element A. This linearly polarized light passes through polarization-selective transmission / reflection element A and is guided to the imaging plane IM.

[0059] Due to the above-mentioned action, only the light that is transmitted through the half mirror C, reflected by the polarization-selective transmission / reflection element PBS, reflected by the half mirror C, and transmitted through the polarization-selective transmission / reflection element PBS is guided to the imaging plane IM.

[0060] In this arrangement, a linear polarizer A' may be placed between the polarization-selective transflector A and the imaging plane IM. In this case, the transmission axes of the linear polarizer A' and the polarization-selective transflector A are made to coincide with each other. In this way, it is possible to absorb light that is reflected by the imaging plane IM, reflected by the polarization-selective transflector A, and then enters the imaging plane IM again to cause ghosts and flares.

[0061] In this configuration, a quarter-wave plate may be disposed between the linear polarizer E and the object. In this case, the quarter-wave plate is disposed so that the fast axis or slow axis of the quarter-wave plate forms an angle of 45° with the transmission axis of the linear polarizer E. In this way, even if the light incident from the object side is linearly polarized, it is possible to capture an image regardless of its polarization direction. Also, a depolarizing element may be disposed instead of the quarter-wave plate. For example, "Cosmoshine SRF" by Toyobo Co., Ltd. can be used as the depolarizing element.

[0062] In the above description of the configuration, terms such as orthogonal, parallel, and 45° are used, but these do not have to be strictly 90°, 0°, and 45°. However, these should be within ±5° of the desired angle, more preferably within ±2°, and even more preferably within ±1°.

[0063] In the imaging optical system of each embodiment, the lens may be made of a polymer material or a glass material. However, it is preferable that the lens disposed between the first transmissive-reflective surface and the second transmissive-reflective surface has low birefringence.

[0064] The configuration of the imaging optical system of each embodiment will be described below. EXAMPLES

[0065] 3 is a cross-sectional view of the imaging optical system of this embodiment. The imaging optical system of this embodiment has, in order from the object side to the image side, at least one concave lens, an F-number determination member (aperture stop) STO, a first transmissive reflecting surface HM1, a quarter-wave plate QWP, and a second transmissive reflecting surface HM2. The imaging optical system of this embodiment has an aperture ratio of about 0.7 and a half angle of view of about 42 degrees.

[0066] The first focus lens group F1 is configured to be movable along the optical axis OA during focusing. The F-number determination member STO determines (limits) the open F-number (Fno) light flux. IM is an image plane, on which the imaging surface of an imaging element (photoelectric conversion element) such as a CCD sensor or a CMOS sensor is disposed.

[0067] FIG. 4 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at infinity. FIG. 5 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at a close distance. In the spherical aberration diagram, Fno is the F-number, and shows the amount of spherical aberration for the d-line (wavelength 587.6 nm) and the g-line (wavelength 435.8 nm). In the astigmatism diagram, S shows the amount of astigmatism in the sagittal image plane, and M shows the amount of astigmatism in the meridional image plane. In the distortion aberration diagram, the amount of distortion for the d-line is shown. In the chromatic aberration diagram, the amount of chromatic aberration at the g-line is shown. ω is the imaging half angle of view (°). EXAMPLES

[0068] 6 is a cross-sectional view of the imaging optical system of this embodiment. The basic configuration of the imaging optical system of this embodiment is similar to that of the imaging optical system of embodiment 1, so a description thereof will be omitted. The imaging optical system of this embodiment is an optical system with an aperture ratio of about 1.0 and a half angle of view of about 50.2 degrees.

[0069] The first focus lens group F1 and the second focus lens group F2 are configured to be independently movable along the optical axis OA during focusing.

[0070] Fig. 7 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at infinity, and Fig. 8 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at a close distance. EXAMPLES

[0071] 9 is a cross-sectional view of the imaging optical system of this embodiment. The basic configuration of the imaging optical system of this embodiment is similar to that of the imaging optical system of embodiment 1, so a description thereof will be omitted. The imaging optical system of this embodiment is an optical system with an aperture ratio of about 2.03 and a half angle of view of about 63 degrees.

[0072] The first focus lens group F1 and the second focus lens group F2 are configured to be independently movable along the optical axis OA during focusing.

