JP2008164989A - Imaging optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact imaging optical system which attains a closeup shot and can photograph a wide area and in which incident light rays on an object side is almost telecentric. <P>SOLUTION: The imaging optical system includes an imaging element and an imaging optical system 1 composed of at least one positive lens. At least one fresnel optical element 2 is disposed closer to the object than to the imaging optical system 1. At least one fresnesl optical element 2 has positive refractive power and satisfies conditional inequality (1): 0.05<L<SB>P</SB>/ϕ<SB>P</SB><1, wherein L<SB>P</SB>is a distance from the fresnel optical element 2 to the imaging optical system 1 and ϕ<SB>P</SB>is the effective diameter of the fresnel optical element 2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an imaging optical system, and more particularly to a small wide-angle optical system that captures images at a relatively close distance.

  2. Description of the Related Art In recent years, devices that obtain desired information by photographing pattern images such as barcodes and QR codes using an imaging optical system and processing the acquired images have become widespread. There are also devices that authenticate individuals by acquiring not only pattern images but also images such as fingerprints, veins, and eye irises used in biometric authentication. These devices are often used by being mounted on, for example, a mobile phone, a mobile terminal, or a notebook computer. These apparatuses have an imaging optical system. A demand for an imaging optical system is that it is small and thin. Another demand is that a wide angle of view is taken because a wide area is photographed while being close to the object. Furthermore, from the viewpoint of ease of image processing after shooting, aberrations generated in the optical system, particularly distortion, must be corrected well. Furthermore, when the distance to the object at the time of shooting is not constant, the distance may change slightly due to, for example, the hand shake. Even in such a case, it is desired that the incident light beam on the object side has a substantially telecentric incidence in order to suppress a change in photographing magnification.

As a wide-angle compact optical system that captures images in a relatively close range, those described in Patent Document 1 and Patent Document 2 are known. These are wide-angle and comparatively compact optical systems, but they are not able to secure substantially telecentricity on the object side. Moreover, since the distortion is extremely large, it is not desirable.
JP-A-2-90118 Japanese Patent No. 3450543

  The present invention has been made in view of these problems of the prior art, and an object thereof is a small imaging optical system in which incident light on the object side is substantially telecentric, and can perform close-up photography and wide-area photography. An imaging optical system capable of maintaining good imaging performance while being possible is provided.

  An imaging optical system of the present invention that achieves the above object includes an imaging optical system and an imaging element composed of at least one positive lens, and at least one Fresnel optical element is closer to the object side than the imaging optical system. It is arranged.

  The at least one Fresnel optical element has a positive refractive power.

  Further, the following condition (1) is satisfied.

(1) 0.05 <L _P / φ _P <1
Where L _P is the distance from the Fresnel optical element to the imaging optical system, and φ _P is the effective diameter of the Fresnel optical element.

  According to the present invention described above, a compact imaging optical system in which the incident light beam on the object side is substantially telecentric, and the imaging optical system maintaining good imaging performance while being capable of close-up photography and wide-area photography. Obtainable.

  The imaging optical system of this embodiment will be described. The imaging optical system of the present embodiment includes an imaging optical system including at least one positive lens and an imaging element, and at least one Fresnel optical element is disposed closer to the object side than the imaging optical system. It is characterized by.

  In general, a wide angle of view can be achieved by shortening the focal length of the optical system. However, in order to make the object-side incident light beam substantially telecentric in a wide field angle state, it is necessary to arrange an optical element on the object side to control the incident light beam angle. Furthermore, in order to achieve a reduction in thickness, it is desirable that the optical element disposed on the object side is not a normal lens but a Fresnel optical element. A Fresnel optical element (Fresnel lens) is a lens element in which the entire thickness is reduced, leaving only a portion that contributes to the refraction of the lens. Therefore, by arranging the Fresnel optical element on the object side, it is possible to make the object-side incident light beam substantially telecentric while achieving miniaturization.

  In the imaging optical system of the present embodiment, it is desirable that the Fresnel optical element has a positive refractive power. Since the Fresnel optical element has a positive refractive power, it is possible to make incident light on the object side substantially telecentric while maintaining good imaging performance. If the Fresnel optical element has a negative refractive power, the angle of the incident light beam becomes tight. In addition, it is necessary to increase the refractive power of the imaging optical system. Therefore, it becomes very difficult to correct various aberrations satisfactorily. For this reason, it is not desirable for the Fresnel optical element to have negative refractive power.

