Optical system and imaging apparatus

Abstract

To provide a bright and compact optical system.SOLUTION: The optical system includes a first lens disposed closest to the object side, a first transmissive-reflective surface HM1, a first wave plate QWP1, a second transmissive-reflective surface HM2, a second wave plate QWP2, and a third transmissive-reflective surface HM3, which are disposed in order from the object side to the image side, and an aperture diaphragm STO. The optical system forms an image of light incident from the object side on an image plane IM through two optical paths including transmission and reflection on the first, second, and third transflective surfaces.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system suitable for an imaging device such as a digital camera. [Background technology]

[0002] As such an optical system, a so-called folded optical system using a plurality of transmissive and reflective surfaces has been proposed. Summary of the Invention [Problem to be solved by the invention]

[0003] A folded optical system for imaging is desired to include an aperture stop, be small, and be bright. [Means for solving the problem]

[0004] An optical system according to one aspect of the present invention includes a first lens disposed closest to the object, and a first transmission-reflection surface, a first wave plate, a second transmission-reflection surface, a second wave plate, and a third transmission-reflection surface disposed in this order from the object side to the image side, and further includes an aperture stop. The optical system forms an image of light incident from the object side on an image plane via two optical paths including transmission and reflection at the first, second, and third transmission-reflection surfaces. When the distance on the optical axis from the object-side surface of the first lens to the image plane is L and the distance on the optical axis from the object-side surface of the first lens to the aperture stop is L1s, -0.20 <L1s / L<0.95 The present invention is characterized in that the following conditions are satisfied:

[0005] Another aspect of the present invention is an optical system that includes a first lens disposed closest to the object, and a first transmission-reflection surface, a first wave plate, a second transmission-reflection surface, a second wave plate, and a third transmission-reflection surface, arranged in this order from the object side to the image side, and further includes an aperture stop. The optical system is characterized in that light incident from the object side is imaged on an image plane via two optical paths that include transmission and reflection at the first, second, and third transmission-reflection surfaces. Note that an imaging device including each of the above optical systems also constitutes another aspect of the present invention. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 2 is a diagram showing the optical path of the optical system of the embodiment. [Figure 2] FIG. 2 is a cross-sectional view (first optical path) of the imaging optical system of the first embodiment. [Figure 3] FIG. 3 is a cross-sectional view (second optical path) of the imaging optical system of the first embodiment. [Figure 4] 3A and 3B are longitudinal aberration diagrams of the imaging optical system of Example 1 (infinity focus state). [Figure 5] 3 is a longitudinal aberration diagram of the imaging optical system of Example 1 (close focus state). [Figure 6] FIG. 10 is a cross-sectional view (first optical path) of the imaging optical system of the second embodiment. [Figure 7] FIG. 10 is a cross-sectional view (second optical path) of the imaging optical system of the second embodiment. [Figure 8] 10A and 10B are longitudinal aberration diagrams of the imaging optical system of Example 2 (infinity focus state). [Figure 9] FIG. 10 is a longitudinal aberration diagram of the imaging optical system of Example 2 (close focus state). [Figure 10] FIG. 11 is a cross-sectional view (first optical path) of the imaging optical system of the third embodiment. [Figure 11] FIG. 11 is a cross-sectional view (second optical path) of the imaging optical system of the third embodiment. [Figure 12] 10A and 10B are longitudinal aberration diagrams of the imaging optical system of Example 3 (infinity focus state). [Figure 13] FIG. 11 is a longitudinal aberration diagram of the imaging optical system of Example 3 (close focus state). [Figure 14] FIG. 10 is a cross-sectional view (first optical path) of the imaging optical system of Example 4. [Figure 15] FIG. 10 is a cross-sectional view (second optical path) of the imaging optical system of Example 4. [Figure 16] 10A and 10B are longitudinal aberration diagrams of the imaging optical system of Example 4 (infinity focus state). [Figure 17] FIG. 10 is a longitudinal aberration diagram of the imaging optical system of Example 4 (close focus state). [Figure 18] FIG. 10 is a cross-sectional view (first optical path) of the imaging optical system of the fifth embodiment. [Figure 19] FIG. 10 is a cross-sectional view (second optical path) of the imaging optical system of the fifth embodiment. [Figure 20] FIG. 10 is a longitudinal aberration diagram of the imaging optical system of Example 5 (infinity focus state). [Figure 21] FIG. 11 is a longitudinal aberration diagram of the imaging optical system of Example 5 (close focus state). [Figure 22] FIG. 13 is a cross-sectional view (first optical path) of the imaging optical system of the sixth embodiment. [Figure 23] FIG. 13 is a cross-sectional view (second optical path) of the imaging optical system of Example 6. [Figure 24] FIG. 13 is a longitudinal aberration diagram of the imaging optical system of Example 6 (infinity focus state). [Figure 25] FIG. 13 is a longitudinal aberration diagram of the imaging optical system of Example 6 (close focus state). [Figure 26] FIG. 1 is a diagram showing an imaging device using the imaging optical systems of Examples 1 to 6. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, embodiments of the present invention will be described with reference to the drawings. First, matters common to embodiments 1 to 6 will be described.

[0008] The imaging optical system (hereinafter simply referred to as the optical system) in each embodiment forms an image of light from an object on an image plane. A solid-state image sensor such as a CCD sensor or a CMOS sensor or a photosensitive film such as a silver halide film is disposed on the image plane, and an image of the object is obtained by capturing the image. The optical system of each embodiment includes a first lens positioned closest to the object, and a first transmissive-reflective surface (HM1), a first wave plate (QWP1), a second transmissive-reflective surface (HM2), a second wave plate (QWP2), and a third transmissive-reflective surface (HM3) arranged in this order from the object side to the image side. The first and second wave plates are, for example, quarter-wave plates. The optical system configured in this manner guides light incident from the object side to an image plane (IM) via the following two optical paths. The optical system of each embodiment also includes an aperture stop (STO).

[0009] [1st optical path] Light from the object side passes through the first transmission-reflection surface, the first wave plate, the second transmission-reflection surface, and the second wave plate in that order, is reflected by the third transmission-reflection surface toward the object side, passes through the second wave plate, is reflected by the second transmission-reflection surface toward the image side, passes through the second wave plate and the third transmission-reflection surface, and reaches the image plane.

[0010] [Second optical path] Light from the object side passes through the first transmissive-reflective surface and the first wave plate in that order, is reflected by the second transmissive-reflective surface toward the object side, passes through the first wave plate, is reflected by the first transmissive-reflective surface toward the image side, passes through the first wave plate, the second transmissive-reflective surface, the second wave plate, and the third transmissive-reflective surface, and then reaches the image plane.

