JP7194979B2 - Imaging optical system and imaging device - Google Patents

Description

The present invention relates to an imaging optical system and an imaging apparatus.

An imaging optical system called a wide-angle foveal lens is known, which is modeled on the human eye and has a wide field of view and high resolution at the center. In such an imaging optical system, by making the optical magnification higher in the center of the field of view than in the periphery, a large amount of image information about the object of interest can be obtained from the center and a wide range of information can be obtained from the periphery. there is

An imaging optical system as a wide-angle foveal optical lens is known from Patent Document 1. The imaging optical system of Patent Document 1 consists of a front group with an aperture therebetween and a rear group having positive refractive power. It is a configuration in which a first lens having an aspherical surface having a negative refractive power in the peripheral portion of the lens is arranged.

JP-A-2005-10521

By the way, in recent years, along with the improvement of the resolution of image sensors and the improvement of the processing capability of image processing apparatuses, the accuracy of object recognition, distance measurement, etc., based on information obtained from captured images has increased. In contrast to the resolution of an image sensor capable of highly accurate object recognition and distance measurement, the conventional imaging optical system described above has a low resolution at the center of the field of view. High optical magnification and sufficient optical imaging performance were not obtained. On the other hand, in the configuration of the imaging optical system as described above, if an attempt is made to improve the optical performance by increasing the optical magnification in the central portion of the field of view while maintaining a wide angle of view, the aspherical shape of the first lens is required. increases in order and the shape becomes complicated. In addition, it becomes difficult to achieve optical imaging performance that is compatible with a high-resolution image pickup device with improved resolution as described above. For this reason, problems such as the fact that actual lens processing cannot be performed even if it is feasible in terms of design arise.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances. The purpose is to provide an apparatus.

The imaging optical system of the present invention is an imaging optical system having, in order from the object side, a front group and a rear group having positive refractive power as a whole, wherein the front group comprises, in order from the object side, a first group and a second group. The first group includes at least a first lens and a second lens closer to the image side than the first lens, and the center portion of one of the first lens and the second lens is The lens has a positive refractive power and a peripheral portion with a negative refractive power, and the other lens has a central portion with a negative refractive power and a peripheral portion with a positive refractive power. The width of the parallel light rays on the most object side surface of the second group with respect to the width of the incident parallel light rays is narrowed for the parallel light rays that are incident on the central portion of the first lens at a small angle, and is incident on the peripheral portion at a large angle. It has a refractive power that spreads out parallel rays.

An image pickup apparatus of the present invention includes the image pickup optical system described above and an image pickup device that converts an optical image formed by the image pickup optical system into an electrical signal.

According to the present invention, the center portion of one of the first lens in the first group disposed closest to the object side in the front group and the second lens closer to the image side than the first lens has a positive refractive power. and the peripheral portion has negative refractive power, and the central portion of the other lens has negative refractive power and the peripheral portion has positive refractive power. The width of light rays incident at a small angle on the central portion is narrowed, and the width of light rays incident on the peripheral portion at a large angle is widened. Higher optical magnification can be obtained in the central portion of the field of view.

2 is a cross-sectional view showing the configuration of an imaging optical system of a first configuration example; FIG. FIG. 11 is a cross-sectional view showing the configuration of an imaging optical system of a second configuration example; FIG. 11 is a cross-sectional view showing the configuration of an imaging optical system of a third configuration example; FIG. 4 is an aberration diagram showing aberration of the imaging optical system of the first configuration example; FIG. 11 is an aberration diagram showing aberration of the imaging optical system of the second configuration example; FIG. 11 is an image showing an example of imaging by the imaging optical system of the second configuration example; FIG. It is an image which shows the example of imaging with the conventional imaging optical system. It is an example of imaging by the imaging optical system of the second configuration example, and is an image in which a pedestrian appears in the periphery of the field of view. FIG. 11 is an aberration diagram showing aberration of the imaging optical system of the third configuration example;