  In the imaging optical system of the present embodiment, it is preferable that the following condition (1) is satisfied.

(1) 0.05 <L _P / φ _P <1
Where L _P is the distance from the Fresnel optical element to the imaging optical system, and φ _P is the effective diameter of the Fresnel optical element.

  When the above conditions are satisfied, a small imaging optical system can be realized while maintaining good imaging performance while photographing a wide area. If the lower limit of 0.05 of the condition (1) is exceeded, the imaging optical system requires an extremely wide field angle optical system, so that it is particularly difficult to correct off-axis aberrations such as astigmatism. In addition, since it is necessary to increase the refractive power of the Fresnel optical element, it is difficult to correct off-axis aberrations. If the upper limit of 1 is exceeded, the imaging optical system becomes large, and in order to reduce the size forcibly, the imaging performance is deteriorated.

  In the imaging optical system of the present embodiment, it is preferable that the following condition (2) is satisfied.

(2) 0.01 <f / f _P <0.1
Here, f is the focal length of the imaging optical system, and f _P is the focal length of the Fresnel optical element.

  If the above conditions are satisfied, it is possible to realize a small imaging optical system with good imaging performance by correcting various aberrations while taking into account the manufacturability of the Fresnel optical element. If the lower limit of 0.01 of the condition (2) is exceeded, the focal length of the imaging optical system becomes short (refractive power is strong). In this case, it is particularly difficult to correct off-axis aberrations. In addition, the optical system becomes large in order to satisfactorily correct various aberrations. If the upper limit of 1 is exceeded, the focal length of the Fresnel optical element will be shortened with respect to the imaging optical system, and correction of astigmatism becomes particularly difficult. In addition, since the Fresnel optical element needs to be formed into a groove with a tight angle, manufacturing is also difficult.

  Further, in the present embodiment, it is desirable that one Fresnel optical element is used and both surfaces of the Fresnel optical element have a Fresnel shape. In this way, various aberrations generated in the Fresnel optical element can be corrected satisfactorily.

FIG. 1 is a schematic diagram in which both sides of a Fresnel optical element are Fresnel shaped in the present embodiment. In the figure, α _j is the edge angle of the image-side annular refracting surface at a distance j from the optical axis OO ′, and β _k is the object-side annular refracting at a distance k from the optical axis OO ′. The edge angle of the face. Here, it is possible to make only one surface a Fresnel shape. However, if one surface has a refractive power, the amount of off-axis aberration generated particularly in the Fresnel optical element increases. Further, in the case of one side, the edge angle becomes large and processing becomes difficult. Therefore, it is desirable that both sides have a Fresnel shape. By doing so, it is possible to obtain a Fresnel optical element that is excellent in processability and maintains good imaging performance.

  Moreover, in this embodiment, when both surfaces of a Fresnel optical element are Fresnel shape, it is desirable to satisfy the following conditions (3).

(3) α _j <β _k
Where α _j is the edge angle of the image side surface at a distance j from the center to the periphery of the Fresnel optical element, and β _k is the edge of the object side surface at a distance k from the center to the periphery of the Fresnel optical element. Is an angle. Here, j and k are distances of 50% or more of the radius and satisfy 0.9 <k / j <1.1.

  In this embodiment, since the image forming optical system has a wide angle of view, if the edge angle on the image side of the Fresnel optical element is large, the efficiency is reduced due to reflection of this surface. In addition, the amount of off-axis aberrations increases, which causes deterioration in imaging performance. Therefore, in the present embodiment, it is desirable that the condition (3) is satisfied particularly at a distance of 50% or more of the radius where the angle of view of the imaging optical system is large.

  In the optical system of the present embodiment, it is desirable that the condition (4) is satisfied.

(4) 0 <D _P / f <5
Where D _P is the distance from the Fresnel optical element to the object, and f is the focal length of the imaging optical system.

  When the above conditions are satisfied, downsizing can be achieved in view of close-up photography. If the lower limit of 0 of the condition (4) is exceeded, the Fresnel optical element will not function. If the upper limit of 5 is exceeded, the apparatus becomes larger because the object distance becomes longer than the focal length of the imaging optical system.

