JP6222116B2 - Imaging optical system, imaging optical device and digital equipment - Google Patents

Description

  The present invention relates to an imaging optical system, an imaging optical device, and a digital device. For example, an imaging optical system compatible with an ultra-wide angle of 160 ° or more, an imaging optical device that captures an image obtained by the imaging optical system with an imaging device, an in-vehicle camera, a surveillance camera, and the like equipped with the imaging optical device The present invention relates to a digital device with an image input function.

  Conventionally, there has been a wide-angle lens composed of a first lens having negative power, a second lens having negative power, a third lens having positive power, and a fourth lens having positive power. Many have been proposed. Many proposals have been made in which the first lens is made of a glass spherical surface with particular emphasis on reliability (see, for example, Patent Documents 1 and 2). On the other hand, the first lens made of resin is also proposed. (For example, see Patent Document 3).

  For example, Patent Document 1 proposes a fish-eye lens including a first lens formed of a glass spherical surface and three resin lenses. Patent Document 2 proposes a fish-eye lens composed of a first lens that is also formed of a glass spherical surface and three resin lenses. In Patent Document 3, a wide-angle lens composed of four resin lenses is proposed.

JP 2009-3343 A JP 2011-048334 A JP 2008-281859 A

  However, in the fish-eye lens described in Patent Document 1, off-axis light that has passed through the aperture is not significantly bent by the fourth lens and is incident obliquely on the sensor surface. For this reason, there is a problem that there is no degree of freedom in sensor selection. In the fisheye lens described in Patent Document 2, the power of the second lens is weak, so that the force for bending the peripheral rays is small. For this reason, the space | interval of a 2nd lens and a 3rd lens is long, and there exists a subject in shortening of a full length.

  Many of the first lenses made of resin cannot be said to be super-wide. For example, in the fisheye lens described in Patent Document 3, the power of the first lens is weak, so that the force for bending the light beam is insufficient. Therefore, there is a problem that it is impossible to capture light having an angle of view of 160 ° or more, and it is impossible to achieve super wide angle.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide a photographing optical system that is compact and capable of widening an angle of view of 160 ° or more while correcting various aberrations satisfactorily. It is an object to provide an imaging optical device and a digital device provided.

In order to achieve the above object, a photographic optical system according to a first aspect of the present invention includes, in order from the object side to the image plane side, a first lens having a meniscus shape that is convex on the object side and having negative power; A fisheye lens having a field angle of 160 ° or more, comprising a second lens having a negative power, a third lens having a positive power, a diaphragm, and a fourth lens having a positive power,
The first lens is a double-sided aspheric lens, and satisfies the following conditional expressions (1) to (4) and conditional expression (6) .
−16 <f1 / f <−5 (1)
1.0 <f4 / f <1.9 (2)
-1.9 <f2 / f <-0.9 (3)
0.6 <(r7 + r8) / (r7−r8) <1.2 (4)
1 <f3 / f <2 (6)
However,
f1: focal length of the first lens,
f4: focal length of the fourth lens,
f2: focal length of the second lens,
f3: focal length of the third lens,
f: focal length of the entire system,
r7: radius of curvature of the object side surface of the fourth lens,
r8: radius of curvature of the image side surface of the fourth lens,
It is.

The photographic optical system of the second invention is characterized in that, in the first invention, the following conditional expression (5) is satisfied.
1 <BF / f <2 (5)
However,
BF: Back focus (air equivalent length),
f: focal length of the entire system,
It is.

A photographic optical system according to a third aspect of the invention is characterized in that, in the first or second aspect of the invention, the following conditional expression (7) is satisfied.
4 <f1 / f2 <16 (7)
However,
f1: focal length of the first lens,
f2: focal length of the second lens,
It is.

According to a fourth aspect of the present invention, in any one of the first to third aspects, the first lens, the second lens, the third lens, and the fourth lens are all plastic lenses. And

According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the second lens, the third lens, and the fourth lens are all double-sided aspheric lenses. .