[0011] The two optical paths are configured so that their focal lengths and back focal lengths (the air-equivalent distance on the optical axis from the lens surface closest to the image side of the optical system to the image plane) are identical. "Identical" here refers to identical in terms of design, and is considered identical even if there is a difference within the manufacturing tolerance range in the actual optical system (for example, a difference of 5%, 2%, or 1%). This allows the light passing through the two optical paths to be superimposed on the image plane, achieving an effect equivalent to doubling the transmittance of the entire optical system, i.e., obtaining an object image that is twice as bright.

[0012] The ratio of transmittance to reflectance of each of the first to third transmission-reflection surfaces may be 50%:50% (1:1), or may be another ratio. Specifically, the ratio of transmittance to 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. The first transmission-reflection surface, second transmission-reflection surface, and third transmission-reflection surface may have a light absorbing function. Furthermore, lenses may be formed on or cemented to both or one side of each transmission-reflection surface.

[0013] The first and second wave plates can be, for example, birefringent polymer films or liquid crystal alignment layers. Also, laminates of such polymer films or liquid crystal alignment layers can be used. By properly laminating these, a phase difference of nearly one-quarter of the wavelength can be obtained over a wide wavelength range. For example, Nippon Kayaku Co., Ltd.'s "WA-140T" or Colorlink Japan Co., Ltd.'s "Polar Correct" can be used. In addition to the above products, inorganic wave plates from Dexerials Corporation can also be used as the first and second wave plates.

[0014] The first and second wave plates can be placed by bonding them to one of the transmission-reflection surfaces. They can also be placed as separate entities from the transmission-reflection surfaces. For example, they can be placed in the optical path as films, or films bonded to glass plates and then placed in the optical path. Lenses may also be formed or bonded to one or both sides of each wave plate. For example, lenses may be formed on one or both sides of an inorganic wave plate using wafer-level optics technology as a substrate.

[0015] The above configuration makes it possible to provide a compact optical system with a bright F-number.

[0016] It is also preferable that the optical system of each embodiment satisfies at least one of the following conditions:

[0017] It is preferable that the imaging optical system of each embodiment satisfies the condition of the following formula (1), where L is the distance (total length) from the object-side surface (front surface) of the first lens closest to the object in the optical system to the image plane, and L1s is the distance on the optical axis from the front surface to the aperture stop.

[0018] -0.20 <L1s / L<0.95 (1) If L1s is increased in the negative direction so that L1s / L falls below the lower limit of formula (1), the height of off-axis rays increases, making it difficult to correct aberrations such as curvature of field, which is not preferable.If L1s is increased so that L1s / L exceeds the upper limit of formula (1), it becomes difficult to ensure the back focus, which is also not preferable.

[0019] It is more preferable to set the lower limit of formula (1) to −0.18, −0.16, −0.15, −0.14, −0.12, or −0.11, and it is more preferable to set the upper limit of formula (1) to 0.90, 0.85, 0.80, 0.75, or 0.70.

[0020] The above configuration makes it possible to provide an optical system that has a bright F-number, is compact, and has high optical performance.

[0021] Next, conditions that are more preferably satisfied in the optical systems of the respective embodiments will be explained.

[0022] The optical system of each embodiment preferably satisfies the condition of the following formula (2), where Nd is the minimum refractive index at the d-line (wavelength 587.6 nm) of the medium (lens, etc.) between the first transmission-reflection surface and the second transmission-reflection surface and between the second transmission-reflection surface and the third transmission-reflection surface.

[0023] 1.05 <Nd<2.50 (2) If Nd is below the lower limit of formula (2), an additional lens is required to correct the Petzval term that occurs during reflection, which is undesirable as the overall optical system becomes larger.If Nd is above the upper limit of formula (2), it is undesirable as high refractive index materials generally have high dispersion, making it difficult to correct the chromatic aberration that occurs during refraction.

[0024] It is more preferable that the lower limit of the formula (2) is 1.10, 1.15, 1.20, 1.25, 1.30, 1.35, or 1.40, and it is more preferable that the upper limit of the formula (2) is 2.40, 2.30, 2.20, 2.10, 2.00, or 1.90.

[0025] The optical system of each embodiment preferably satisfies the condition of the following formula (3), where Zm3 is the distance on the optical axis from the third transmissive-reflective surface to the image plane and f is the focal length of the optical system.

[0026] 0.20 <Zm3 / f<2.00 (3) If Zm3 is shortened so that Zm3 / f falls below the lower limit of formula (3), it becomes difficult to ensure the back focus, which is not preferable, and if Zm3 is lengthened so that Zm3 / f exceeds the upper limit of formula (3), it becomes difficult to correct the curvature of field, which is not preferable.

[0027] It is more preferable that the lower limit of the formula (3) is 0.15, 0.20, 0.25, 0.30, 0.35, or 0.40, and it is more preferable that the upper limit of the formula (3) is 1.90, 1.80, 1.70, 1.60, 1.50, or 1.40.

[0028] It is preferable that the optical system of each embodiment satisfies the condition of the following formula (4), where Dm12 is the distance on the optical axis between the first transmissive-reflective surface and the second transmissive-reflective surface.

[0029] 0.01 <Dm12 / L<0.20 (4) If Dm12 is shortened so that Dm12 / L falls below the lower limit of formula (4), the first and second transmitting-reflecting surfaces may interfere with each other, which is undesirable.If Dm12 is lengthened so that Dm12 / L exceeds the upper limit of formula (4), it becomes difficult to correct coma aberration in particular, which is undesirable.

[0030] It is more preferable to set the upper limit of the formula (4) to 0.19, 0.18, 0.17, 0.16 or 0.15.

[0031] The optical system of each embodiment preferably satisfies the condition of the following formula (5), where f1s is the composite focal length of at least one lens (lens group) arranged closer to the object side than the aperture stop.

[0032] 0.50 <f1s / f<7.00 (5) If f1s is shortened so that f1s / f falls below the lower limit of equation (5), the combined power of the optical system up to the aperture stop becomes too large, which increases spherical aberration and coma in particular, which is undesirable.If f1s is lengthened so that f1s / f exceeds the upper limit of equation (5), the optical system on the image side of the aperture stop becomes large, which is undesirable.

[0033] It is more preferable that the lower limit of formula (5) is 0.60, 0.70, 0.80, 0.90, 1.00, or 1.20, and it is more preferable that the upper limit of formula (5) is 6.80, 6.60, 6.50, 6.40, 6.20, 6.00, or 5.80.