FIG. 1 shows a first configuration example of an imaging optical system according to an embodiment of the present invention. The imaging optical system 10 of the first configuration example is a wide-angle foveal lens that provides a wide field of view, ie, a wide angle, and a higher optical magnification (horizontal magnification) in the central portion of the field of view (imaging screen) than in the peripheral portion. This imaging optical system 10 is attached to or integrally assembled with, for example, an imaging device (not shown). The optical performance of the imaging optical system 10 is such that a high optical magnification can be obtained in the central portion of the field of view, and high resolution can be obtained in the central portion while maintaining a constant resolution in the periphery of the field of view. It is. Such an imaging optical system 10 is suitable for, for example, a system that recognizes and measures the distance to an object in front with high accuracy and recognizes and measures the distance to surrounding objects in a wide range at a constant level. , useful as an in-vehicle camera for automatic driving.

The imaging optical system 10 has a front group Gf, a diaphragm ST, and a rear group Gr arranged in order from the object side (left side in the drawing). An imaging element that converts a formed optical image into an electrical signal is arranged on the image plane IM of the imaging optical system 10 . Further, in this example, a plate-like infrared cut filter P1 is provided between the diaphragm ST and the rear group Gr, and an image pickup device arranged on the image plane IM is provided between the rear group Gr and the image plane IM. A cover plate P2 is arranged to protect the light receiving surface (not shown). The infrared cut filter P1 and the cover plate P2 can be omitted.

The front group Gf includes, in order from the object side, a first group G1 and a second group G2. The first group G1 is for controlling the image magnification, and is composed of a first lens L11 and a second lens L12 in order from the object side. The first lens L11 has positive refractive power (lens power) in the central portion and negative refractive power in the peripheral portion. The second lens L12 has negative refractive power in the central portion and positive refractive power in the peripheral portion. As a whole, the first group G1 composed of the first lens L11 and the second lens L12 is the most object-side surface of the second group G2 (the third lens The width of the parallel rays incident on the central portion of the first lens L11 at a small angle (angle with respect to the optical axis) is narrowed, and the width of the parallel rays at the peripheral portion with a large angle (angle with respect to the optical axis) is narrowed. ), it has a refractive power that expands for parallel light rays incident at . That is, the first group G1 narrows the width of the parallel light rays incident on the central portion of the first lens L11 at a small angle when they are incident on the third lens L13 compared to the width when they are incident on the first lens L11. , the parallel rays incident on the periphery of the first lens L11 at a large angle have a wider width when incident on the third lens L13 than the width when incident on the first lens L11.

In this example, the first group G1 as a whole has positive refractive power in the central portion and negative refractive power in the peripheral portion. should have refractive power for parallel light rays incident on . Further, in the imaging optical system 10 of this example, the first lens L11 is an aspherical lens whose object-side surface has a convex spherical shape toward the object side and whose image plane side has an aspherical shape. Further, the second lens L12 is an aspherical lens in which the object-side and image-plane-side surfaces are aspherical.

The first group G1 reduces distortion in the central portion of the field of view, while largely changing the coefficient of distortion aberration from the central portion to the peripheral portion, thereby generating a large negative distortion aberration in the peripheral portion. As a result, the imaging optical system 10 has a wide angle and a higher optical magnification in the central portion of its field of view. The angle of view of the imaging optical system 10 and the optical magnification of the central portion of the field of view can be adjusted by the respective refractive powers of the central portion and the peripheral portion of the first group G1. The imaging optical system 10 preferably satisfies "-95%<Da<-10%" in the distortion aberration Da at the maximum half angle of view as a wide-angle foveal lens.

As described above, the first lens L11 and the second lens L12 of the first group G1 achieve a wide angle, a high optical magnification at the center, and an improved optical imaging performance. That is, by arranging the second lens L12, which has an opposite positive/negative refractive power between the central portion and the peripheral portion, with an air gap with respect to the first lens L11, a wide-angle field of view and a large change in optical magnification are realized. Astigmatism in the entire field of view is well suppressed, and as a result, a wide angle and high optical magnification and optical imaging performance in the center of the field of view with optical imaging performance sufficiently suitable for application to high-pixel imaging devices are achieved. .