According to a sixth aspect of the present invention, in any one of the first to fifth aspects, the second lens has a meniscus shape that is convex toward the object side.

According to a seventh aspect of the present invention, in any one of the first to sixth aspects, a hard coat is formed on the object side surface of the first lens.

An imaging optical system according to an eighth invention is characterized in that, in any one of the first to seventh inventions, the following conditional expression (8) is satisfied.
nd1 ≦ 1.65 (8)
However,
nd1: refractive index at the d-line of the first lens,
It is.

According to a ninth aspect of the present invention, there is provided the photographing optical system according to any one of the first to eighth aspects, wherein the third lens has a biconvex shape.

An imaging optical device according to a tenth aspect of the invention is an imaging optical system according to any one of the first to ninth aspects, an imaging element that converts an optical image formed on the imaging surface into an electrical signal, The imaging optical system is provided so that an optical image of a subject is formed on the imaging surface of the imaging device.

A digital device according to an eleventh aspect is characterized in that at least one of a still image photographing and a moving image photographing function of a subject is added by including the imaging optical device according to the tenth aspect.

According to a twelfth aspect of the present invention, in the eleventh aspect , the digital device is an in-vehicle camera or a surveillance camera.

  By adopting the configuration of the present invention, it is possible to realize a compact imaging optical system capable of widening an angle of view of 160 ° or more and an imaging optical apparatus including the imaging optical system while various aberrations are corrected well. it can. Then, by using the photographing optical system or the imaging optical device according to the present invention for a digital device such as an in-vehicle camera or a surveillance camera, a high-performance and ultra-wide-angle image input function can be added to the digital device in a compact and low-cost manner. Is possible.

The lens block diagram of 1st Embodiment (Example 1). FIG. 6 is an aberration diagram of Example 1. The lens block diagram of 2nd Embodiment (Example 2). FIG. 6 is an aberration diagram of Example 2. The lens block diagram of 3rd Embodiment (Example 3). FIG. 6 is an aberration diagram of Example 3. The lens block diagram of 4th Embodiment (Example 4). FIG. 6 is an aberration diagram of Example 4. The lens block diagram of 5th Embodiment (Example 5). FIG. 6 is an aberration diagram of Example 5. The lens block diagram of 6th Embodiment (Example 6). FIG. 10 is an aberration diagram of Example 6. FIG. 3 is a schematic diagram illustrating a schematic configuration example of a digital device equipped with a photographing optical system.

Hereinafter, the photographing optical system according to the present invention will be described. A photographing optical system according to the present invention includes, in order from the object side to the image plane side, a first lens having a meniscus shape convex toward the object side and having negative power, and a second lens having negative power. , A fish-eye lens having a field angle of 160 ° or more, composed of a third lens having a positive power, a stop, and a fourth lens having a positive power (power: an amount defined by the reciprocal of the focal length) ). The first lens is a double-sided aspheric lens, and satisfies the following conditional expressions (1) to (4).
−16 <f1 / f <−5 (1)
1.0 <f4 / f <1.9 (2)
-1.9 <f2 / f <-0.9 (3)
0.6 <(r7 + r8) / (r7−r8) <1.2 (4)
However,
f1: focal length of the first lens,
f4: focal length of the fourth lens,
f2: focal length of the second lens,
f: focal length of the entire system,
r7: radius of curvature of the object side surface of the fourth lens,
r8: radius of curvature of the image side surface of the fourth lens,
It is.

  With a negative / negative / positive / aperture / positive power arrangement, the first lens has a convex meniscus shape on the object side. A lens can be achieved. Further, by arranging aspheric surfaces on both surfaces of the first lens, it is possible to effectively correct distortion and astigmatism. Controlling distortion and astigmatism for each angle of view is difficult with a spherical lens, but it can be controlled to some extent by using a double-sided aspheric surface.