[0034] The optical system of each embodiment preferably satisfies the condition of the following formula (6), where Φ3 is the optical power of the entire optical system and the optical power for light reflected at the third transflective surface.

[0035] 0.50<Φ3 / Φ<2.00 (6) Φ3 is given by the following equation, where Nd3 is the refractive index at the d-line of the medium adjacent to the third transmitting-reflecting surface on the object side, and R is the radius of curvature of the third transmitting-reflecting surface: Φ3=-2×Nd3 / R Φ is the reciprocal of the focal length of the entire optical system.

[0036] If Φ3 becomes small so that Φ3 / Φ falls below the lower limit of equation (6), it becomes necessary to add a lens with a large power, which increases the size of the entire optical system, which is undesirable.If Φ3 becomes large so that Φ3 / Φ exceeds the upper limit of equation (6), the curvature of the third transflective surface becomes large, which increases the decentration sensitivity of coma aberration in particular, which is undesirable.

[0037] It is more preferable that the lower limit of the formula (6) is 0.55, 0.60, 0.65, or 0.70, and it is more preferable that the upper limit of the formula (6) is 1.90, 1.80, 1.70, 1.60, or 1.50.

[0038] It is preferable that the optical system of each embodiment satisfies the condition of the following expression (7).

[0039] 1.00 <L / f<5.50 (7) If the distance L is reduced so that L / f falls below the lower limit of equation (7), it becomes difficult to correct spherical aberration, which is undesirable. If L is increased so that L / f exceeds the upper limit of equation (7), the entire optical system becomes large, which is undesirable.

[0040] It is more preferable that the lower limit of the formula (7) is 1.10, 1.20, 1.30, 1.35, or 1.40, and it is more preferable that the upper limit of the formula (7) is 5.25, 5.00, 4.75, or 4.50.

[0041] It is preferable that the optical system of each embodiment satisfies the condition of the following formula (8), where Fno is the maximum open F-number of the optical system.

[0042] 0.55≦Fno≦8.00 (8) If Fno is below the lower limit of formula (7), it is difficult to correct spherical aberration and coma, which is undesirable.If Fno is above the upper limit of formula (8), the amount of light reaching the image plane becomes too small, along with the loss of light at each transmissive / reflective surface, which is undesirable.

[0043] It is more preferable that the lower limit of the formula (8) is 0.60, 0.70, 0.80, or 0.90, and it is more preferable that the upper limit of the formula (8) is 7.00, 6.00, 5.00, 4.50, 4.00, or 3.00.

[0044] Furthermore, it is preferable that the optical system of each embodiment has at least one of the following configurations.

[0045] The second transflective surface is preferably a flat surface. If the second transflective surface has a curvature, it is difficult to make the focal lengths and back focal lengths of the two optical paths in the optical system the same, which is undesirable.

[0046] It is preferable that the first transmissive-reflective surface be a surface convex toward the object side, and the third transmissive-reflective surface be a surface concave toward the object side. This configuration is preferable because it makes the power at the time of reflection positive and enables the optical system to be made compact.

[0047] It is preferable that the absolute values ​​of the curvatures on the optical axis of the first and third transmissive-reflective surfaces are the same. The term "same" here is as defined above. This configuration is advantageous for making the focal lengths and back focal lengths of the two optical paths within the optical system the same.

[0048] It is preferable that the first and third transflective surfaces each be configured with a polarization-selective transmission element. With this configuration, when a polarization-utilizing configuration (described later) is adopted, ghost light (unwanted light) can be reduced and the light guided to the image plane via the two normal optical paths can be increased.

[0049] The third transflective surface is preferably located closer to the image side than the aperture stop. With this configuration, the light that has passed through the aperture stop is reflected (folded) by the transflective surface, thereby increasing the height of off-axial rays and making the angle of incidence on the image plane gentler. As a result, an optical system with a bright F-number can be realized.

[0050] [Polarized configuration] A configuration using polarized light will be described using Figure 1. The transmission-reflection surface located on the object side of the optical system is a polarization-selective transmission-reflection element (PBS1 / HM1) A serving as a reflective polarizer, the transmission-reflection surface located on the image side is a polarization-selective transmission-reflection element (PBS2 / HM3) E, and the transmission-reflection surface located between these two transmission-reflection surfaces is a half mirror (HM2) C. Furthermore, a first quarter-wave plate (QWP1) B serving as a first wave plate is located between the polarization-selective transmission-reflection element A and the half mirror C, and a second quarter-wave plate (QWP2) D serving as a second wave plate is located between the half mirror C and the polarization-selective transmission-reflection element E.

[0051] The polarization-selective transmission-reflection element A is an element configured to transmit a first linearly polarized light and reflect a second linearly polarized light that is orthogonal to the first linearly polarized light. The polarization-selective transmission-reflection element E is an element configured to transmit the second linearly polarized light and reflect the first linearly polarized light. That is, the polarization-selective transmission-reflection elements A and E are arranged so that their polarization transmission axes are orthogonal to each other.

[0052] The polarization-selective transmission / reflection element is, for example, a wire grid polarizer or a reflective polarizer having a laminated retardation film structure. In this case, the wire grid-formed surface or retardation film surface of the polarization-selective transmission / reflection elements A and E functions as a transmission / reflection surface.

[0053] The wire grid polarizer does not necessarily have to be made of aligned metal wires, but may be any element that has thin metal or dielectric layers at predetermined intervals 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.

[0054] 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 axes of the polarization-selective transmission-reflection elements A and E. The first quarter-wave plate B and the second quarter-wave plate D are arranged with their slow axes tilted at 90° with respect to each other. With this arrangement, when light passes through the first quarter-wave plate B and the second quarter-wave plate D, the wavelength dispersion characteristics of the wave plates B and D cancel each other out.

[0055] The half mirror C is a half mirror formed by, for example, a dielectric multilayer film or metal deposition, and functions as a transmissive and reflective surface.