[0073] Fig. 10 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at infinity, and Fig. 11 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focusing at a close distance. EXAMPLES

[0074] 12 is a cross-sectional view of the imaging optical system of this embodiment. The basic configuration of the imaging optical system of this embodiment is similar to that of the imaging optical system of embodiment 1, so a description thereof will be omitted. The imaging optical system of this embodiment is an optical system with an aperture ratio of about 2.0 and a half angle of view of about 105 degrees.

[0075] FIG. 13 is a longitudinal aberration diagram of the imaging optical system of this embodiment when focused at infinity.

[0076] Numerical examples 1 to 4 corresponding to the first to fourth embodiments, respectively, are shown below.

[0077] In the surface data of each numerical example, r represents the radius of curvature of each optical surface, and d (mm) represents the axial spacing (distance on the optical axis) between the mth surface and the (m+1)th surface. Here, m is the surface number counted from the light incident side. Additionally, nd represents the refractive index of each optical member with respect to the d-line, and νd represents the Abbe number of the optical member. The Abbe number νd of a certain material is given by the following, when the refractive indices at the d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm) of the Fraunhofer lines are Nd, NF, and NC, respectively: νd=(Nd-1) / (NF-NC) It is expressed as:

[0078] In each numerical example, d, focal length (mm), F-number, and half angle of view (degrees) are all values ​​when the imaging 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 surface expressed as an air-equivalent length. "Total lens length" is the distance on the optical axis from the foreground (the lens surface closest to the object) of the imaging optical system to the final surface plus the back focus. The term "lens group" is not limited to cases where the group is composed of multiple lenses, but also includes cases where the group is composed of a single lens.

[0079] If the optical surface is aspheric, a * symbol is added to the right of the surface number. The aspheric shape is expressed as follows, where X is the displacement from the apex of the surface in the optical axis direction, h is the height from the optical axis in the direction perpendicular to the optical axis, R is the paraxial radius of curvature, K is the conic constant, and A4, A6, A8, A10, A12, A14, and A16 are the aspheric coefficients of each order: X = (h 2 / R) / [1+{1-(1+K)(h / R) 2} 1 / 2 ]+A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 +A12×h 12 +A14×h 14 +A16×h 16 In addition, "e±XX" in each aspheric coefficient is "×10± XX " It means.