  Conditional expression (1) represents the focal length ratio of the first lens with respect to the entire system. If the lower limit of conditional expression (1) is exceeded, the relative power of the first lens will be reduced, the force to bend the light incident on this photographic optical system will be weak, and an ultra-wide angle of 160 ° or more will be achieved. Becomes difficult. If the upper limit of conditional expression (1) is exceeded, the amount of distortion, lateral chromatic aberration, etc. generated in the first lens will increase, making it difficult to correct it in the second lens and thereafter. Therefore, by satisfying conditional expression (1), it is possible to achieve both super wide angle of view angle of 160 ° or more and good correction of various aberrations.

  Conditional expression (2) represents the focal length ratio of the fourth lens with respect to the entire system. When the upper limit of conditional expression (2) is exceeded, the relative power of the fourth lens decreases, and the distance from the object side surface to the image side surface of the fourth lens increases. If the lower limit of conditional expression (2) is exceeded, axial chromatic aberration, astigmatism, etc. will increase, making it difficult to correct with other lenses. Therefore, by satisfying conditional expression (2), it is possible to achieve both compactness and good correction of various aberrations.

  Conditional expression (3) represents the focal length ratio of the second lens with respect to the entire system. When the lower limit of conditional expression (3) is exceeded, the relative power of the second lens is reduced, and the force for bending the light beam is reduced. In order to make a light beam enter the diaphragm through the third lens, measures such as increasing the distance between the second lens and the third lens are required, which may increase the total length. Alternatively, it becomes difficult to configure the super wide angle itself. If the upper limit of conditional expression (3) is exceeded, the astigmatism and the field curvature difference will increase, making it difficult to correct with other lenses. Therefore, by satisfying conditional expression (3), it is possible to achieve both ultra-wide angle and compactness and good correction of various aberrations.

  Conditional expression (4) defines the shape of the fourth lens and represents the behavior of the light beam that has passed through the stop. When the lower limit of conditional expression (4) is exceeded, the incident angle of the light beam incident on the object side surface of the fourth lens increases, and as a result, the light beam width viewed from the sensor becomes narrower, resulting in a decrease in ambient illuminance. If the upper limit of conditional expression (4) is exceeded, the shape of the front and back surfaces will be close, and off-axis light that has passed through the aperture will not be refracted greatly when passing through the fourth lens, but will be incident obliquely on the sensor. A malfunction occurs. Therefore, a bright image with high image quality can be obtained by satisfying conditional expression (4).

  According to the above characteristic configuration, a compact imaging optical system capable of widening an angle of view of 160 ° or more and an imaging optical device including the same are corrected while various aberrations (particularly longitudinal chromatic aberration) are corrected satisfactorily. Is possible. By using the imaging optical system or imaging optical device for digital devices such as in-vehicle cameras and surveillance cameras, it is possible to add a high-performance, ultra-wide-angle image input function to digital devices in a compact and low-cost manner. It can contribute to the downsizing, higher performance, higher functionality, etc. The conditions for achieving such effects in a well-balanced manner and achieving higher optical performance, downsizing, etc. will be described below.

It is more desirable to satisfy the following conditional expression (2a).
1.5 <f4 / f <1.9 (2a)
This conditional expression (2a) defines a more preferable condition range based on the above viewpoints, etc., among the condition ranges defined by the conditional expression (2). Therefore, the above effect can be further increased preferably by satisfying conditional expression (2a).

It is desirable to satisfy the following conditional expression (5).
1 <BF / f <2 (5)
However,
BF: Back focus (air equivalent length),
f: focal length of the entire system,
It is.

  Conditional expression (5) defines a preferable condition range relating to appropriate back focus (the distance from the lens final surface to the paraxial image surface is a length in terms of air). If the lower limit of conditional expression (5) is exceeded, it will be difficult to dispose a sensor cover glass, filter, or the like between the photographing optical system and the image plane. If the upper limit of conditional expression (5) is exceeded, the back focus becomes too long relative to the focal length. In that case, it is necessary to secure the back focus by adjusting the power arrangement of the first to fourth lenses. Therefore, the power arrangement is different from that suitable for aberration correction, and it becomes difficult to perform good aberration correction.