[0056] [Polarized light in the first optical path] Light entering the optical system from the object side is converted into linearly polarized light by polarization-selective transmission / reflection element A, and this linearly polarized light is converted into circularly polarized light by first quarter-wave plate B and enters half mirror C. The circularly polarized light that enters half mirror C and is transmitted through it is converted by second quarter-wave plate D into linearly polarized light in the same polarization direction as when it passed through polarization-selective transmission / reflection element A and reflected by polarization-selective transmission / reflection element E. The reflected linearly polarized light is converted back into circularly polarized light by second quarter-wave plate D and enters half mirror C. A portion of the circularly polarized light that entered half mirror C is reflected and becomes circularly polarized in the reverse direction and returns to second quarter-wave plate D. The circularly polarized light that returned to second quarter-wave plate D is converted by second quarter-wave plate D into linearly polarized light in a polarization direction orthogonal to when it first passed through polarization-selective transmission / reflection element A and enters polarization-selective transmission / reflection element E. At this time, the polarization direction of the linearly polarized light coincides with the transmission axis of the polarization-selective transmission / reflection element E, so the linearly polarized light incident on the polarization-selective transmission / reflection element E is transmitted through it and guided to the imaging plane IM.

[0057] [Polarized light in the second optical path] Light entering the optical system from the object side is converted into linearly polarized light by the polarization-selective transmission / reflection element A, and this linearly polarized light is then converted into circularly polarized light by the first quarter-wave plate B and enters the half mirror C. Up to this point, the process is the same as the first optical path.

[0058] Of the circularly polarized light that enters half mirror C, the circularly polarized light reflected by it is converted by first quarter-wave plate B into linearly polarized light that is orthogonal to the light that initially passed through polarization-selective transmission-reflector A, and then reflected by polarization-selective transmission-reflector A. The linearly polarized light reflected by polarization-selective transmission-reflector A is converted back into circularly polarized light by first quarter-wave plate B and enters half mirror C. A portion of the circularly polarized light that enters half mirror C is transmitted through and enters second quarter-wave plate D, where it is converted by second quarter-wave plate D into linearly polarized light with the same polarization direction as the linearly polarized light reflected by polarization-selective transmission-reflector A. The linearly polarized light from second quarter-wave plate D enters polarization-selective transmission-reflector E. Because the polarization direction of the linearly polarized light at this time coincides with the transmission axis of polarization-selective transmission-reflector E, the linearly polarized light that passes through polarization-selective transmission-reflector E is transmitted through it and directed to the imaging plane IM.

[0059] As described above, light incident from the object side is guided to the image plane IM via two optical paths, making it possible to provide a compact optical system with a bright F-number.

[0060] Note that the above-mentioned "orthogonal (90°)", "45°" and "same" do not mean 90°, 45° and same in the strict sense, and there may be a difference of within ±5° (or within ±2°, or even within ±1°).

[0061] In the optical systems of the respective embodiments, the lenses may be made of either a polymer material or a glass material, although the lenses disposed between the first and second transmission-reflection surfaces and the lenses disposed between the second and third transmission-reflection surfaces are preferably made of a material with low birefringence.

[0062] The optical systems of Examples 1 to 6 will be specifically described below. After Example 6, Numerical Examples 1 to 6 corresponding to Examples 1 to 6, respectively, are listed. [Example]

[0063] 2 and 3 respectively show cross sections of the first and second optical paths of the optical system of Example 1 (Numerical Example 1). The optical system of Example 1 has, arranged in order from the object side to the image side, an aperture stop STO, a first transmissive-reflective surface HM1, a first wave plate QWP1, a second transmissive-reflective surface HM2, a second wave plate QWP2, and a third transmissive-reflective surface HM3. The optical system of this example further has multiple lenses including a first lens.

[0064] The first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located closer to the image than the aperture stop STO, and the third transmissive-reflective surface HM3 is provided on the object-side concave surface of the lens located further to the image side. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the image-side flat surface of the lens on which the first transmissive-reflective surface HM1 is provided.

[0065] An image sensor is disposed on the image plane IM, and a sensor protection glass CG is disposed on the display surface of the image sensor.

[0066] The optical system of Numerical Example 1 has an aperture ratio of about 0.95 and a half angle of view of about 24.38°.

[0067] In this optical system, focusing can be achieved by moving some of the lenses in the direction of the optical axis. The arrows in the figure indicate the direction in which the lenses move when focusing from infinity to close range. This also applies to the other embodiments described below.

[0068] FIG. 4 shows longitudinal aberrations (spherical aberration, astigmatism, distortion, and chromatic aberration) of the optical system of Numerical Example 1 when it is focused on an object at infinity (hereinafter referred to as an infinity focus state). FIG. 5 shows longitudinal aberrations of the optical system of Numerical Example 1 when it is focused on an object at a close distance (hereinafter referred to as a close distance focus state). The value obj in FIG. 5 is the distance (mm) from the object plane to the focus plane (image plane).

[0069] In the spherical aberration diagrams, Fno indicates the F-number, the solid line indicates spherical aberration at the d-line (wavelength 587.6 nm), and the two-dot chain line indicates spherical aberration at the g-line (wavelength 435.8 nm). In the astigmatism diagrams, the solid line S indicates astigmatism at the sagittal image plane, and the dashed line M indicates astigmatism at the meridional image plane. The distortion diagrams show distortion at the d-line. The chromatic aberration diagrams show chromatic aberration of magnification at the g-line. ω is the half angle of view (°). The above explanations regarding the aberration diagrams apply to other embodiments described later. [Example]

[0070] 6 and 7 show cross sections of the first and second optical paths of the optical system of Example 2 (Numerical Example 2), respectively. The basic configuration of the optical system of this example is the same as that of Example 1.

[0071] In this embodiment as well, the first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located closer to the image than the aperture stop STO, and the third transmissive-reflective surface HM3 is provided on the object-side concave surface of a lens located further to the image side. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the image-side flat surface of the lens on which the first transmissive-reflective surface HM1 is provided.

[0072] The optical system of Numerical Example 2 has an aperture ratio of about 0.95 and a half angle of view of about 15.27°.

[0073] 8 and 9 show longitudinal aberrations of the optical system of Numerical Example 2 in the infinity focus state and the close distance focus state, respectively. [Example]

[0074] 10 and 11 show cross sections of the first and second optical paths of the optical system of Example 3 (Numerical Example 3), respectively. The basic configuration of the optical system of this example is similar to that of Example 1.

[0075] In this embodiment as well, the first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located closer to the image than the aperture stop STO, and the third transmissive-reflective surface HM3 is provided on the object-side concave surface of a lens further closer to the image. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the image-side flat surface of a lens cemented to the lens on which the first transmissive-reflective surface HM1 is provided.

[0076] The optical system of Numerical Example 3 has an aperture ratio of about 0.95 and a half angle of view of about 39.86°.

[0077] 12 and 13 show longitudinal aberrations of the optical system of Numerical Example 3 in the infinity focus state and the close distance focus state, respectively. [Example]

[0078] 14 and 15 show cross sections of the first and second optical paths of the optical system of Example 4 (Numerical Example 4), respectively. The basic configuration of the optical system of this example is similar to that of Example 1.