[0080] [Numerical Example 1] Unit: mm Surface Data Surface number rd nd νd 1 75.153 4.00 1.48749 70.2 2 38.626 11.54 3* -153.438 5.00 1.85135 40.1 4* -110.398 7.34 5* 37.919 8.54 1.49710 81.6 6* 903.753 7.42 7(Aperture) ∞ 5.00 8* -203.188 12.06 1.59201 67.0 9* -32.219 0.70 10* -115.357 2.40 2.00178 19.3 11* -617.948 3.23 12* -74.487 -3.23 13* -617.948 3.23 14* -74.487 6.46 1.59201 67.0 15* -31.878 0.70 16 70.760 5.61 1.49700 81.5 17∞5.00 Image plane ∞ Aspheric Data 3rd page K = 0.00000e+00 A 4= 6.90310e-06 A 6=-1.18669e-10 A 8=-1.45462e-14 A10= 1.85849e-15 A12=-1.90182e-19 Side 4 K = 0.00000e+00 A 4= 1.07004e-05 A 6=-5.16331e-09 A 8= 9.53971e-12 A10=-6.99885e-15 A12= 7.47937e-18 5th page K = 0.00000e+00 A 4= 6.23717e-06 A 6=-1.03357e-08 A 8= 6.34562e-12 A10= 3.18774e-14 A12=-3.17854e-17 Side 6 K = 0.00000e+00 A 4=-8.13525e-07 A 6=-5.00015e-09 A 8= 1.68190e-11 A10=-1.88163e-14 A12= 6.32007e-18 Side 8 K = 0.00000e+00 A 4=-1.43963e-05 A 6=-3.55063e-09 A 8=-5.68413e-11 A10= 1.68695e-13 A12=-4.12289e-16 9th page K = 0.00000e+00 A 4=-1.55561e-06 A 6= 1.09550e-08 A 8=-1.24225e-10 A10= 2.97939e-13 A12=-2.92015e-16 Side 10 K = 0.00000e+00 A 4=-1.22186e-05 A 6= 5.22298e-09 A 8=-1.27765e-10 A10= 1.32336e-13 A12=-9.94539e-17 Page 11 K = 0.00000e+00 A 4=-1.06247e-05 A 6=-4.24834e-09 A 8=-1.49135e-11 A10=-3.70202e-14 A12= 7.22317e-17 Side 12 K = 0.00000e+00 A 4=-6.52096e-06 A 6= 2.60342e-09 A 8=-4.67165e-11 A10= 6.11641e-14 A12=-3.22546e-17 Page 13 K = 0.00000e+00 A 4=-1.06247e-05 A 6=-4.24834e-09 A 8=-1.49135e-11 A10=-3.70202e-14 A12= 7.22317e-17 Side 14 K = 0.00000e+00 A 4=-6.52096e-06 A 6= 2.60342e-09 A 8=-4.67165e-11 A10= 6.11641e-14 A12=-3.22546e-17 Page 15 K = 0.00000e+00 A 4= 3.96573e-07 A 6=-1.25386e-08 A 8= 7.36277e-12 A10=-4.81591e-15 A12=-2.78757e-17 Various data Focal length 24.00 F-number: 0.70 Half angle of view [deg] 42.04 Lens total length 85.00 BF 5.00 First focus lens group F1 Starting point 8 End surface 15 F1 drive amount when focusing at close range (-0.6m from the image plane) -1.26 [Numerical Example 2] Unit: mm Surface Data Surface number rd nd νd 1 76.753 2.50 1.48749 70.2 2 20.301 6.55 3* 70.537 4.46 1.53160 55.8 4* 16.630 5.19 5 30.648 8.76 1.80810 22.8 6 38.051 1.78 7* 23.414 3.26 1.53160 55.8 8* 27.910 2.67 9 57.589 7.00 1.49700 81.5 10 -76.924 2.61 11(Aperture) ∞ 2.43 12 17.443 8.57 1.49700 81.5 13 -35.398 0.50 14 -43.619 1.50 1.85478 24.8 15∞1.52 16* -103.975 -1.52 17 ∞ 1.52 18* -103.975 1.83 1.53160 55.8 19* -33.803 0.70 20 -2391.149 3.16 1.49700 81.5 21 -37.807 11.50 Image plane ∞ Aspheric Data 3rd page K = 0.00000e+00 A 4= 7.18789e-05 A 6=-2.67733e-07 A 8= 7.79197e-10 A10=-1.30731e-12 A12= 1.01400e-15 Side 4 K = 0.00000e+00 A 4= 5.88394e-05 A 6=-1.66317e-07 A 8=-1.61763e-09 A10= 1.04955e-11 A12=-3.01415e-14 Side 7 K = 0.00000e+00 A 4=-1.19790e-04 A 6=-6.17280e-07 A 8= 4.41932e-09 A10=-3.35374e-11 A12= 1.10260e-13 Side 8 K = 0.00000e+00 A 4=-1.24953e-04 A 6=-3.77720e-07 A 8= 3.14488e-09 A10=-1.52028e-11 A12= 5.44430e-14 Page 16 K = 0.00000e+00 A 4= 1.94438e-05 A 6= 4.18663e-08 A 8=-1.44043e-09 A10= 9.06092e-12 A12=-1.92562e-14 Side 18 K = 0.00000e+00 A 4= 1.94438e-05 A 6= 4.18663e-08 A 8=-1.44043e-09 A10= 9.06092e-12 A12=-1.92562e-14 Page 19 K = 0.00000e+00 A 4= 5.98622e-05 A 6=-6.63837e-08 A 8= 5.55235e-09 A10=-3.24060e-11 A12= 1.35603e-13 Various data Focal