In particular, when it is mounted on a mobile phone, a personal computer, etc. and priority is given to downsizing, the distance from the Fresnel optical element to the object, D _P, is preferably 0 mm to 5 mm.

  Further, in this embodiment, the imaging optical system is configured in a so-called retrofocus type in order from the object side, with a negative group having at least one negative lens and a positive group having at least one positive lens. Is desirable. With such a configuration, a wide range of photographing is possible.

  In the present embodiment, it is desirable that an aperture stop be disposed between the negative group and the positive group. With such a configuration, it is possible to reduce the size of the optical system in the radial direction. By adopting such a configuration, it is possible to reduce the size of the imaging optical system.

  In this embodiment, at least one aspheric lens is used for the negative group of the imaging optical system, and at least one surface of the aspheric lens has a shape that weakens the negative refractive power from the center to the periphery of the optical axis. Is desirable. With such a configuration, it is possible to correct distortion particularly well.

  In this embodiment, at least one aspheric lens is used for the positive group of the imaging optical system, and at least one surface of the aspheric lens has a shape that weakens the positive refractive power from the center to the periphery of the optical axis. Is desirable. With such a configuration, it is possible to correct distortion particularly well.

  In the optical system of the present embodiment, it is desirable that at least one surface of the lens immediately after the aperture stop has an aspheric shape. With such a configuration, it is possible to particularly correct spherical aberration.

  In the present embodiment, it is desirable to satisfy the following condition (1 ′) instead of the condition (1).

(1 ′) 0.1 <L _P / φ _P <0.5
By satisfying the above conditions, it is possible to realize a small imaging optical system while maintaining good imaging performance while photographing a wide area.

  In the present embodiment, it is desirable to satisfy the following condition (2 ′) instead of the condition (2).

(2 ′) 0.02 <f / f _P <0.08
By satisfying the above conditions, it is possible to realize a small imaging optical system with good imaging performance by correcting various aberrations while taking into account the manufacturability of the Fresnel optical element.
In the optical system of the present embodiment, it is desirable that the condition (4 ′) is satisfied instead of the condition (4).

(4 ′) 0 <D _P / f <3
By satisfying the above conditions, downsizing can be achieved in view of close-up photography.

  Examples 1 to 4 of the imaging optical system of the present invention will be described below with reference to the drawings.

Example 1
Example 1 is a structure which shows a cross section in Fig.2 (a).

  In the figure, 1 is an imaging optical system, 2 is a Fresnel optical element, and 3 is an object plane. An image sensor such as a CCD is disposed on the object plane 3.

  The Fresnel optical element 2 is disposed closer to the object side than the imaging optical system 1 and has a positive refractive power. Moreover, in the Fresnel optical element 2 of the present embodiment, both surfaces are Fresnel surfaces.

  The imaging optical system 1 has a configuration shown in a cross section in FIG. 2B. In order from the object side, a negative group composed of two negative lenses and a positive lens, an aperture stop, a positive lens, and a positive lens. It is composed of a positive group composed of two lenses. In addition, aspherical surfaces are used on both surfaces of the negative first lens, and in particular, the aspherical shape on the image side is configured such that the negative refractive power decreases from the optical axis to the periphery. In addition, aspheric surfaces on both sides of the positive third lens are used, and in particular, the aspheric shape on the object side has a shape in which the negative refractive power increases from the optical axis to the periphery, that is, the positive refractive power decreases. It is configured.

  All lenses having an aspheric surface in Example 1 are made of organic materials.

  In the first embodiment, in particular, since the negative group is composed of two lenses, a negative lens and a positive lens, the chromatic aberration is particularly well corrected, and an imaging optical system suitable for obtaining a color image is provided. It has become.

Numerical data of this embodiment will be described later. R ₁ , r ₂ ... Are the radius of curvature of each lens surface, d ₁ , d ₂ ... _Are the distances between the lens surfaces, n _d1 , n _d2 . The refractive index of the line, ν _d1 , ν _d2 ... Is the Abbe number of each lens. R ₀ is the radius of curvature of the object surface, and d ₀ is the distance between the object surface and the first surface of the Fresnel optical element. The aspherical shape is represented by the following expression, where x is an optical axis with the light traveling direction being positive, and y is a direction orthogonal to the optical axis.

x = (y ² / r) / [1+ {1- (K + 1) (y / r) ² } ^1/2 ]
+ A ₄ y ⁴ + A ₆ y ⁶ + A ₈ y ⁸
Here, r is a paraxial radius of curvature, K is a conical coefficient, and A ₄ , A ₆ , and A ₈ are fourth-order, sixth-order, and eighth-order aspheric coefficients, respectively.