[0079] In this embodiment as well, the first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located closer to the image side than the aperture stop STO, and the third transmissive-reflective surface HM3 is provided as a surface having a concave shape facing the object side on the image-side convex surface of a lens cemented to the above-mentioned lens on the image side. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the flat surface that serves as the cemented surface of these lenses.

[0080] The optical system of Numerical Example 4 has an aperture ratio of about 1.2 and a half angle of view of about 9.68°.

[0081] 16 and 17 show longitudinal aberrations of the optical system of Numerical Example 4 in the infinity focus state and the close distance focus state, respectively. [Example]

[0082] 18 and 19 respectively show cross sections of the first optical path and the second optical path of the optical system of Example 5 (Numerical Example 5). The basic configuration of the optical system of this example is similar to that of Example 1.

[0083] In this embodiment as well, the first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located closer to the image side than the aperture stop STO, and the third transmissive-reflective surface HM3 is provided as a surface having a concave shape facing the object side on the image-side convex surface of a lens cemented to the above-mentioned lens on the image side. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the flat surface that serves as the cemented surface of these lenses.

[0084] The optical system of Numerical Example 5 has an aperture ratio of about 2.8 and a half angle of view of about 24.38°.

[0085] 20 and 21 show longitudinal aberrations of the optical system of Numerical Example 5 in the infinity focus state and the close distance focus state, respectively. [Example]

[0086] 22 and 23 show cross sections of the first and second optical paths of the optical system of Example 6 (Numerical Example 6), respectively. The basic configuration of the optical system of this example is similar to that of Example 1.

[0087] In this embodiment as well, the first transmissive-reflective surface HM1 is provided on the object-side convex surface of a lens located on the image side of the aperture stop STO, and the third transmissive-reflective surface HM3 is provided as a surface concave toward the object side on the image-side convex surface of a lens located further to the image side. Furthermore, the first wave plate QWP1, the second transmissive-reflective surface HM2, and the second wave plate QWP2 are provided on the image-side flat surface of the lens on which the first transmissive-reflective surface HM1 is provided.

[0088] The optical system of Numerical Example 6 has an aperture ratio of about 2.8 and a half angle of view of about 33.82°.

[0089] 24 and 25 show longitudinal aberrations of the optical system of Numerical Example 6 in the infinity focus state and the close distance focus state, respectively.

[0090] Numerical values ​​for the first optical path in Numerical Examples 1 to 6 are listed below. In the surface data for each numerical example, surface number i indicates the order of the surface when counted from the object side. r is the radius of curvature of the i-th surface (mm), d is the lens thickness or air gap (mm) between the i-th and (i+1)-th surfaces, and nd is the refractive index at the d-line of the material of the i-th optical element. νd is the Abbe number based on the d-line of the material of the i-th optical element. The Abbe number based on the d-line, νd, is expressed as νd = (Nd-1) / (NF-NC), where Nd, NF, and NC are the refractive indices at the Fraunhofer d-line (587.6 nm), F-line (486.1 nm), and C-line (656.3 nm), respectively.

[0091] In each numerical example, d, focal length (mm), F-number, and half angle of view (°) are all values ​​when the optical system is focused at infinity. BF represents back focus (mm). Back focus is the distance on the optical axis from the surface closest to the image (the final surface) of the optical system to the paraxial image plane, expressed as an air-equivalent length.

[0092] An "*" next to a surface number indicates that the surface has an aspherical shape. The aspherical shape is expressed by the following formula, where x is the amount of displacement in the optical axis direction from the vertex of the surface at a height h from the optical axis, R is the paraxial radius of curvature, K is the conic constant, and Ai (i=2, 4, 6, 8, ...) are the aspherical coefficients of each order. The conic constant and the aspherical coefficient "e±M" are multiplied by 10 ±M means.