It is desirable to satisfy the following conditional expression (6).
1 <f3 / f <2 (6)
However,
f3: focal length of the third lens,
f: focal length of the entire system,
It is.

  Conditional expression (6) defines a preferable condition range regarding the focal length ratio between the third lens and the entire system. If the upper limit of conditional expression (6) is exceeded, the relative power of the third lens will be small, both of which are insufficiently corrected for the longitudinal chromatic aberration generated by the first and second lenses having negative power, which is good Correct aberration correction becomes difficult. If the lower limit of conditional expression (6) is exceeded, axial chromatic aberration will be excessively corrected, and this will also make it difficult to correct aberrations satisfactorily.

It is desirable to satisfy the following conditional expression (7).
4 <f1 / f2 <16 (7)
However,
f1: focal length of the first lens,
f2: focal length of the second lens,
It is.

  Conditional expression (7) defines a preferable condition range regarding a focal length ratio between the first lens and the second lens. When the lower limit of conditional expression (7) is exceeded, the refractive power of the light beam at the first lens increases, and the aberration generated at the first lens increases. In particular, it is difficult to correct distortion or the like occurring in the peripheral portion of the first lens having a high light beam height with another lens. If the upper limit of conditional expression (7) is exceeded, the power of the first lens will be too weak to achieve a super wide angle.

  It is desirable that all of the first lens, the second lens, the third lens, and the fourth lens are plastic lenses. By configuring all of the first lens, the second lens, the third lens, and the fourth lens from plastic (resin), it is possible to easily add an aspherical surface to the lens surface. Moreover, since mass production is possible, it is also effective in cost reduction.

  It is desirable that all of the second lens, the third lens, and the fourth lens are double-sided aspheric lenses. By arranging aspheric surfaces on both surfaces of the second lens, the third lens, and the fourth lens, it is possible to effectively correct astigmatism, distortion, coma, and the like.

  It is desirable that the second lens has a meniscus shape that is convex toward the object side. By making the second lens a meniscus lens convex toward the object side, the space between the first lens and the second lens can be narrowed. As a result, the light passing position of the first lens can be lowered, and the effect of reducing the diameter of the first lens can be obtained. If the light beam passing position becomes high, the lens diameter increases, and as a result, the amount of off-axis aberrations such as distortion and lateral chromatic aberration increases. That is, by using the second lens as a meniscus lens convex toward the object side, it is possible to suppress the occurrence of off-axis aberrations such as distortion and lateral chromatic aberration.

  It is desirable that a hard coat be formed on the object side surface of the first lens. When the first lens is made of resin, disposing a hard coat on the object side surface of the first lens is effective in improving reliability such as scratch resistance.

It is desirable to satisfy the following conditional expression (8).
nd1 ≦ 1.65 (8)
However,
nd1: refractive index at the d-line of the first lens,
It is.

  Conditional expression (8) defines a preferable condition range relating to the refractive index of the first lens, thereby defining the reliability of the resin material. If a resin material having a refractive index exceeding the upper limit of conditional expression (8) is left in the sun for a long time, the transmittance on the single wavelength side is lowered and the image becomes yellow.

  It is desirable that the third lens has a biconvex shape. By making the third lens a biconvex lens having convex surfaces facing the object side and the image surface side, correction of various aberrations (for example, spherical aberration) can be shared between the object side surface and the image side surface. For example, if the meniscus shape is used, the error sensitivity and aberration tend to increase due to the lens surface shape for obtaining strong power. However, if the biconvex shape is used, an increase in error sensitivity and aberration can be effectively suppressed.