  Less than. the same.

Example 2
Example 2 is a structure which shows a cross section in Fig.3 (a).

  In the figure, 1 is an imaging optical system, 2 is a Fresnel optical element, and 3 is an object plane. An image sensor such as a CCD is disposed on the object plane 3.

  The Fresnel optical element 2 is disposed closer to the object side than the imaging optical system 1 and has a positive refractive power. Moreover, in the Fresnel optical element 2 of the present embodiment, both surfaces are Fresnel surfaces.

  The imaging optical system 1 has a configuration shown in a cross section in FIG. 3B, and in order from the object side, a negative group composed of one negative lens, an aperture stop, a positive lens, and a positive lens. It consists of a positive group consisting of In addition, aspherical surfaces are used on both surfaces of the negative first lens, and in particular, the aspherical shape on the image side is configured such that the negative refractive power decreases from the optical axis to the periphery. In addition, aspherical surfaces are used on both surfaces of the positive third lens, and in particular, the aspherical shape on the image side is configured such that the positive refractive power decreases from the optical axis to the periphery.

All the lenses having an aspherical surface in Example 2 are made of organic materials.
Numerical data of this embodiment will be described later.

Example 3
Example 3 is a structure which shows a cross section in FIG.

In FIG. 4, the Fresnel optical element is omitted, but it is arranged on the most object side and satisfies each condition, as in the first and second embodiments. The imaging optical system includes, in order from the object side, a negative group composed of two lenses, a negative lens and a negative lens, and a positive group composed of an aperture stop, a positive lens, and a positive lens. ing. In this embodiment, aspherical surfaces are used on both surfaces of the negative first lens, and in particular, the aspherical shape on the image side is configured such that the negative refractive power decreases from the optical axis to the periphery. In addition, aspherical surfaces are used on both surfaces of the positive third lens, and in particular, the aspherical shape on the image side is configured such that the positive refractive power decreases from the optical axis to the periphery.
Numerical data of this embodiment will be described later.

Example 4
Example 4 is a structure which shows a cross section in FIG.

  In FIG. 5, the Fresnel optical element is omitted, but it is disposed closest to the object side and satisfies each condition, as in the first and second embodiments. The imaging optical system is composed of a total of three elements including, in order from the object side, a negative group composed of one negative lens, an aperture stop, a positive lens, and a cemented lens of a positive lens and a negative lens. It consists of a positive group. In this embodiment, aspherical surfaces are used on both surfaces of the negative first lens, and in particular, the aspherical shape on the image side is configured such that the negative refractive power decreases from the optical axis to the periphery. In addition, aspherical surfaces are used on both surfaces of the positive second lens, and in particular, the aspherical shape on the image side is configured such that the positive refractive power decreases from the optical axis to the periphery.

  The fourth embodiment is characterized in that the chromatic aberration is particularly favorably corrected by using a cemented lens, and is an imaging optical system suitable for obtaining a color image.

  Note that the edge angle α of the image-side annular refracting surface and the edge angle β of the object-side annular refracting surface of the Fresnel optical element 2 of Examples 1 to 4 are as shown in FIG.

