JP2025027164A - Imaging Optical System - Google Patents

Abstract

To appropriately downsize an entire imaging optical system.SOLUTION: An imaging optical system 10 is a single-focus imaging optical system which forms an image of a subject image on a photoelectric conversion part of an imaging element 51, and includes at least two transmissive reflection surfaces R for controlling transmission and reflection of light by controlling polarization of light. The imaging optical system 10 satisfies the following conditional expression. 0.50<D12/f<0.85 ... (1) where D12 is a distance between the two transmissive reflection surfaces R on an optical axis Ax, and f is a focal length of the entire imaging optical system 10.SELECTED DRAWING: Figure 1

Description

The present invention relates to an imaging optical system.

In recent years, there has been a constant demand for miniaturization of imaging optical systems mounted on portable terminals such as smartphones and notebook PCs.
In addition, with the spread of remote work, there is an increasing demand for higher pixel count cameras installed in notebook PCs. Increasing the pixel count of a camera requires a larger sensor size, but in notebook PCs, the camera is often placed in the bezel of the display, which can be a factor in determining the thickness of the PC body, so the overall optical length needs to be shorter.

However, once the sensor size or the focal length of the optical system is determined, there is a limit to shortening the total optical length, and it is difficult to realize an optical system with TTL/Y (TTL: total optical length, Y: diagonal image height) or TTL/f (f: focal length) generally less than 1.0. Even in optical systems for smartphones, which are in high demand for miniaturization, the TTL/Y is about 1.1 even in the most compact design solution (see, for example, Patent Document 1).
Furthermore, in order to shorten the overall optical length, it is necessary to shorten the focal length as well, which poses the problem of the camera's angle of view becoming wider. It is not necessarily the case that the wider the angle of view of a camera is, but rather that an appropriate angle of view is determined according to the shooting application, and there is a strong demand to shorten the overall optical length while maintaining a certain angle of view.

In response to this demand for shortening the optical system, for example, the technology described in Patent Document 2 creates a folded optical path by controlling the reflection and transmission of light rays. This ensures a sufficient optical path length with a limited number of lenses, and keeps the overall optical length small.

US Patent Application Publication No. 2021/0223515 JP 2005-352273 A

However, in the imaging optical system described in Patent Document 2, it is difficult to say that the size of the optical system has been suitably reduced, for example, because aberration correction is insufficient.
The present invention has been made in consideration of the above problems, and has an object to suitably reduce the size of the entire imaging optical system.

In order to achieve the above object, the present invention provides a single-focus imaging optical system that forms a subject image on a photoelectric conversion portion of an image sensor, comprising:
The optical element has at least two transmissive/reflective surfaces that control the transmission and reflection of light by controlling the polarization of the light,
It is characterized in that the following conditional expression is satisfied.
0.50<D12/f<0.85...(1)
however,
D12: Distance on the optical axis between the two transmissive/reflective surfaces f: Focal length of the entire imaging optical system

The present invention makes it possible to effectively miniaturize the entire imaging optical system.

1 is a schematic cross-sectional view of an imaging device according to an embodiment. 2 is a block diagram showing a schematic control configuration of the imaging apparatus according to the embodiment; FIG. 1A to 1C are diagrams for explaining the principle of polarization control in an imaging device according to an embodiment. FIG. 2A is a diagram showing an optical path of the imaging optical system according to the first embodiment, and FIG. 11A is a diagram showing an optical path of an imaging optical system according to a second embodiment, and FIG. 13A is a diagram showing an optical path of an imaging optical system according to a third embodiment, and FIG. 13A is a diagram showing an optical path of an imaging optical system according to a fourth embodiment, and FIG. 13A is a diagram showing an optical path of an imaging optical system according to a fifth embodiment, and FIG. 13A is a diagram showing an optical path of an imaging optical system according to a sixth embodiment, and FIG.

The following describes an embodiment of the present invention with reference to the drawings.

[Overall configuration of the imaging device]
FIG. 1 is a schematic cross-sectional view of an imaging device 100 according to the present embodiment.
As shown in this figure, the imaging device 100 includes a camera module 30 for forming an image signal. The camera module 30 includes an imaging optical system 10 and a sensor unit 50.

The imaging optical system 10 is a single-focus optical system for forming a subject image on an imaging surface (projection surface) I of the imaging element 51, and is housed within the lens barrel 41. The imaging optical system 10 includes a plurality of lenses (first lens L1 to fourth lens L4).
The configuration of the imaging optical system 10 will be described in detail later.