[0093] x=(h 2 / R) / [1+√{1-(1+K)(h / R) 2}] +A4×h 4 +A6×h 6 +A8×h 8 +A10×h 10 The various data also includes focal length (mm), F-number, half angle of view (°), and image height (mm). (Numerical Example 1) Unit: mm Surface Data Surface number rd nd νd 1* -94.107 1.80 1.58913 61.1 2* 76.271 0.20 3 52.548 3.30 1.80518 25.4 4 210.007 1.16 5 -488.286 1.50 2.00100 29.1 6 58.168 6.84 1.75500 52.3 7 -55.281 0.50 8 (Aperture) ∞ 14.75 9 102.432 2.32 1.78470 26.3 10 ∞ 2.32 1.78470 26.3 11 -102.432 -2.32 12∞2.32 13 -102.432 1.70 2.00100 29.1 14* 30.774 5.13 15 -40.943 1.62 2.00100 29.1 16 -38.906 1.00 17 48.199 8.07 1.49700 81.5 18 -26.326 2.04 1.83481 42.7 19 -910.871 0.10 20* 53.194 8.67 1.59522 67.7 21* -36.271 9.40 22 ∞ 3.00 1.51633 64.1 23∞0.10 Image plane ∞ Aspheric data Front page K = 0.00000e+00 A 4=-2.01384e-06 A 6= 1.05466e-09 2nd side K = 0.00000e+00 A 4= 1.45010e-06 A 6= 1.70305e-09 Page 14 K = 0.00000e+00 A 4=-6.41282e-07 A 6=-2.57751e-09 A 8= 7.90939e-12 A10=-2.56976e-14 Page 20 K = 0.00000e+00 A 4=-4.70322e-07 A 6= 4.45271e-09 Page 21 K = 0.00000e+00 A 4= 7.89133e-06 A 6=-9.88331e-09 A 8= 3.68077e-11 A10=-3.74355e-14 Various data Focal length 32.00 F-number 0.95 Half angle of view (°) 24.38 Image height 14.50 BF 0.10 (Numerical Example 2) Unit: mm Surface Data Surface number rd nd νd 1* -285.181 1.80 1.51742 52.4 2* 55.862 0.20 3 53.636 8.52 1.80400 46.6 4 497.664 4.93 5 907.970 1.50 1.80400 46.6 6 78.163 10.20 1.60300 65.4 7 -103.443 0.50 8 (Aperture) ∞ 19.94 9 153.852 3.92 1.80518 25.4 10 ∞ 3.92 1.80518 25.4 11 -153.852 -3.92 12∞3.92 13 -153.852 1.70 2.00330 28.3 14* 40.984 6.88 15 -57.350 1.65 1.80518 25.4 16 -54.518 1.00 17 669.365 6.47 1.59522 67.7 18 -31.952 2.40 1.88300 40.8 19 -278.234 2.85 20* 54.403 10.66 1.51823 58.9 21* -43.863 15.47 22 ∞ 3.00 1.51633 64.1 23∞0.10 Image plane ∞ Aspheric data Front page K = 0.00000e+00 A 4=-5.42118e-07 A 6=-2.96568e-11 2nd side K = 0.00000e+00 A 4= 4.58806e-07 A 6= 1.21547e-10 Page 14 K = 0.00000e+00 A 4= 1.28694e-07 A 6=-1.93586e-10 A 8= 8.69864e-13 A10=-6.61327e-16 Page 20 K = 0.00000e+00 A 4=-1.08573e-06 A 6=-7.72692e-10 Page 21 K = 0.00000e+00 A 4= 3.94929e-06 A 6=-4.67606e-09 A 8= 8.04931e-12 A10=-6.11647e-15 Various data Focal length 53.13 F-number 0.95 Half angle of view (°) 15.27 Image height 14.50 BF 0.10 (Numerical Example 3) Unit: mm Surface Data Surface number rd nd νd 1 55.751 1.00 1.62041 60.3 2 23.340 7.70 3* 70.604 2.35 1.51633 64.1 4* 34.019 3.99 5 29.326 3.75 1.80518 25.4 6 41.944 10.93 7 22.456 8.98 1.49700 81.5 8 -104.157 0.10 9 (Aperture) ∞ 3.76 10 -47.717 1.00 1.72916 54.7 11 9405.818 0.50 12* 83.368 1.00 1.88300 40.8 13* 46.532 2.43 1.51633 64.1 14 ∞ 2.43 1.51633 64.1 15* -46.532 1.00 1.88300 40.8 16* -83.368 -1.00 17* -46.532 -2.43 1.51633 64.1 18∞2.43 19* -46.532 1.00 1.88300 40.8 20* -83.368 1.34 21 -33.281 1.85 1.51633 64.1 22 -25.687 0.50 23 -42.255 1.00 1.95375 32.3 24 -1992.251 0.10 25* 45.697 6.78 1.77250 49.6 26* -32.935 9.40 27 ∞ 3.00 1.51633 64.1 28∞0.10 Image plane ∞ Aspheric data 3rd page K = 0.00000e+00 A 4= 2.41089e-05 A 6=-4.66339e-08 A 8= 3.98433e-11 A10=-2.36767e-14 Side 4 K = 0.00000e+00 A 4= 2.56034e-05 A 6=-4.76636e-08 Side 12 K = 0.00000e+00 A 4=-2.07388e-06 A 6=-3.24551e-09 A 8=-1.93377e-11 Page 13 K = 0.00000e+00 A 4= 2.69899e-06 A 6= 8.95419e-09 A 8=-5.45146e-11 Page 15 K = 0.00000e+00 A 4=-2.69899e-06 A 6=-8.95419e-09 A 8= 5.45146e-11 Page 16 K = 0.00000e+00 A 4= 2.07388e-06 A 6= 3.24551e-09 A 8= 1.93377e-11 Page 17 K = 0.00000e+00 A 4=-2.69899e-06 A 6=-8.95419e-09 A 8= 5.45146e-11 Page 19 K = 0.00000e+00 A 4=-2.69899e-06 A 6=-8.95419e-09 A 8= 5.45146e-11 Page 20 K = 0.00000e+00 A 4= 2.07388e-06 A 6= 3.24551e-09 A 8= 1.93377e-11 Page 25 K = 0.00000e+00 A 4=-8.43406e-06 A 6= 2.76741e-08 A 8=-2.77767e-11 Page 26 K = 0.00000e+00 A 4= 1.09800e-05 A 6=-9.64897e-09 A 8= 9.45847e-11 A10=-1.75246e-13 Various data Focal length 17.37 F-number 0.95 Half angle of view (°) 39.86 Image height 14.50 BF 0.10 (Numerical Example 4) Unit: mm Surface Data Surface number rd nd νd 1 71.616 10.13 1.60300 65.4 2 2625.506 3.00 1.65412 39.7 3 294.518 9.18 4 269.737 3.00 2.00100 29.1 5 499.753 0.10 6 78.041 5.07 1.53775 74.7 7 237.310 17.17 8* -312.370 3.00 1.80400 46.6 9 72.936 4.20 10 (Aperture) ∞ 3.00 11 219.573 3.00 1.51633 64.1 12 ∞ 3.00 1.51633 64.1 13 -219.573 -3.00 14∞3.00 15 -219.573 0.50 16 -698.013 2.00 1.95375 32.3 17 37.027 4.48 1.51633 64.1 18 277.750 17.98 19 68.324 11.20 1.80518 25.4 20 -184.668 20.00 21 ∞ 3.00 1.51633 64.1 22∞0.10 Image plane ∞ Aspheric data Side 8 K = 0.00000e+00 A 4=-7.54205e-07 A 6= 1.65340e-10 A 8=-7.14643e-15 Various data Focal length 85.00 F-number 1.20 Half angle of view (°) 9.68 Image height 14.50 BF 0.10 (Numerical Example 5) Unit: mm Surface Data Surface number rd nd νd 1 (Aperture) ∞ 4.50 2* 20.430 8.34 1.51742 52.4 3 -52.701 0.69 4 58.909 1.00 1.91650 31.6 5* 19.763 2.98 6 119.924 6.20 1.49700 81.5 7 ∞ 6.20 1.49700 81.5 8 -119.924 -6.20 9∞6.20 10 -119.924 12.00 11 ∞ 3.00 1.51633 64.1 12 ∞ 0.10 Image plane ∞ Aspheric data 2nd side K = 0.00000e+00 A 4=-2.65515e-05 A 6=-2.97402e-08 A 8= 2.10475e-10 5th page K = 0.00000e+00 A 4=-8.08934e-06 A 6=-4.80167e-08 A 8= 1.20263e-10 Various data Focal length 32.00 F-number 2.80 Half angle of view (°) 24.38 Image height 14.50 BF 0.10 (Numerical Example 6) Unit: mm Surface Data Surface number rd nd νd 1 29.653 5.00 1.43875 94.9 2 19.530 12.69 3* 152.941 1.91 1.43875 94.9 4* 67.284 4.83 5 18.891 3.02 1.67270 32.1 6 19.241 10.70 7 16.685 10.11 1.53775 74.7 8 -61.074 0.50 9 (Aperture) ∞ 0.69 10 -32.800 1.00 1.89190 37.1 11 301.503 0.50 12* 87.959 1.00 1.72916 54.7 13* 56.015 1.34 1.43875 94.9 14 ∞ 1.34 1.43875 94.9 15* -56.015 1.00 1.72916 54.7 16* -87.959 -1.00 17* -56.015 -1.34 1.43875 94.9 18∞1.34 19* -56.015 1.00 1.72916 54.7 20* -87.959 0.50 21 46.022 1.03 1.84666 23.8 22 51.041 0.50 23 135.106 1.00 1.80400 46.6 24 32.485 0.10 25* 26.095 3.75 1.56732 42.8 26* -86.901 9.40 27 ∞ 3.00 1.51633 64.1 28∞0.10 Image plane ∞ Aspheric data 3rd page K = 0.00000e+00 A 4= 2.41089e-05 A 6=-4.66339e-08 A 8= 3.98433e-11 A10=-2.36767e-14 Side 4 K = 0.00000e+00 A 4= 2.56034e-05 A 6=-4.76636e-08 Side 12 K = 0.00000e+00 A 4=-1.57730e-06 A 6= 9.96129e-08 A 8=-6.61859e-09 Page 13 K = 0.00000e+00 A 4= 1.51977e-05 A 6= 4.44825e-07 A 8=-2.73136e-08 Page 15 K = 0.00000e+00 A 4=-1.51977e-05 A 6=-4.44825e-07 A 8= 2.73136e-08 Page 16 K = 0.00000e+00 A 4= 1.57730e-06 A 6=-9.96129e-08 A 8= 6.61859e-09 Page 17 K = 0.00000e+00 A 4=-1.51977e-05 A 6=-4.44825e-07 A 8= 2.73136e-08 Page 19 K = 0.00000e+00 A 4=-1.51977e-05 A 6=-4.44825e-07 A 8= 2.73136e-08 Page 20 K = 0.00000e+00 A 4= 1.57730e-06 A 6=-9.96129e-08 A 8= 6.61859e-09 Page 25 K = 0.00000e+00 A 4=-8.43406e-06 A 6= 2.76741e-08 A 8=-2.77767e-11 Page 26 K = 0.00000e+00 A 4= 1.09800e-05 A 6=-9.64897e-09 A 8= 9.45847e-11 A10=-1.75246e-13 Various data Focal length 21.65 F-number 2.80 Half angle of view (°) 33.82 Image height 14.50 BF 0.10 The values ​​of the formulas (1) to (8) in the respective numerical examples are summarized in Table 1 below.