By configuring the first group G1 with the first lens L11 and the second lens L12 having different positivity between the central portion and the peripheral portion, the imaging optical system 10 has a wide angle and the optical magnification of the central portion of the field of view. is higher. Since the image magnification is controlled by the first lens L11 and the second lens L12 as described above, the degree of freedom in designing each surface of the first lens L11 and the second lens L12 is increased. It is possible to increase the optical magnification in the central portion of the field of view while suppressing an increase in the aspherical order of the second lens L12 and suppressing an increase in the number of lenses, which is advantageous for making the total length compact.

In addition, by controlling the image magnification using the first lens L11 and the second lens L12 as in the imaging optical system 10, focusing on the surface shape of one lens as in the conventional lens configuration, Higher image magnification change over the entire field of view (distortion -95%<Da<-10%) and high optical imaging performance compared to the method that uses positive refractive power in the center and negative refractive power in the periphery. While realizing the above, it becomes easy to increase the aperture, and a bright optical system having an F value of 3.5 or less is possible. This makes the imaging optical system 10 suitable for imaging at night or in a dark place.

In this example, the object-side surface of the first lens L11 is spherical, but any one of the four surfaces of the first lens L11 and the second lens L12 can be spherical. Since the aspherical order of the first lens L11 and the second lens L12 is suppressed from increasing, even if one of the four surfaces of the first lens L11 and the second lens L12 is spherical, this Like the first group G1 of the imaging optical system 10 of the example, the other three surfaces can be made as aspherical as possible. It is preferable to form one of the four surfaces of the first lens L11 and the second lens L12 into a spherical shape from the viewpoint of manufacturing cost reduction and lens productivity.

The first lens L11 and the second lens L12 each have at least one aspherical surface to obtain the refractive power in the central portion and the peripheral portion as described above, but the aspherical shape has a low aspherical surface order. To achieve this, it is preferable that three or more of the four surfaces of the first lens L11 and the second lens L12 have an aspherical shape. Therefore, as in an example to be described later, each surface of the first lens L11 and the second lens L12 is formed to have an aspherical shape. This is a more preferable embodiment from the viewpoint of facilitating processing by suppressing the increase in order.

The first lens L11 has positive refractive power in the central portion and negative refractive power in the peripheral portion, and has a convex surface facing the object side. The second lens L12 has negative refractive power in the central portion and negative refractive power in the peripheral portion. The configuration in which the refracting power of the portion is positive is a preferred mode for increasing the aperture, that is, for decreasing the F value.

The first group G1 in this example consists of one first lens L11 and one second lens L12, respectively. At least one of each of the two types of lenses with different positive and negative forces is sufficient, and both or one of these two types of lenses may be provided in plural. Further, the refractive power of the central portion of the first lens L11 is negative, the refractive power of the peripheral portion is positive, the refractive power of the central portion of the second lens L12 is positive, the refractive power of the peripheral portion is negative, and the first group G1 As a whole, the width of the parallel light beam on the surface of the second group G2 closest to the object side with respect to the width of the parallel light beam entering the first lens L11 is expressed as It may be configured such that it is narrowed, and parallel rays incident on the peripheral portion at a large angle are widened.

In this example, the second group G2 is composed of one third lens L13, and the third lens L13 is a meniscus lens having negative refractive power with a concave surface facing the image side. The refractive power of the front group Gf as a whole becomes negative due to the third lens L13 as the second group G2 in this example. Also, the combination of the first group G1 and the third lens L13 makes it possible to increase the aperture diameter, which contributes to improvement in brightness. The second group G2 may be composed of a plurality of lenses.

The rear group Gr comprises, in order from the object side, a fourth lens L14 to a seventh lens L17, which are spherical lenses, and is an imaging optical system for forming an image of the light from the front group Gf, and has positive refractive power as a whole. have. The fourth lens L14 and the fifth lens L15 form a cemented lens that corrects chromatic aberration. The fourth lens L14 is a concave meniscus lens with the concave surface facing the image side, and the fifth lens L15 to the seventh lens L17 are all biconvex lenses.

Next, another configuration example of the imaging optical system will be described. It should be noted that the configuration is the same as that of the first configuration example except for the following description.