A lens barrel 41 that houses the imaging optical system 10 has an opening OP through which light from the object side is incident.
The lens barrel 41 is also provided with a drive mechanism 42 (see FIG. 2) that moves at least some of the lenses or lens groups (e.g., the second lens L2 to the fourth lens L4) among the first lens L1 to the fourth lens L4 along the optical axis Ax. The drive mechanism 42 enables the focusing operation of the imaging optical system 10 by moving the some of the lenses or lens groups on the optical axis Ax. The drive mechanism 42 includes, for example, a voice coil motor and a guide. Note that the drive mechanism 42 may be configured with a stepping motor or the like instead of a voice coil motor or the like.

The sensor unit 50 includes an image sensor (solid-state image sensor) 51 that photoelectrically converts the subject image formed by the imaging optical system 10 .
The imaging element 51 is, for example, a CMOS type image sensor. The imaging element 51 is fixed in a state where it is positioned with respect to the optical axis Ax. This imaging element 51 has a photoelectric conversion section as an imaging surface I, and a signal processing circuit (not shown) is formed around the periphery of the photoelectric conversion section. Pixels, that is, photoelectric conversion elements, are two-dimensionally arranged in the photoelectric conversion section. Note that the imaging element 51 is not limited to the above-mentioned CMOS type image sensor, and may be one incorporating other imaging elements such as a CCD.

FIG. 2 is a block diagram showing a schematic control configuration of the imaging apparatus 100. As shown in FIG.
As shown in this figure, the imaging device 100 includes a processing unit 60 that operates the camera module 30 .
The processing unit 60 includes a lens driving unit 61 , an element driving unit 62 , an input unit 63 , a storage unit 64 , an image processing unit 65 , a display unit 66 , and a control unit 67 .

The lens driving unit 61 operates the driving mechanism 42 to move some of the lenses or lens groups (e.g., the second lens L2 to the fourth lens L4) among the first lens L1 to the fourth lens L4 along the optical axis Ax, thereby performing operations such as focusing of the imaging optical system 10.
The element driving section 62 receives a voltage and a clock signal for driving the image sensor 51 from the control section 67 and outputs them to a circuit associated with the image sensor 51 , thereby operating the image sensor 51 .
The input unit 63 is a section that accepts operations by the user or commands from an external device.
The storage unit 64 is a section that stores information necessary for the operation of the imaging device 100, image data acquired by the camera module 30, lens correction data used in image processing, and the like.
The image processing unit 65 performs image processing on the image signal output from the imaging element 51. The image processing unit 65 processes the frame images that compose the image signal, assuming that the image signal corresponds to, for example, a moving image. In addition to normal image processing such as color correction, gradation correction, and zooming, the image processing unit 65 performs distortion correction processing on the image signal based on lens correction data read out from the storage unit 64.
The display unit 66 is a section that displays information to be presented to the user, captured images, etc. The display unit 66 can also function as the input unit 63.
The control unit 67 comprehensively controls the operations of the lens driving unit 61, the element driving unit 62, the input unit 63, the storage unit 64, the image processing unit 65, the display unit 66, etc., and performs various image processing on image data obtained by the camera module 30, for example.

[Specific configuration of the imaging optical system]
Next, the imaging optical system 10 will be described in more detail.
As shown in FIG. 1, in this embodiment, the imaging optical system 10 is substantially composed of, in order from the object side, a first parallel plate P, an aperture stop S, a first lens L1, a second lens L2, a third lens L3, a fourth lens L4, and a second parallel plate F.

The first parallel plate P has a polarizing function and is an optical element for aligning the polarization state of light to linearly polarized light in a predetermined direction. The first parallel plate P in this embodiment is, for example, a 0.5 mm-thick parallel plate that combines a 0.2 mm-thick parallel plate assumed to be a linear polarizer and a 0.3 mm-thick parallel plate assumed to be a λ/4 plate.
The second parallel plate F is a parallel plate assumed to be an optical low-pass filter, an IR cut filter, a seal glass for the image pickup element 51, or the like.