[0094] [Table 1]

[0095] (imaging device) 26 shows a digital still camera as an imaging device equipped with the optical system of each of the above-described embodiments. 20 denotes the camera body, and 21 denotes the imaging optical system constituted by any of the zoom lenses of Examples 1 to 5. 22 denotes a solid-state imaging element such as a CCD sensor or CMOS sensor that is built into the camera body 20 and photoelectrically converts the optical image (subject image) formed by the imaging optical system 21, i.e., captures the subject image through the imaging optical system 21. 23 denotes a recording unit that records image data generated by processing the imaging signal from the imaging element 22, and 24 denotes a rear display that displays the image data.

[0096] By using the zoom lens of each embodiment, a small camera with high optical performance can be obtained.

[0097] The camera may be a single-lens reflex camera having a quick-turn mirror, or may be a mirrorless camera having no quick-turn mirror.

[0098] The above embodiment includes the following configurations.

[0099] (Configuration 1) The first lens element is located closest to the object, a first transmissive-reflective surface, a first wave plate, a second transmissive-reflective surface, a second wave plate, and a third transmissive-reflective surface, which are arranged in this order from the object side to the image side; an aperture stop; Light incident from the object side is imaged on an image plane via two optical paths including transmission and reflection at the first, second, and third transmissive-reflective surfaces; When the distance on the optical axis from the object-side surface of the first lens to the image plane is L and the distance on the optical axis from the object-side surface of the first lens to the aperture stop is L1s, -0.20 <L1s / L<0.95 An optical system characterized by satisfying the following conditions: (Configuration 2) The light from the object side the light reaches the image plane via a first optical path of transmitting through the first transmission-reflection surface, transmitting through the first wave plate, transmitting through the second transmission-reflection surface, transmitting through the second wave plate, being reflected at the third transmission-reflection surface, transmitting through the second wave plate, being reflected at the second transmission-reflection surface, transmitting through the second wave plate, and transmitting through the third transmission-reflection surface; the optical system described in Configuration 1, characterized in that the light reaches the image plane via a second optical path in which it transmits through the first transmission-reflection surface, transmits through the first wave plate, is reflected at the second transmission-reflection surface, transmits through the first wave plate, is reflected at the first transmission-reflection surface, transmits through the first wave plate, transmits through the second transmission-reflection surface, transmits through the second wave plate, and transmits through the third transmission-reflection surface. (Configuration 3) When the minimum refractive index at the d-line of the medium between the first transmission-reflection surface and the second transmission-reflection surface and between the second transmission-reflection surface and the third transmission-reflection surface is Nd, 1.05 <Nd<2.50 3. The 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 third transmissive-reflective surface to the image plane is Zm3 and the focal length of the optical system is f, 0.20 <Zm3 / f<2.00 4. The optical system according to any one of configurations 1 to 3, wherein the following condition is satisfied: (Configuration 5) When the distance on the optical axis between the first transmissive-reflective surface and the second transmissive-reflective surface is Dm12, 0.01 <Dm12 / L<0.20 5. The optical system according to any one of configurations 1 to 4, wherein the following condition is satisfied: (Configuration 6) When the composite focal length of at least one lens arranged on the object side of the aperture stop is f1s and the focal length of the optical system is f, 0.50 <f1s / f<7.00 6. The imaging optical system according to any one of configurations 1 to 5, wherein the following condition is satisfied: (Configuration 7) When the optical power of the light reflected by the third transmitting-reflecting surface is Φ3, the optical power of the optical system is Φ, the refractive index at the d-line of the medium adjacent to the third transmitting-reflecting surface on the object side is Nd3, and the radius of curvature of the third transmitting-reflecting surface is R, then Φ3=-2×Nd3 / R: 0.50<Φ3 / Φ<2.00 7. The optical system according to any one of configurations 1 to 6, wherein the following condition is satisfied: (Configuration 8) When the focal length of the optical system is f, 1.00 <L / f<5.50 8. The optical system according to any one of configurations 1 to 7, wherein the following condition is satisfied: (Configuration 9) When the open F-number of the optical system is Fno, 0.55≦Fno≦8.00 9. The optical system according to any one of configurations 1 to 8, wherein the following condition is satisfied: (Configuration 10) 10. The optical system according to any one of configurations 1 to 9, wherein the second transmitting-reflecting surface is a flat surface. (Configuration 11) 11. The optical system according to any one of configurations 1 to 10, wherein the first transmitting-reflecting surface is a surface that is convex toward the object side, and the third transmitting-reflecting surface is a surface that is concave toward the object side. (Configuration 12) 12. The optical system according to any one of configurations 1 to 11, wherein the absolute values ​​of the curvatures on the optical axis of the first transmissive-reflective surface and the third transmissive-reflective surface are the same. (Configuration 13) 13. The optical system according to any one of configurations 1 to 12, wherein the first transmission-reflection surface and the third transmission-reflection surface are configured by reflective polarizers. (Configuration 14) 14. The optical system according to any one of configurations 1 to 13, wherein the second transmitting-reflecting surface is configured by a half mirror. (Configuration 15) 15. The optical system according to any one of configurations 1 to 14, wherein the third transmissive-reflective surface is disposed on the image side of the aperture stop. (Configuration 16) The first lens element is located closest to the object, a first transmissive-reflective surface, a first wave plate, a second transmissive-reflective surface, a second wave plate, and a third transmissive-reflective surface, which are arranged in this order from the object side to the image side; an aperture stop; An optical system characterized in that light incident from the object side is imaged on an image plane via two optical paths including transmission and reflection at the first, second, and third transmissive-reflective surfaces. (Configuration 17) The optical system according to any one of configurations 1 to 16; and an imaging element for capturing an image of an object through the optical system.