  As can be seen from the above description, the photographing optical system according to the present invention is suitable for use as a photographing optical system for a digital device with an ultra-wide-angle image input function (for example, an in-vehicle camera, a surveillance camera, and a portable terminal). . That is, the photographic optical system according to the present invention is suitable for an application in which an optical image of an object (that is, a subject image) is formed at an ultra-wide angle on an imaging surface (for example, a photoelectric conversion unit of a solid-state imaging device) of an imaging device (sensor). Is. By combining the imaging optical system according to the present invention with an imaging device or the like, it is possible to configure an imaging optical apparatus that optically captures a subject image and outputs it as an electrical signal. The imaging optical device is an optical device that is a main component of a camera used for still image shooting and moving image shooting of a subject. For example, an imaging optical system that forms an optical image of an object in order from the object (that is, subject) side. And an image sensor that converts an optical image formed by the photographing optical system into an electrical signal. Then, the photographic optical system having the above-described characteristic configuration is arranged so that the optical image of the subject is formed on the light receiving surface (that is, the imaging surface) of the imaging device, so that high performance can be achieved with a small size and low cost. It is possible to realize an imaging optical device having the above and a digital device including the same.

  Examples of digital devices with an image input function include cameras such as surveillance cameras, security cameras, vehicle-mounted cameras (for example, back view cameras), aircraft cameras, digital cameras, video cameras, videophone cameras, and personal computers. , Mobile terminals (for example, mobile phones, smart phones (high-function mobile phones), small and portable information device terminals such as mobile computers), peripheral devices (scanners, printers, etc.), other digital devices (drive recorders, A camera built in or externally attached to a defense device or the like). As can be seen from these examples, it is possible not only to configure a camera by using an imaging optical device, but also to add a camera function by mounting the imaging optical device on various devices. For example, a digital device with an image input function such as a mobile phone with a camera can be configured.

  As an example of a digital device with an image input function, FIG. 13 shows a schematic configuration example of a digital device DU in a schematic cross section. The imaging optical device LU mounted on the digital device DU shown in FIG. 13 includes a photographing optical system LN (AX: optical axis) that forms an optical image (image plane) IM of the object in order from the object (namely, subject) side. , A parallel plate PT (cover glass of the image sensor SR; corresponding to an optical low-pass filter, an optical filter such as an infrared cut filter, etc. disposed as necessary) and a light receiving surface (imaging surface) by the photographing optical system LN. And an image sensor SR that converts the optical image IM formed on the SS into an electrical signal. When a digital device DU with an image input function is constituted by this imaging optical device LU, the imaging optical device LU is usually arranged inside the body, but when necessary to realize the camera function, a form as necessary is adopted. Is possible. For example, the unitized imaging optical device LU can be configured to be detachable or rotatable with respect to the main body of the digital device DU.

  The photographic optical system LN is a four-lens single-focus lens composed of first to fourth negative, positive and positive lenses in order from the object side, and forms an optical image IM on the light receiving surface SS of the image sensor SR. It has become. As the image sensor SR, for example, a solid-state image sensor such as a CCD (Charge Coupled Device) type image sensor having a plurality of pixels or a CMOS (Complementary Metal-Oxide Semiconductor) type image sensor is used. Since the photographic optical system LN is provided so that the optical image IM of the subject is formed on the light receiving surface SS which is a photoelectric conversion unit of the image sensor SR, the optical image IM formed by the photographic optical system LN is It is converted into an electrical signal by the image sensor SR. As described above, since it is assumed that the image sensor SR such as a CCD image sensor or a CMOS image sensor is used, a cover glass is formed as a parallel plate PT between the photographing optical system LN and the image plane IM. Of course, depending on the type of sensor, a cover glass may not be arranged.