The imaging optical system 10 has two transmission-reflection surfaces R that control the transmission and reflection of light by controlling the polarization of light. In this embodiment, a polarizing element that controls the polarization of light is added to two lens surfaces (surfaces having power) among the first lens L1 to the fourth lens L4, and functions as the two transmission-reflection surfaces R (first transmission-reflection surface R1, second transmission-reflection surface R2).
Specifically, in this embodiment, the object side surface of the second lens L2 is the first transmissive reflecting surface R1, which is coated with a half-mirror coating. The half mirror is an optical element that separates incident light into transmitted light and reflected light at a predetermined light amount ratio (e.g., 50:50). The image side surface of the fourth lens L4 is the second transmissive reflecting surface R2, which is provided with the function of a polarizing beam splitter. The polarizing beam splitter is a linear polarization selection optical element that transmits a specific linearly polarized light and reflects linearly polarized light whose vibration plane is perpendicular to the linearly polarized light.
In addition, the object side surface of the fourth lens L4 is provided with the function of a λ/4 plate. The λ/4 plate is an optical element that can convert linearly polarized light into circularly polarized light (or circularly polarized light into linearly polarized light) by imparting a phase difference of λ/4 (90°).

The principle of polarization control in the imaging optical system 10 will be described. Fig. 3 is a diagram for explaining the principle of polarization control in the imaging optical system 10. However, in Fig. 3, for ease of understanding, the polarizing element added to the lens surface in this embodiment is illustrated as an independent optical element.
In order to shorten the overall length of the imaging optical system 10, it is necessary to ensure that the optical path length is as long as possible within a limited space. Therefore, in this embodiment, an optical system with a folded optical path is created by controlling the transmission and reflection of light rays according to the polarization state of the light, thereby achieving a significant reduction in the overall length.

Specifically, in the imaging optical system 10, as shown in FIG. 3, the randomly polarized light from the outside world is converted to linearly polarized light (e.g., vertically polarized light) by the polarizing plate arranged closest to the object side, and then converted to circularly polarized light (e.g., circularly polarized in the right direction) by the first λ/4 plate. The light converted to circularly polarized light passes through a half mirror and then through a lens optical system, and is converted to linearly polarized light (e.g., vertically polarized light) in the same direction as when it entered the first λ/4 plate by the second λ/4 plate arranged behind it (on the image side). The light converted to linearly polarized light is reflected by the polarizing beam splitter toward the object side, re-enters the second λ/4 plate, and is converted to circularly polarized light again (e.g., circularly polarized in the right direction). After passing through the lens optical system, this light is reflected by the half mirror to be circularly polarized in the opposite direction (e.g., left direction), and heads toward the image side again. The light reflected by the half mirror passes through the lens optical system and then passes through the second λ/4 plate, where it is converted into linearly polarized light (e.g., horizontally polarized light) in a direction perpendicular to the direction of the first pass. Therefore, this light passes through the polarizing beam splitter without being reflected, and is imaged on the imaging element.
In the configuration of Figure 3 above and the imaging optical system 10 of this embodiment, the polarizing plate and the first λ/4 plate correspond to the first parallel plate P, the half mirror corresponds to the object side surface (first transmitting and reflecting surface R1) of the second lens L2, the lens optical system corresponds to the first lens L1 to the fourth lens L4, the second λ/4 plate corresponds to the object side surface of the fourth lens L4, and the polarizing beam splitter corresponds to the image side surface (second transmitting and reflecting surface R2) of the fourth lens L4.

In this way, in the imaging optical system 10, the light beam passes through the optical surface (lens surface) of the lens optical system multiple times by repeatedly transmitting and reflecting the light beam through the half mirror and the polarizing beam splitter (i.e., the two transmission-reflection surfaces R). For example, if the lens optical system is configured with two lenses, the light beam passes through the optical surface a total of 12 times, making it possible to provide an aberration correction effect equivalent to that of a normal optical system configured with six lenses. Therefore, it is possible to obtain optical performance equivalent to three times the number of lenses while keeping the total optical length short.
Since the same lens surface is passed over multiple times, strictly speaking there is not as much freedom in terms of the surface as with a six-element structure; however, by using aspherical lenses, for example, the freedom in shape can be increased, making it possible to approach the performance of a six-element structure.

Moreover, the imaging optical system 10 satisfies the following conditional expression (1).
0.50<D12/f<0.85...(1)
Here, D12 is the distance on the optical axis Ax between the two transmissive and reflective surfaces R, and f is the focal length of the entire imaging optical system 10.