[0100] The embodiments described above are merely representative examples, and various modifications and alterations are possible to each embodiment when implementing the present invention. [Explanation of symbols]

[0101] HM1 1st transmissive reflective surface HM2 2nd transmissive reflective surface HM3 3rd transmissive reflective surface QWP1 1st wave plate QWP2 2nd wave plate STO aperture stop IM image plane

Claims

1. a first lens arranged closest to the object; a first transmission-reflection surface, a first wave plate, a second transmission-reflection surface, a second wave plate, and a third transmission-reflection surface, which are arranged in this order from the object side to the image side; an aperture stop; light incident from the object side is imaged on an image plane via two optical paths including transmission and reflection at the first, second and third transmissive-reflective surfaces; When the distance on the optical axis from the object-side surface of the first lens to the image plane is L and the distance on the optical axis from the object-side surface of the first lens to the aperture stop is L1s, -0.20<L1s / L<0.95 An optical system characterized by satisfying the following conditions:

2. The light from the object side the light reaches the image plane via a first optical path of being transmitted through the first transmission-reflection surface, being transmitted through the first wave plate, being transmitted through the second transmission-reflection surface, being transmitted through the second wave plate, being reflected through the third transmission-reflection surface, being transmitted through the second wave plate, being reflected through the second transmission-reflection surface, being transmitted through the second wave plate, and being transmitted through the third transmission-reflection surface; 2. The optical system according to claim 1, wherein the light reaches the image plane via a second optical path in which the light passes through the first transmission-reflection surface, passes through the first wave plate, is reflected at the second transmission-reflection surface, passes through the first wave plate, is reflected at the first transmission-reflection surface, passes through the first wave plate, is transmitted at the second transmission-reflection surface, passes through the second wave plate, and passes through the third transmission-reflection surface.

3. When the minimum refractive index of the medium between the first transmission-reflection surface and the second transmission-reflection surface and between the second transmission-reflection surface and the third transmission-reflection surface is Nd, 1.05<Nd<2.50 2. The optical system according to claim 1, wherein the following condition is satisfied:

4. When the distance on the optical axis from the third transmissive-reflective surface to the image plane is Zm3 and the focal length of the optical system is f, 0.20<Zm3 / f<2.00 2. The optical system according to claim 1, wherein the following condition is satisfied:

5. When the distance on the optical axis between the first transmissive-reflective surface and the second transmissive-reflective surface is Dm12, 0.01<Dm12 / L<0.20 2. The optical system according to claim 1, wherein the following condition is satisfied:

6. When the composite focal length of at least one lens arranged on the object side of the aperture stop is f1s and the focal length of the optical system is f, 0.50<f1s / f<7.00 2. The imaging optical system according to claim 1, wherein the following condition is satisfied:

7. When the optical power of the light reflected by the third transmitting-reflecting surface is Φ3, the optical power of the optical system is Φ, the refractive index at the d-line of the medium adjacent to the third transmitting-reflecting surface on the object side is Nd3, and the radius of curvature of the third transmitting-reflecting surface is R, then Φ3=-2×Nd3 / R: 0.50<Φ3 / Φ<2.00 2. The optical system according to claim 1, wherein the following condition is satisfied:

8. When the focal length of the optical system is f, 1.00<L / f<5.50 2. The optical system according to claim 1, wherein the following condition is satisfied:

9. When the open F-number of the optical system is Fno, 0.55≦Fno≦8.00 2. The optical system according to claim 1, wherein the following condition is satisfied:

10. 2. The optical system according to claim 1, wherein the second transmissive-reflective surface is a flat surface.

11. 2. The optical system according to claim 1, wherein the first transmitting-reflecting surface is a surface that is convex toward the object side, and the third transmitting-reflecting surface is a surface that is concave toward the object side.

12. 2. The optical system according to claim 1, wherein the absolute values ​​of the curvatures on the optical axis of the first transmissive-reflective surface and the third transmissive-reflective surface are the same.

13. 2. The optical system according to claim 1, wherein the first transmission-reflection surface and the third transmission-reflection surface are configured by reflective polarizers.

14. 2. The optical system according to claim 1, wherein the second transmitting and reflecting surface is formed of a half mirror.

15. 2. The optical system according to claim 1, wherein the third transmissive-reflective surface is disposed closer to the image side than the aperture stop.

16. a first lens arranged closest to the object; a first transmission-reflection surface, a first wave plate, a second transmission-reflection surface, a second wave plate, and a third transmission-reflection surface, which are arranged in this order from the object side to the image side; an aperture stop; an optical system that forms an image on an image plane by transmitting light incident from the object side through two optical paths including transmission and reflection at said first, second and third transmissive-reflective surfaces;

17. An optical system according to any one of claims 1 to 16; and an imaging element for capturing an image of an object through the optical system.

Images