Below, the numerical data of the said Examples 1-4 are shown.
Example 1
L _P = 13.539 φ _P = 49 f = 0.828 f _P = 25 D _P = 0.5
FNo. 2.6
r ₀ = ∞ (object surface) d ₀ = 0.5000
r ₁ = ∞ d ₁ = 1.0000 n _d1 = 1.51633 ν _d1 = 64.06
r ₂ = ∞ d ₂ = 13.5391
r ₃ = ∞ (aspherical surface) d ₃ = 0.5000 n _d2 = 1.52542 ν _d2 = 55.78
r ₄ = 1.1254 (aspherical surface) d ₄ = 1.4736
r ₅ = 11.1208 d ₅ = 0.9376 n _d3 = 1.92286 ν _d3 = 18.90
r ₆ = -9.0196 d ₆ = 1.1564
r ₇ = ∞ (aperture) d ₇ = 0.6053
r ₈ = ∞ (aspherical surface) d ₈ = 1.3180 n _d4 = 1.52542 ν _d4 = 55.78
r ₉ = -0.9895 (aspherical surface) d ₉ = 0.1000
r ₁₀ = 581.7635 d ₁₀ = 0.7866 n _d5 = 1.77250 ν _d5 = 49.60
r ₁₁ = -7.4892 d ₁₁ = 0.5000
r ₁₂ = ∞ d ₁₂ = 0.6000 n _d6 = 1.51633 ν _d6 = 64.06
r ₁₃ = ∞ d ₁₃ = 0.5083
r ₁₄ = ∞ (image plane)
Aspheric coefficient 3rd surface K = 0
A ₄ = -2.9028 × 10 ^-3
A ₆ = 2.6496 × 10 ^-4
A ₈ = 0.0000
The fourth side K = -2.3562
A ₄ = 1.0761 × 10 ^-1
A ₆ = -4.7455 × 10 ^-2
A ₈ = 8.8730 × 10 ^-3
8th surface K = 0
A ₄ = -1.2036 × 10 ^-1
A ₆ = 4.2284 × 10 ^-2
A ₈ = 0.0000
9th surface K = -0.9212
A ₄ = 1.6002 × 10 ^-3
A ₆ = -1.8590 × 10 ^-2
A ₈ = 0.0000
(1) L _P / φ _P = 0.28
(2) f / f _P = 0.033
(4) D _P /f=0.6.

Example 2
L _P = 7.136 φ _P = 43 f = 0.615 f _P = 10 D _P = 1
FNo. 2.1
r ₀ = ∞ (object surface) d ₀ = 1.0000
r ₁ = ∞ d ₁ = 1.0000 n _d1 = 1.56864 ν _d1 = 30.30
r ₂ = ∞ d ₂ = 7.1359
r ₃ = ∞ (aspherical surface) d ₃ = 1.3009 n _d2 = 1.52268 ν _d2 = 55.78
r ₄ = 1.1828 (aspherical surface) d ₄ = 4.1698
r ₅ = ∞ (aperture) d ₅ = 0.1872
r ₆ = -3.5252 (aspherical surface) d ₆ = 1.6658 n _d3 = 1.52268 ν _d3 = 55.78
r ₇ = -1.7327 (aspherical surface) d ₇ = 0.1500
r ₈ = 8.2450 (aspherical surface) d ₈ = 1.7714 n _d4 = 1.52268 ν _d4 = 55.78
r ₉ = -1.5458 (aspherical surface) d ₉ = 0.5000
r ₁₀ = ∞ d ₁₀ = 1.0000 n _d5 = 1.50864 ν _d5 = 64.06
r ₁₁ = ∞ d ₁₁ = 0.8465
r ₁₂ = ∞ (image plane)
Aspheric coefficient 3rd surface K = 0
A ₄ = 5.3699 × 10 ^-4
A ₆ = -8.2971 × 10 ^-6
A ₈ = 7.0979 × 10 ^-8
4th surface K = -1.0056
A ₄ = -1.4489 × 10 ^-2
A ₆ = -8.6417 × 10 ^-6
A ₈ = -1.0112 × 10 ^-5
6th page K = 2.3738
A ₄ = -8.9123 × 10 ^-2
A ₆ = -1.3915 × 10 ^-1
A ₈ = 0.0000
7th surface K = 0.5112
A ₄ = 6.5724 × 10 ^-3
A ₆ = 2.1670 × 10 ^-2
A ₈ = -6.5081 × 10 ^-3
8th surface K = 0
A ₄ = -2.5421 × 10 ^-2
A ₆ = 1.8108 × 10 ^-2
A ₈ = -3.2991 × 10 ^-3
Surface 9 K = -1.6150
A ₄ = -1.1575 × 10 ^-3
A ₆ = -4.3372 × 10 ^-4
A ₈ = 1.7498 × 10 ^-3
(1) L _P / φ _P = 0.17
(2) f / f _P = 0.062
(4) D _P /f=1.63.