FIG. 2 shows a second configuration example of the imaging optical system according to the embodiment of the present invention. In the imaging optical system 20 of the second configuration example, each surface of the first lens L21 and the second lens L22 that constitute the first group G1 of the front group Gf is aspherical. The imaging optical system 20 includes, in order from the object side, a front group Gf, an infrared cut filter P1, a diaphragm ST, a rear group Gr, and a cover plate P2. The front group Gf is composed of, in order from the object side, a first lens L21 and a second lens L22, which constitute the first group G1, and a third lens L23, which constitutes the second group G2. The rear group Gr consists of a plurality of spherical lenses, and from the object side, a fourth lens L24 and a fifth lens L25 constituting a cemented lens for correcting chromatic aberration, and a sixth lens L26 to an eighth lens L28. This constitutes an imaging optical system for imaging the light from the front group Gf.

As described above, each surface of the first lens L21 and the second lens L22 is aspherical. The first lens L21 has positive refractive power in the central portion and negative refractive power in the peripheral portion, and the second lens L22 has negative refractive power in the central portion and positive refractive power in the peripheral portion. doing. In the first group G1, as a whole, the width of the parallel light rays on the most object-side surface of the second group G2 (the object-side lens surface of the third lens L23) with respect to the width of the parallel light rays incident on the first lens L21 is The first lens L21 has a refractive power that narrows parallel rays incident on the central portion at a small angle and widens parallel rays incident on the peripheral portion at a large angle.

In the imaging optical system 20 of this example, similarly to the imaging optical system 10, the first group G1 as a whole has a positive refractive power in the central portion and a negative refractive power in the peripheral portion. Instead, it is sufficient that the first group G1 as a whole has a refractive power with respect to incident parallel rays as described above.

The third lens L23 is a meniscus lens (concave meniscus lens) having negative refractive power with a concave surface facing the image side. The front group Gf has negative refractive power as a whole.

The fourth lens L24 is a concave meniscus lens with the concave surface facing the image side, the fifth lens L25 is a convex meniscus lens with the convex surface facing the object side, the sixth lens L26 is a plano-convex lens with the convex surface facing the object side, The seventh lens L27 is a biconvex lens, and the seventh lens L27 is a convex meniscus lens with the convex surface facing the object side.

FIG. 3 shows a third configuration example of the imaging optical system according to the embodiment of the present invention. In the imaging optical system 30 of the third configuration example, each surface of the first lens L31 and the second lens L32 constituting the first group G1 of the front group Gf is aspherical, as in the imaging optical system 20 described above. For example. The imaging optical system 30 includes, in order from the object side, a front group Gf, a diaphragm ST, an infrared cut filter P1, a rear group Gr, and a cover plate P2.

The front group Gf consists of a first lens L31 to a third lens L33 in order from the object side, the first lens L31 and the second lens L32 constitute the first group G1, and the third lens L33 constitutes the second group G2. is provided. The rear group Gr consists of a plurality of spherical lenses, and from the object side, a fourth lens L34 and a fifth lens L35 constituting a cemented lens for correcting chromatic aberration, and a sixth lens L36 to an eighth lens L38. This constitutes an imaging optical system for imaging the light from the front group Gf.

As described above, each surface of the first lens L31 and the second lens L32 has an aspherical shape, and the first lens L31 has positive refractive power in the central portion and negative refractive power in the peripheral portion. The second lens L32 has negative refractive power in the central portion and positive refractive power in the peripheral portion. In the first group G1, as a whole, the width of the parallel light rays on the most object-side surface of the second group G2 (the lens surface of the third lens L33 on the object side) with respect to the width of the parallel light rays incident on the first lens L31 is The first lens L31 has a refractive power that narrows parallel rays incident on the central portion at a small angle and expands parallel rays incident on the peripheral portion at a large angle. The first group G1 of the imaging optical system 30 as a whole has positive refractive power in the central portion and negative refractive power in the peripheral portion. It suffices if it has a refractive power for incident parallel light rays, such as The third lens L33 has negative refractive power and is a biconcave lens. The front group Gf has negative refractive power as a whole.

The fourth lens L34 is a concave meniscus lens with the concave surface facing the image side, the fifth lens L35 is a plano-convex lens with the convex surface facing the object side, the sixth lens L36 is a biconvex lens, the seventh lens L37 and the eighth lens L38 is a plano-convex lens with a convex surface facing the object side.