length 12.06 F-number 1.00 Half angle of view [deg] 50.23 Lens length 76.50 BF 11.50 First focus lens group F1 Starting point 12 Final page 19 Second focus lens group F2 Starting point 9 End surface 10 When focusing at close range (-0.2m from the image plane), the F1 drive amount is -1.11, and the F2 drive amount is 1.38. [Numerical Example 3] Unit: mm Surface Data Surface number rd nd νd 1* -1292.458 3.50 1.48749 70.2 2 34.444 13.15 3 99.379 2.50 1.48749 70.2 4 22.729 11.66 5* 35.646 2.00 1.59201 67.0 6* 17.268 5.09 7 24.382 5.03 1.95375 32.3 8 63.807 12.36 9(Aperture) ∞ 2.30 10 31.493 12.13 1.49700 81.5 11 -15.201 1.20 1.80810 22.8 12 -23.009 0.50 13 -102.804 1.50 1.80810 22.8 14 -39380.616 2.50 15* -182.700 -2.50 16 -39380.616 2.50 17* -182.700 4.00 1.59201 67.0 18* -61.894 0.70 19* -152.590 6.30 1.59201 67.0 20* -21.028 13.60 Image plane ∞ Aspheric Data Front page K = 0.00000e+00 A 4= 6.35973e-06 A 6=-4.69667e-09 A 8= 2.56033e-12 A10=-7.77952e-16 A12= 1.12638e-19 5th page K = 0.00000e+00 A 4= 8.12933e-05 A 6=-2.98011e-07 A 8= 7.85905e-10 A10=-2.20542e-12 A12= 2.68019e-15 Side 6 K = 0.00000e+00 A 4= 8.52668e-05 A 6=-2.16917e-07 A 8=-1.25803e-10 A10=-2.48945e-12 A12= 4.79686e-15 Page 15 K = 0.00000e+00 A 4= 7.55612e-06 A 6= 5.10821e-08 A 8=-3.59935e-10 A10= 1.35344e-12 A12=-2.35031e-15 Page 17 K = 0.00000e+00 A 4= 7.55612e-06 A 6= 5.10821e-08 A 8=-3.59935e-10 A10= 1.35344e-12 A12=-2.35031e-15 Side 18 K = 0.00000e+00 A 4= 8.49549e-06 A 6= 7.19326e-08 A 8=-8.15232e-10 A10= 2.85603e-12 A12=-3.66330e-15 Page 19 K = 0.00000e+00 A 4=-5.26526e-05 A 6= 4.46114e-07 A 8=-1.98536e-09 A10= 4.22671e-12 A12=-3.60703e-15 Page 20 K = 0.00000e+00 A 4=-1.24404e-05 A 6= 3.08673e-07 A 8=-9.40054e-10 A10= 1.39679e-12 A12=-4.24441e-16 Various data Focal length 11.00 F-number 2.03 Half angle of view [deg] 63.04 Lens total length 100.00 BF 13.60 First focus lens group F1 Starting point 10 End surface 18 Second focus lens group F2 Starting point 7 End surface 8 When focusing at close range (-0.4m from the image plane), F1 drive amount is -1.12, F2 drive amount is -0.20 [Numerical Example 4] Unit: mm Surface Data Surface number rd nd νd 1 50.186 2.20 1.72916 54.7 2 13.491 5.81 3 23.081 1.50 1.72916 54.7 4 8.511 6.82 5* -22.785 1.52 1.53160 55.8 6* 17.057 0.52 7 17.659 3.21 1.80810 22.8 8 -36.916 4.94 9(Aperture) ∞ 1.64 10 13.259 3.16 1.49700 81.5 11 -17.587 0.82 12 -17.100 1.05 1.92286 18.9 13∞1.10 14* -296.941 -1.10 15∞1.10 16* -296.941 1.41 1.59201 67.0 17* -74.408 0.70 18 61.343 5.78 1.49700 81.5 19 -12.256 0.60 20 57.193 2.20 1.49700 81.5 21 -219.428 13.60 Image plane ∞ Aspheric Data 5th page K = 0.00000e+00 A 4= 9.41582e-05 A 6= 1.58368e-06 A 8=-1.22848e-07 A10= 2.37154e-09 A12=-1.69053e-11 Side 6 K = 0.00000e+00 A 4= 1.88314e-06 A 6= 4.96667e-06 A 8=-3.28152e-07 A10= 7.32711e-09 A12=-6.13397e-11 Side 14 K = 0.00000e+00 A 4= 2.42656e-05 A 6= 2.89608e-07 A 8= 8.23640e-09 A10=-1.25973e-09 A12= 2.83740e-11 Page 16 K = 0.00000e+00 A 4= 2.42656e-05 A 6= 2.89608e-07 A 8= 8.23640e-09 A10=-1.25973e-09 A12= 2.83740e-11 Page 17 K = 0.00000e+00 A 4= 1.86570e-04 A 6= 7.03375e-07 A 8=-3.75096e-09 A10=-4.21870e-10 A12= 9.23085e-12 Various data Focal length 6.21 F-number 2.00 Half angle of view [deg] 105.0 Lens length 58.60 BF 13.60 Various values ​​in each numerical example are summarized in Table 1 below.