Example 3
f = 0.516
FNo. 1.61
r ₀ = ∞ (object surface) d ₀ = 0.5000
r ₁ = ∞ d ₁ = 1.0000 n _d1 = 1.50864 ν _d1 = 64.06
r ₂ = ∞ d ₂ = 7.4323
r ₃ = 227.2766 (aspherical surface) d ₃ = 0.6000 n _d2 = 1.52268 ν _d2 = 55.78
r ₄ = 2.0382 (aspherical surface) d ₄ = 3.3829
r ₅ = -50 150 d ₅ = 0.8915 n _d3 = 1.50871 ν _d3 = 64.14
r ₆ = 3.2564 d ₆ = 1.9592
r ₇ = ∞ (aperture) d ₇ = 0.1000
r ₈ = 26.7432 (aspherical surface) d ₈ = 1.7854 n _d4 = 1.72963 ν _d4 = 49.34
r ₉ = -1.4130 (aspherical surface) d ₉ = 0.1000
r ₁₀ = -721.7818 d ₁₀ = 1.2977 n _d5 = 1.75849 ν _d5 = 49.60
r ₁₁ = -3.5792 d ₁₁ = 0.4000
r ₁₂ = ∞ d ₁₂ = 0.6000 n _d6 = 1.50864 ν _d6 = 64.06
r ₁₃ = ∞ d ₁₃ = 0.5093
r ₁₄ = ∞ (image plane)
Aspheric coefficient 3rd surface K = 0
A ₄ = 5.9945 × 10 ^-4
A ₆ = -2.7490 × 10 ^-6
A ₈ = 0.0000
4th surface K = -0.8411
A ₄ = -6.3458 × 10 ^-3
A ₆ = -1.5177 × 10 ^-4
A ₈ = 0.0000
8th surface K = 0
A ₄ = 1.4902 × 10 ^-17
A ₆ = 1.8855 × 10 ^-17
A ₈ = 0.0000
9th surface K = 0
A ₄ = 6.4225 × 10 ^-2
A ₆ = 2.1097 × 10 ^-2
A ₈ = 0.0000.

Example 4
f = 1.44
FNo. 2.61
r ₀ = ∞ (object surface) d ₀ = 0.5000
r ₁ = ∞ d ₁ = 1.0000 n _d1 = 1.51633 ν _d1 = 64.06
r ₂ = ∞ d ₂ = 9.9052
r ₃ = ∞ (aspherical surface) d ₃ = 0.7000 n _d2 = 1.52542 ν _d2 = 55.78
r ₄ = 1.7765 (aspherical surface) d ₄ = 5.3441
r ₅ = ∞ (aperture) d ₅ = 0.1557
r ₆ = -1.2415 (aspherical surface) d ₆ = 1.2891 n _d3 = 1.52542 ν _d3 = 55.78
r ₇ = -1.1203 (aspherical surface) d ₇ = 0.1000
r ₈ = 4.0153 d ₈ = 2.5616 n _d4 = 1.77250 ν _d4 = 49.60
r ₉ = -1.4957 d ₉ = 0.4000 n _d5 = 1.92286 ν _d5 = 18.90
r ₁₀ = -3.2619 d ₁₀ = 0.5000
r ₁₁ = ∞ d ₁₁ = 0.6000 n _d6 = 1.51633 ν _d6 = 64.06
r ₁₂ = ∞ d ₁₂ = 0.5081
r ₁₃ = ∞ (image plane)
Aspheric coefficient 3rd surface K = 0
A ₄ = 2.4999 × 10 ^-4
A ₆ = 2.1410 × 10 ^-6
A ₈ = 0.0000
4th surface K = -0.9230
A ₄ = -1.2303 × 10 ^-2
A ₆ = 1.2013 × 10 ^-4
A ₈ = 4.7406 × 10 ^-6
6th surface K = 0
A ₄ = -9.3040 × 10 ^-2
A ₆ = -1.3515 × 10 ^-1
A ₈ = 0.0000
Surface 7 K = -0.0940
A ₄ = 1.2079 × 10 ^-2
A ₆ = 3.4421 × 10 ^-2
A ₈ = 0.0000.