  The digital device DU includes a signal processing unit 1, a control unit 2, a memory 3, an operation unit 4, a display unit 5 and the like in addition to the imaging optical device LU. The signal generated by the image sensor SR is subjected to predetermined digital image processing, image compression processing, and the like in the signal processing unit 1 as necessary, and recorded as a digital video signal in the memory 3 (semiconductor memory, optical disc, etc.) In some cases, it is transmitted to other devices via a cable or converted into an infrared signal or the like (for example, a communication function of a mobile phone). The control unit 2 is composed of a microcomputer, and controls functions such as a shooting function (still image shooting function, moving image shooting function, etc.), an image reproduction function, etc .; a lens moving mechanism for focusing, etc. For example, the control unit 2 controls the imaging optical device LU so as to perform at least one of still image shooting and moving image shooting of a subject. The display unit 5 includes a display such as a liquid crystal monitor, and performs image display using an image signal converted by the image sensor SR or image information recorded in the memory 3. The operation unit 4 is a part including operation members such as an operation button (for example, a release button) and an operation dial (for example, a shooting mode dial), and transmits information input by the operator to the control unit 2.

  1, 3, 5, 7, 9, and 11 show first to sixth embodiments of the photographing optical system LN in an infinitely focused state in optical cross sections, respectively. The j-th lens Lj (j = 1, 2, 3, 4) is a lens located at the jth position from the object side, and the parallel plate PT disposed on the image side of the photographing optical system LN is a cover of the image sensor SR. This assumes glass.

  In the imaging optical systems LN of the first to sixth embodiments, in order from the object side, the first lens L1 having negative power, the second lens L2 having negative power, and the third having positive power. The lens L3, the stop ST, and a fourth lens L4 having a positive power are configured. The first lens L1 and the second lens L2 have a convex meniscus shape on the object side, the third lens L3 has a biconvex shape, and the fourth lens L4 has a concave meniscus shape on the object side or It has a biconvex shape.

  In addition, all lens surfaces constituting the photographing optical system LN are aspherical surfaces, and all lenses constituting the photographing optical system LN are assumed to be made of a plastic material as an optical material. Since the first lens L1 is a plastic lens, a cover member may be disposed on the object side of the photographing optical system LN. However, since it is difficult to install a cover member on the object side of the first lens L1 with an ultra-wide-angle lens, it is preferable to form a hard coat on the object side surface of the first lens L1 instead of providing a cover member.

  Hereinafter, the configuration and the like of the photographic optical system in which the present invention is implemented will be described more specifically with reference to the construction data of the examples. Examples 1 to 6 (EX1 to 6) listed here are numerical examples corresponding to the first to sixth embodiments, respectively, and are lens configuration diagrams illustrating the first to sixth embodiments. (FIGS. 1, 3, 5, 7, 9, and 11) show the lens cross-sectional shapes and the like of the corresponding Examples 1 to 6, respectively.

In the construction data of each embodiment, as surface data, in order from the left column, the surface number, the radius of curvature r (mm), the axial upper surface distance d (mm), and the refractive indexes nd and d regarding the d-line (wavelength 587.56 nm). The Abbe number vd for the line is shown. A surface with * in the surface number is an aspheric surface, and the surface shape is defined by the following expression (AS) using a local orthogonal coordinate system (x, y, z) with the surface vertex as the origin. . As aspheric data, an aspheric coefficient or the like is shown. It should be noted that the coefficient of the term not described in the aspherical data of each example is 0, and E−n = × 10 ⁻ⁿ for all data.
z = (C · h ² ) / [1 + √ {1− (1 + k) C ² · h ² }] + A4 · h ⁴ + A6 · h ⁶ + A8 · h ⁸ + A10 · h ¹⁰ + A12 · h ¹² (AS)
However,
h: height in the direction perpendicular to the z axis (optical axis AX) (h ² = x ² + y ² ),
z: displacement in the direction of the optical axis AX at the position of the height h (based on the surface vertex),
C: Paraxial curvature at the surface vertex (the reciprocal of the radius of curvature r),
k: cone coefficient,
A4, A6, A8, A10, A12: 4th order, 6th order, 8th order, 10th order, and 12th order aspheric coefficients,
It is.