Conditional expression (1) is a conditional expression for appropriately setting the distance between the two transmitting and reflecting surfaces R. As described above, while ensuring an optical path length that is folded as much as possible is effective in terms of aberration correction, making the optical path length too long is disadvantageous in terms of the total optical length.
When D12/f is greater than the lower limit of conditional expression (1), a suitable folded optical path length can be ensured, and good aberration correction can be performed. On the other hand, when D12/f is less than the upper limit of conditional expression (1), the total optical length can be prevented from becoming too large.

Moreover, it is preferable that the imaging optical system 10 satisfies the following conditional expression (2) in addition to the above conditional expression (1).
0≦DAS/f<0.15...(2)
Here, DAS is the distance on the optical axis Ax between the aperture stop S and the first lens L1, and f is the focal length of the entire imaging optical system 10.

Conditional expression (2) is a conditional expression for appropriately setting the distance on the optical axis Ax between the aperture stop S and the first lens L1 closest to the object side, thereby achieving both a small lens diameter and aberration correction.
When DAS/f exceeds the lower limit of conditional expression (2), the distance between the aperture stop S and the first lens L1 can be appropriately increased, and the positions of the light rays passing through the first lens L1 can be divided for each angle of view, allowing for good aberration correction. On the other hand, when DAS/f is below the upper limit of conditional expression (2), the distance between the aperture stop S and the first lens L1 is not too large, and the diameter of the first lens L1 can be kept small.

Furthermore, it is preferable that the imaging optical system 10 satisfies the following conditional expression (3).
1.90<f3/f<4.00...(3)
Here, f3 is the focal length of the third lens L3, and f is the focal length of the entire imaging optical system 10.

Conditional formula (3) is a conditional formula for appropriately setting the focal length of the positive third lens L3. The third lens L3 is disposed between two transmissive and reflective surfaces R, and is therefore a lens through which light rays pass a total of three times. Therefore, in order to achieve both a compact imaging optical system 10 and aberration correction, it is necessary to appropriately set the focal length.
When f3/f is greater than the lower limit of conditional expression (3), the refractive power of the third lens L3 does not become too strong, and the aberration generated in the third lens L3 can be suppressed to a small value. On the other hand, when f3/f is less than the upper limit of conditional expression (3), the positive refractive power of the third lens L3 can be appropriately maintained, and the imaging optical system 10 can be made compact.

[Technical effect of the present embodiment]
As described above, according to this embodiment, by having two transmission-reflection surfaces R that control the transmission and reflection of light by controlling the polarization of the light, it is possible to make a light beam pass through the lens surface multiple times, thereby making it possible to keep the total optical length short while achieving the desired optical performance.
Furthermore, by making the imaging optical system 10 satisfy conditional expression (1), it is possible to appropriately set the distance between the two transmissive-reflective surfaces R. In other words, by making D12/f exceed the lower limit of conditional expression (1), it is possible to ensure an appropriately folded optical path length and perform favorable aberration correction. Furthermore, by making D12/f fall below the upper limit of conditional expression (1), it is possible to prevent the total optical length from becoming too large.
As a result, the entire imaging optical system 10 can be suitably miniaturized.
Although not particularly limited, in this specification, an index of "compactness" of the imaging optical system 10 is that TTL/Y (TTL: total optical length, Y: diagonal image height) is less than 1.0.

In addition, according to this embodiment, an optical element (first parallel plate P) for aligning the polarization state of light to only linearly polarized light in a specific direction is arranged closest to the object. This allows for efficient polarization control within the imaging optical system 10.

In addition, according to this embodiment, the imaging optical system 10 satisfies conditional expression (2), so that the distance on the optical axis Ax between the aperture stop S and the first lens L1 closest to the object side can be appropriately set, and it is possible to appropriately achieve both a small lens diameter and aberration correction. In other words, by making DAS/f exceed the lower limit of conditional expression (2), the distance between the aperture stop S and the first lens L1 can be appropriately set, and the position of the light passing through the first lens L1 can be divided for each angle of view, and good aberration correction can be performed. Also, by making DAS/f lower than the upper limit of conditional expression (2), the distance between the aperture stop S and the first lens L1 is not too large, and the diameter of the first lens L1 can be kept small.

In addition, according to this embodiment, a polarizing element that controls the polarization of light (in this embodiment, a second λ/4 plate and a polarizing beam splitter) is added to the lens surface. This makes it possible to shorten the total optical length compared to when these polarizing elements are provided separately.