[0081] [Table 1]

[0082] [Imaging device] An imaging device having the imaging optical system of each embodiment will be described with reference to Fig. 14. Fig. 14 is a schematic diagram of a single-lens reflex camera, which is an example of an imaging device. The lens device 10 has an imaging lens 1, which is one of the imaging optical systems described in the first to fourth embodiments, and a lens barrel 2 that holds the imaging lens 1. The camera body 20 has a quick-return mirror 3 that reflects a light beam from the lens device 10 upward, a focusing screen 4 that is disposed at an image forming position of the lens device 10, and a pentagonal roof prism 5 that converts an inverse image formed on the focusing screen 4 into an erect image. The camera body 20 also has an eyepiece 6 for observing the erect image.

[0083] An imaging element (photoelectric conversion element) such as a CCD sensor or a CMOS sensor is disposed on the imaging surface 7. When capturing an image, the quick return mirror 3 is retracted from the optical path, and an image is formed on the imaging surface 7 by the lens device 10. The imaging element receives the optical image formed via the imaging lens 1, performs photoelectric conversion, and outputs image data. Note that the imaging lens 1 can also be applied to an imaging device without the quick return mirror 3.

[0084] In this way, by applying the imaging optical system of each embodiment to an imaging device such as a video camera or a digital still camera, it is possible to realize an imaging device having high optical performance.

[0085] The disclosure of this embodiment includes the following configuration. (Configuration 1) An imaging optical system having at least one concave lens, an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, which are arranged in this order from an object side to an image side, light from the object side passes through the first transmission-reflection surface and the quarter-wave plate in this order, is reflected by the second transmission-reflection surface toward the object side, passes through the quarter-wave plate, is reflected by the first transmission-reflection surface toward the image side, passes through the quarter-wave plate and the second transmission-reflection surface in this order, and proceeds toward the image side; When the distance on the optical axis from the first transmissive-reflective surface to the image plane is zm1 and the focal length of the imaging optical system is f, 0.55≦zm1 / f≦4.80 An imaging optical system characterized by satisfying the following conditional expression: (Configuration 2) An imaging optical system having an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, which are arranged in this order from an object side to an image side, light from the object side passes through the first transmission-reflection surface and the quarter-wave plate in this order, is reflected by the second transmission-reflection surface toward the object side, passes through the quarter-wave plate, is reflected by the first transmission-reflection surface toward the image side, passes through the quarter-wave plate and the second transmission-reflection surface in this order, and proceeds toward the image side; When the distance on the optical axis from the first transmissive-reflective surface to the image plane is zm1 and the focal length of the imaging optical system is f, 0.80≦zm1 / f≦4.80 An imaging optical system characterized by satisfying the following conditional expression: (Configuration 3) When the distance on the optical axis from the aperture stop to the image plane is zp, 1.2≦zp / f≦6.5 3. The imaging optical system according to configuration 1 or 2, wherein the following condition is satisfied: (Configuration 4) When the distance on the optical axis from the lens surface closest to the object side of the imaging optical system to the aperture stop is Dp, 1.0≦Dp / f≦6.5 4. The imaging optical system according to any one of configurations 1 to 3, wherein the following condition is satisfied: (Configuration 5) Let L be the total length of the imaging optical system, and Dm be the distance on the optical axis from the first transmissive-reflective surface to the second transmissive-reflective surface, 0.005≦Dm / L≦0.060 5. The imaging optical system according to any one of configurations 1 to 4, wherein the following condition is satisfied: (Configuration 6) When the distance on the optical axis from the second transmissive-reflective surface to the image plane is zm2, 0.50≦zm2 / f≦4.50 6. The imaging optical system according to any one of configurations 1 to 5, wherein the following condition is satisfied: (Configuration 7) Let L be the total length of the imaging optical system, and Dm1 be the distance on the optical axis from the aperture stop to the first transmitting / reflecting surface, 0.07≦Dm1 / L≦0.40 7. The imaging optical system according to any one of configurations 1 to 6, wherein the following condition is satisfied: (Configuration 8) Let L be the total length of the imaging optical system, and Dm2 be the distance on the optical axis from the aperture stop to the second transmissive-reflective surface. 0.09≦Dm2 / L≦0.42 8. The imaging optical system according to any one of configurations 1 to 7, wherein the following condition is satisfied: (Configuration 9) When the total length of the imaging optical system is L, 2.5≦L / f≦11.5 9. The imaging optical system according to any one of configurations 1 to 8, wherein the following condition is satisfied: (Configuration 10) When the F-number of the imaging optical system is Fno, 0.55≦Fno≦2.50 10. The imaging optical system according to any one of configurations 1 to 9, wherein the following condition is satisfied: (Configuration 11) The imaging optical system according to any one of configurations 1 to 10, wherein the second transmitting and reflecting surface has a concave shape facing the object side. (Configuration 12) 12. An imaging apparatus comprising: an imaging optical system according to any one of configurations 1 to 11; and an imaging element that receives an image formed by the imaging optical system.