  Table 1 shows values corresponding to the conditional expressions of each example, and Table 2 shows the surface shape (notation based on paraxial curvature) and power (Lj, j = 1, 2, 3, 4) of each lens. Expressed as positive or negative.) Table 3 shows various data (values relating to the d-line) as the focal length (f, mm) of the entire system and the focal lengths (f1, f2, f3) of the lenses (Lj, j = 1, 2, 3, 4). , F4; mm), total lens length (TL, mm), F number (FNO), back focus (BF, mm), radius of curvature r7 of the object side surface of the fourth lens L4, radius of curvature of the image side surface of the fourth lens L4. r8, the total angle of view (2ω, °), and the maximum image height (Y ′, mm; corresponding to half the diagonal length of the imaging surface SS of the imaging element SR). Note that the back focus BF represents the distance from the lens final surface to the paraxial image surface by an air conversion length, and the lens total length TL is obtained by adding the back focus BF to the distance from the lens front surface to the lens final surface. Is.

  2, 4, 6, 8, 10, and 12 are aberration diagrams of Examples 1 to 6 (EX 1 to 6), where (A) is spherical aberration (mm), and (B) is non-aberration. Point aberrations (mm) and (C) indicate distortion aberration (%). In the spherical aberration diagram (A), the solid line indicates the amount of spherical aberration with respect to the d-line (wavelength 587.56 nm), the two-dot chain line indicates the amount of spherical aberration with respect to the g-line (wavelength 435.84 nm), and the dotted line indicates the C-line (wavelength 656.28 nm). The amount of spherical aberration with respect to is expressed by the amount of deviation in the optical axis AX direction from the paraxial image plane, and the vertical axis represents the F number. In the astigmatism diagram (B), the dotted line T represents the tangential image plane with respect to the d line, and the solid line S represents the sagittal image plane with respect to the d line, respectively, by the amount of deviation in the optical axis AX direction from the paraxial image plane. The vertical axis represents the image height Y ′ (mm). In the distortion diagram (C), the horizontal axis represents the distortion with respect to the d-line, and the vertical axis represents the image height Y ′ (mm). This distortion is a value based on the projection method of Y ′ = 2f · tan (ω / 2). A normal lens uses a relational expression of Y ′ = f · tan ω as a reference, but this formula cannot be applied to an ultra-wide angle lens in which the half angle of view ω exceeds 90 degrees.

  Here, the schematic configuration (Table 2) of each example will be described. However, all power is based on paraxial values. In Example 1 (FIG. 1), in order from the object side, a negative power first lens L1 having a meniscus shape with a convex surface facing the object side and a second negative power having a meniscus shape with a convex surface facing the object side. It comprises a lens L2, a positive-power third lens L3 having a biconvex shape, an aperture stop ST, and a positive-power fourth lens L4 having a meniscus shape with a concave surface facing the object side. All the lenses L1 to L4 are made of plastic, and all the surfaces are aspherical.

  In Examples 2, 3, 4, 5, and 6 (FIGS. 3, 5, 5, 7, and 11), a negative power first having a meniscus shape with a convex surface facing the object side in order from the object side. Lens L1, negative power second lens L2 having a meniscus shape with a convex surface facing the object side, positive power third lens L3 having a biconvex shape, aperture stop ST, and positive power having a biconvex shape The fourth lens L4. All the lenses L1 to L4 are made of plastic, and all the surfaces are aspherical.