  The aberration diagrams of Examples 1 to 4 are shown in FIGS. 6 to 9, respectively. In these aberration diagrams, DZY represents lateral aberration, AS represents astigmatism, DT represents distortion, and CC represents lateral chromatic aberration. In each figure, “FIY” indicates the image height. Further, λ = 890 nm is a curve showing aberration at a wavelength of 890 nm.

  The above imaging optical system of the present invention can be configured as follows, for example.

    [1] An imaging system including an imaging optical system including at least one positive lens and an imaging element, wherein at least one Fresnel optical element is disposed closer to the object side than the imaging optical system. Optical system.

    [2] The imaging optical system according to [1], wherein the at least one Fresnel optical element has a positive refractive power.

    [3] The imaging optical system as described in 2 above, wherein the following condition (1) is satisfied.

(1) 0.05 <L _P / φ _P <1
Where L _P is the distance from the Fresnel optical element to the imaging optical system, and φ _P is the effective diameter of the Fresnel optical element.

    [4] The imaging optical system as described in 2 above, wherein the following condition (2) is satisfied.

(2) 0.01 <f / f _P <0.1
Here, f is the focal length of the imaging optical system, and f _P is the focal length of the Fresnel optical element.

    [5] The imaging optical system as described in 2 above, wherein one Fresnel optical element is used, and both surfaces of the Fresnel optical element have a Fresnel shape.

    [6] The imaging optical system as described in 5 above, wherein the Fresnel optical element satisfies the following condition (3).

(3) α _j <β _k
Where α _j is the edge angle of the image side surface at a distance j from the center to the periphery of the Fresnel optical element, and β _k is the edge of the object side surface at a distance k from the center to the periphery of the Fresnel optical element. Is an angle. Here, j and k are distances of 50% or more of the radius and satisfy 0.9 <k / j <1.1.

    [7] The imaging optical system as described in 2 above, wherein the following condition (4) is satisfied.

(4) 0 <D _P / f <5
Where D _P is the distance from the Fresnel optical element to the object, and f is the focal length of the imaging optical system.

    [8] The above-described imaging optical system includes the negative group having at least one negative lens and the positive group having at least one positive lens in order from the object side. 4. The imaging optical system according to any one of 3 above.

    [9] The imaging optical system according to [8], wherein an aperture stop is disposed between the negative group and the positive group of the imaging optical system.

    [10] At least one aspheric lens is used for the negative group of the imaging optical system, and at least one surface of the aspheric lens has a shape that weakens negative refractive power from the center to the periphery of the optical axis. 9. The imaging optical system according to 8 above.

    [11] The at least one aspheric lens is used for the positive group of the imaging optical system, and at least one surface of the aspheric lens has a shape that weakens the positive refractive power from the center to the periphery of the optical axis. 9. The imaging optical system according to 8 above.

It is the schematic diagram which made the both surfaces of the Fresnel optical element the Fresnel shape in this invention. 1A is a cross-sectional view of a first embodiment of an imaging optical system according to the present invention, and FIG. 2B is a cross-sectional view of an imaging optical system. FIG. 6 is a cross-sectional view (a) of Embodiment 2 of the imaging optical system of the present invention and a cross-sectional view (b) of the imaging optical system. It is sectional drawing of the imaging optical system of Example 3 of the imaging optical system of this invention. It is sectional drawing of the imaging optical system of Example 4 of the imaging optical system of this invention. FIG. 6 is an aberration diagram of Example 1. FIG. 6 is an aberration diagram of Example 2. FIG. 6 is an aberration diagram of Example 3. FIG. 6 is an aberration diagram of Example 4. It is a figure which shows the edge angle of the ring-shaped refractive surface by the side of an image of the Fresnel optical element of Examples 1-4, and the edge angle of the ring-shaped refractive surface by the side of an object.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Imaging optical system 2 ... Fresnel optical element 3 ... Object surface

Claims (3)

An imaging optical system comprising an imaging optical system comprising at least one positive lens and an imaging element, wherein at least one Fresnel optical element is disposed closer to the object side than the imaging optical system. The imaging optical system according to claim 1, wherein the at least one Fresnel optical element has a positive refractive power. The imaging optical system according to claim 2, wherein the following condition (1) is satisfied.
(1) 0.05 <L _P / φ _P <1
Where L _P is the distance from the Fresnel optical element to the imaging optical system, and φ _P is the effective diameter of the Fresnel optical element.

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