[0086] 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]

[0087] HM1 First transflective surface QWP 1 / 4 wave plate HM2 Second transflective surface STO aperture stop IM Imaging surface

Claims

1. An imaging optical system having at least one concave lens, an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, which are arranged in this order from an object side to an image side, light from the object side passes through the first transmission-reflection surface and the quarter-wave plate in this order, is reflected by the second transmission-reflection surface toward the object side, passes through the quarter-wave plate, is reflected by the first transmission-reflection surface toward the image side, passes through the quarter-wave plate and the second transmission-reflection surface in this order, and proceeds toward the image side; When the distance on the optical axis from the first transmitting / reflecting surface to the image plane is zm1 and the focal length of the imaging optical system is f, 0.55≦zm1 / f≦4.80 An imaging optical system characterized by satisfying the following conditional expression:

2. An imaging optical system having an aperture stop, a first transmissive-reflective surface, a quarter-wave plate, and a second transmissive-reflective surface, which are arranged in this order from an object side to an image side, light from the object side passes through the first transmission-reflection surface and the quarter-wave plate in this order, is reflected by the second transmission-reflection surface toward the object side, passes through the quarter-wave plate, is reflected by the first transmission-reflection surface toward the image side, passes through the quarter-wave plate and the second transmission-reflection surface in this order, and proceeds toward the image side; When the distance on the optical axis from the first transmitting / reflecting surface to the image plane is zm1 and the focal length of the imaging optical system is f, 0.80≦zm1 / f≦4.80 An imaging optical system characterized by satisfying the following conditional expression:

3. When the distance on the optical axis from the aperture stop to the image plane is zp, 1.2≦zp / f≦6.5 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

4. When the distance on the optical axis from the lens surface closest to the object side of the imaging optical system to the aperture stop is Dp, 1.0≦Dp / f≦6.5 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

5. Let L be the total length of the imaging optical system, and Dm be the distance on the optical axis from the first transmissive-reflective surface to the second transmissive-reflective surface, 0.005≦Dm / L≦0.060 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

6. When the distance on the optical axis from the second transmissive-reflective surface to the image plane is zm2, 0.50≦zm2 / f≦4.50 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

7. Let L be the total length of the imaging optical system, and Dm1 be the distance on the optical axis from the aperture stop to the first transmitting / reflecting surface, 0.07≦Dm1 / L≦0.40 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

8. Let L be the total length of the imaging optical system, and Dm2 be the distance on the optical axis from the aperture stop to the second transmitting / reflecting surface. 0.09≦Dm2 / L≦0.42 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

9. When the total length of the imaging optical system is L, 2.5≦L / f≦11.5 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

10. When the F-number of the imaging optical system is Fno, 0.55≦Fno≦2.50 3. The imaging optical system according to claim 1, wherein the following condition is satisfied:

11. 3. The imaging optical system according to claim 1, wherein the second transmitting and reflecting surface has a concave shape facing the object side.

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

Images