  When the lens is made of plastic, there is a drawback that the hardness is low and the scratch resistance and weather resistance are poor. As an effective method for solving this problem, it is now possible to increase the hardness of the surface without impairing the lightness and workability of the plastic by forming a cured film (hard coat film) on the surface of the plastic member. . Since it is difficult to install a cover member on the object side of the first lens L1 with an ultra-wide-angle lens, for example, when used for an in-vehicle camera or a surveillance camera, it is sufficient that the object side surface of the first lens L1 is exposed to the outside. Conceivable. For this reason, in any of Examples 1 to 6, a hard coat is formed on the object side surface of the first lens L1. By forming a transparent hard coat film having a thickness of about 2 to 15 μm on the object side surface of the first lens by a technique such as dip coating, spray coating, or spin coating, the scratch resistance and weather resistance can be improved. In order to prevent water droplets attached due to rain or the like, a water repellent coat or a hydrophilic coat may be added on the hard coat. In order to further improve the light resistance, the first lens 11 made of plastic is made of a material. A UV-cutting agent may be added to.

  In Examples 1 to 6, since it is assumed that the image sensor SR such as a CCD image sensor or a CMOS image sensor is used, a cover glass (parallel plate) is provided between the fourth lens L4 and the imaging plane IM. PT) is arranged, but of course, there is a case where a cover glass is not arranged depending on the type of sensor.

DU Digital equipment LU Image pickup optical device LN Image pickup optical system L1 to L4 First to fourth lenses ST Aperture stop (stop)
SR Image sensor SS Light-receiving surface (imaging surface)
IM image plane (optical image)
AX Optical axis 1 Signal processing unit 2 Control unit 3 Memory 4 Operation unit 5 Display unit

Claims (12)

A first lens having a meniscus shape convex toward the object side and having negative power, a second lens having negative power, and a third lens having positive power in order from the object side to the image plane side A fisheye lens having an angle of view of 160 ° or more, comprising a diaphragm and a fourth lens having a positive power,
The first lens is a double-sided aspheric lens, and satisfies the following conditional expressions (1) to (4) and conditional expression (6) ;
−16 <f1 / f <−5 (1)
1.0 <f4 / f <1.9 (2)
-1.9 <f2 / f <-0.9 (3)
0.6 <(r7 + r8) / (r7−r8) <1.2 (4)
1 <f3 / f <2 (6)
However,
f1: focal length of the first lens,
f4: focal length of the fourth lens,
f2: focal length of the second lens,
f3: focal length of the third lens,
f: focal length of the entire system,
r7: radius of curvature of the object side surface of the fourth lens,
r8: radius of curvature of the image side surface of the fourth lens,
It is. The photographing optical system according to claim 1, wherein the following conditional expression (5) is satisfied:
1 <BF / f <2 (5)
However,
BF: Back focus (air equivalent length),
f: focal length of the entire system,
It is. According to claim 1 or 2, wherein the photographic optical system, characterized by satisfying the following conditional expression (7);
4 <f1 / f2 <16 (7)
However,
f1: focal length of the first lens,
f2: focal length of the second lens,
It is. The first lens, second lens, third lens and the imaging optical system according to any one of claims 1 to 3 in which the fourth lens is equal to or both a plastic lens. The second lens, third lens and the imaging optical system according to any one of claims 1 to 4, the fourth lens is equal to or either a double-sided aspheric lens. Imaging optical system according to any one of claims 1 to 5, wherein the second lens and having a convex meniscus shape on the object side. Imaging optical system according to any one of claims 1 to 6, characterized in that a hard coat is formed on the object side surface of the first lens. Claim 1-7 photographing optical system according to any one of which is characterized by satisfying the following conditional expression (8);
nd1 ≦ 1.65 (8)
However,
nd1: refractive index at the d-line of the first lens,
It is. Imaging optical system according to any one of claims 1 to 8, wherein said third lens is characterized by having a biconvex shape. An imaging optical system according to any one of claims 1 to 9 , and an imaging device that converts an optical image formed on the imaging surface into an electrical signal, and on the imaging surface of the imaging device An imaging optical apparatus, wherein the imaging optical system is provided so that an optical image of a subject is formed. 11. A digital apparatus comprising the imaging optical device according to claim 10 to which at least one function of still image shooting and moving image shooting of a subject is added. The digital device according to claim 11 , wherein the digital device is an in-vehicle camera or a surveillance camera.

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