Furthermore, according to this embodiment, some of the lenses or lens groups are moved when focusing. This reduces the load on the actuator and allows the amount of movement to be kept smaller than when the entire lens group is extended.

In addition, according to this embodiment, the imaging optical system 10 satisfies conditional expression (3), so that the focal length of the positive third lens L3 can be appropriately set. That is, by making f3/f exceed the lower limit of conditional expression (3), the refractive power of the third lens L3 does not become too strong, and the aberration generated by the third lens L3 can be kept small. Also, by making f3/f below the upper limit of conditional expression (3), the positive refractive power of the third lens L3 can be maintained at an appropriate level, and the imaging optical system 10 can be made more compact.

Although one embodiment of the present invention has been described above, embodiments to which the present invention can be applied are not limited to the above-described embodiment and its variations, and can be modified as appropriate without departing from the spirit of the present invention.

For example, in the above embodiment, among the polarizing elements (linear polarizing plate, two λ/4 plates, and polarizing beam splitter) that control the polarization of light, the second λ/4 plate and the polarizing beam splitter are added to the lens surface. However, although it is preferable that at least one of these polarizing elements is added to the lens surface, they may be disposed as independent optical elements without being added to the lens surface.
In addition, in the above embodiment, a half mirror and a polarizing beam splitter have been described as examples of optical elements corresponding to the transmissive-reflective surface R, but the transmissive-reflective surface R may be any surface that controls the transmission and reflection of light by controlling the polarization of light, and may be, for example, a cholesteric liquid crystal.
Furthermore, the specific configuration of the imaging optical system 10 is not limited to that in the above embodiment, but may be any configuration having at least two transmissive and reflective surfaces R (i.e., having a folded optical path).

ただし、
Ａｉ：ｉ次の非球面係数
Ｒ ：曲率半径
Ｋ ：円錐定数 Examples of the imaging optical system of the present invention will be described below. The symbols used in each example are as follows.
f: focal length of the entire imaging optical system fB: back focus Fno: F-number 2Y: diagonal length of the imaging surface of the solid-state imaging element R: radius of curvature D: axial surface spacing Nd: refractive index of the lens material for the d-line vd: Abbe number of the lens material In each example, a surface having an "*" after the surface number in the lens surface data is a surface having an aspheric shape, and the shape of the aspheric surface is expressed by the following "Equation 1" with the vertex of the surface as the origin, the X-axis in the direction of the optical axis, and h as the height perpendicular to the optical axis.

however,
Ai: ith aspheric coefficient R: radius of curvature K: conic constant

Example 1
4A and 4B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of the first embodiment.
The imaging optical system 10 of Example 1 corresponds to the imaging optical system 10 of the above embodiment. Unless otherwise specified, the imaging optical system 10 of Examples 2 and onwards has the same optical configuration as that of the above embodiment.

The overall specifications of the imaging optical system of the first embodiment are shown below.
f = 8.78 mm
Fno = 1.90
2Y = 15.86 mm

The lens surface data for Example 1 is shown in Table I below.

The aspheric coefficients of the lens surfaces of Example 1 are shown in Table II below. Note that hereinafter (including the lens data in the table), powers of 10 (e.g., 2.5×10-02) will be represented by E (e.g., 2.5E-02).

The values of the conditional expressions (1) to (3) in the imaging optical system of the first embodiment are shown below.
Conditional expression (1): D12/f=0.67
Conditional expression (2): DAS/f=0.01
Conditional expression (3): f3/f=2.54

Example 2
5A and 5B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of the second embodiment.

The overall specifications of the imaging optical system of the second embodiment are shown below.
f = 8.60 mm
Fno = 1.00
2Y = 15.86 mm

The lens surface data for Example 2 is shown below in Table III.

The aspheric coefficients of the lens surfaces of Example 2 are shown in Table IV below.

The values of the conditional expressions (1) to (3) in the imaging optical system of the second embodiment are as follows.
Conditional expression (1): D12/f=0.67
Conditional expression (2): DAS/f=0.00
Conditional expression (3): f3/f=2.60

Example 3
6A and 6B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of the third embodiment.

The overall specifications of the imaging optical system of the third embodiment are shown below.
f = 7.43 mm
Fno = 1.90
2Y = 15.86 mm

The lens surface data for Example 3 is shown in Table V below.

The aspheric coefficients of the lens surfaces of Example 3 are shown in Table VI below.

The values of the conditional expressions (1) to (3) in the imaging optical system of the third embodiment are shown below.
Conditional expression (1): D12/f=0.62
Conditional expression (2): DAS/f=0.07
Conditional expression (3): f3/f=2.23

Example 4
7A and 7B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of the fourth embodiment.
In the imaging optical system 10 of the fourth embodiment, the second transmissive reflective surface R2 is the object side surface of the second parallel plate F, not the image side surface of the fourth lens L4.

The overall specifications of the imaging optical system of the fourth embodiment are shown below.
f = 8.89 mm
Fno = 1.90
2Y = 15.86 mm

The lens surface data for Example 4 is shown below in Table VII.

The aspheric coefficients of the lens surfaces of Example 4 are shown in Table VIII below.

The values of the conditional expressions (1) to (3) in the imaging optical system of the fourth embodiment are as follows.
Conditional expression (1): D12/f=0.66
Conditional expression (2): DAS/f=0.02
Conditional expression (3): f3/f=2.61

Example 5
8A and 8B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of the fifth embodiment.
The imaging optical system 10 of Example 5 includes three lenses, a first lens L1 to a third lens L3, and the second transmissive reflective surface R2 is the image side surface of the third lens L3.

The overall specifications of the imaging optical system of the fifth embodiment are shown below.
f = 5.88 mm
Fno = 1.90
2Y = 15.86 mm

The lens surface data for Example 5 is shown below in Table IX.

The aspheric coefficients of the lens surfaces of Example 5 are shown in Table X below.

The values of the conditional expressions (1) to (3) in the imaging optical system of the fifth embodiment are as follows.
Conditional expression (1): D12/f=0.53
Conditional expression (2): DAS/f=0.09
Conditional expression (3): f3/f=3.98

Example 6
9A and 9B show an optical path diagram and longitudinal aberration diagrams (spherical aberration, astigmatism, and distortion) of the imaging optical system of Example 6. FIG.

The overall specifications of the imaging optical system of the sixth embodiment are shown below.
f=9.50mm
Fno = 1.90
2Y = 15.86 mm

The lens surface data for Example 6 is shown in Table XI below.

The aspheric coefficients of the lens surfaces of Example 6 are shown in Table XII below.

The values of the conditional expressions (1) to (3) in the imaging optical system of Example 6 are as follows.
Conditional expression (1): D12/f=0.79
Conditional expression (2): DAS/f=0.01
Conditional expression (3): f3/f=2.43

10 Imaging optical system 51 Imaging element 100 Imaging device Ax Optical axis I Imaging surface L1 First lens L2 Second lens L3 Third lens L4 Fourth lens R Transmissive reflecting surface R1 First transmissive reflecting surface R2 Second transmissive reflecting surface P First parallel plate F Second parallel plate S Aperture stop

Claims (6)

A single-focus imaging optical system that forms a subject image on a photoelectric conversion unit of an imaging element,
The optical element has at least two transmissive/reflective surfaces that control the transmission and reflection of light by controlling the polarization of the light,
An imaging optical system characterized by satisfying the following conditional expression:
0.50<D12/f<0.85...(1)
however,
D12: Distance on the optical axis between the two transmissive/reflective surfaces f: Focal length of the entire imaging optical system The imaging optical system according to claim 1, characterized in that it is provided with an optical element arranged closest to the object and for aligning the polarization state of light to only linearly polarized light in a predetermined direction. an aperture stop; and a first lens disposed closer to an image side than the aperture stop;
3. The imaging optical system according to claim 1, wherein the following condition is satisfied:
0≦DAS/f<0.15...(2)
however,
DAS: Distance on the optical axis between the aperture stop and the first lens f: Focal length of the entire imaging optical system The imaging optical system according to any one of claims 1 to 3, characterized in that it comprises a lens having a lens surface to which a polarizing element that controls the polarization of light is added. The imaging optical system according to any one of claims 1 to 4, characterized in that some lenses or lens groups are moved when focusing. The optical system includes a first lens, a second lens, and a third lens, which are arranged in this order from the object side,
6. The imaging optical system according to claim 1, wherein the following condition is satisfied:
1.90<f3/f<4.00...(3)
however,
f3: focal length of the third lens f: focal length of the entire imaging optical system

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