In each of the above examples, the front group Gf is configured to have a negative refractive power as a whole. good. With such a configuration, for example, a larger magnification change can be produced.

Next, Numerical Examples 1 to 3 of the imaging optical system according to the above embodiment will be described.

[Numerical Example 1]
Table 1 shows the lens data of the imaging optical system 10 . "Surface number" in Table 1 is the number assigned to the surface of the component in order from the object side. "*" attached to the surface number indicates that the surface is an aspherical surface. The column of "radius of curvature" indicates the radius of curvature of each surface, and the column of "spacing between surfaces" indicates the lens thickness or air gap between each surface and the next surface on the optical axis. The unit of radius of curvature and surface spacing is "mm". The "nd" column shows the refractive index of each optical element for the d-line (wavelength: 587.6 nm), and the "νd" column shows the Abbe number of each optical element for the d-line.

The column of "Materials" in Table 1 shows the material (type of glass or resin) of each optical element. "E48R" in the "Materials" column is "ZEONEX (registered trademark) E48R" (manufactured by Nippon Zeon Co., Ltd.), which is a cycloolefin polymer (COP) resin. "OKP-1" is a product name of Osaka Gas Chemicals Co., Ltd. and is an optical polyester resin. "S-LAL18", "S-TIH11", "S-PHM52" and "S-BSL7" are product names of OHARA INC. and are optical glasses. "B270 (registered trademark)" is a glass material manufactured by SCHOTT AG, and a thin film was formed on it to form an infrared cut filter P1.

The aspherical shape of the lens is expressed by the following aspherical formula using conic coefficient k and aspherical coefficients A ₂ , A ₄ , A ₆ . . . shown in Table 2.

Each value in the above formula is as follows.
Z: Amount of sag in the optical axis direction (mm)
s: distance from the optical axis (mm)
C: Curvature (reciprocal of paraxial radius of curvature)
k: conic constant A ₄ , A ₆ , A ₈ .

The imaging optical system 10 shown in the above lens data has a total lens length (distance from the front surface of the first lens L11 to the image plane IM) of 47.75 mm, an F value of 3.5, and a focal length of the entire system of 10.5 mm. , the maximum half angle of view was 47°, and the distortion aberration Da at the maximum half angle of view was -80%. Also, the focal length of the entire system is near the optical axis. The conversion value of the focal length from the maximum half angle of view is 1.07 mm, and the ratio of the focal length between the central portion and the peripheral portion is nearly 10 times. That is, the imaging optical system 10 has an F value of 3.5, and is a very bright lens that can be attached to a visible light camera even at night. It was obtained. The F value and the optical magnification are the same for Numerical Examples 2 and 3 of imaging optical systems 20 and 30, which will be described later.

FIG. 4 shows curvature of field and distortion of the imaging optical system 10, respectively. FIG. 4 shows the curvature of field for the F-line (wavelength 435.8 nm), d-line, and C-line (wavelength 656.3 nm). 4 shows the curvature of field in the plane. Also, since the distortion aberration is almost the same for the F-line, d-line, and C-line, only one aberration curve is drawn.

As can be seen from the field curvature and distortion shown in FIG. 4, the imaging optical system 10 has a wide-angle field of view and intentionally creates large distortion, but the field distortion is kept small. , and achieves high imaging performance not only in the center of the high-magnification field of view, but also over the entire field of view.

[Numerical Example 2]
Lens data of the imaging optical system 20 are shown in Table 3 below. Also, the aspheric shape of the lens is represented by the above aspheric formula using the conic coefficient k and the aspheric coefficients A ₂ , A ₄ , A ₆ . . . shown in Table 4. FIG. 5 shows curvature of field and distortion of the imaging optical system 20 . The items in Tables 3 and 4, and the curvature of field and distortion in FIG.

"S-LAM60" and "S-BSM4" in the column of "Materials" in Table 3 are product names of OHARA INC. and are optical glasses. "BOROFLOAT (registered trademark)" is a glass material manufactured by SCHOTT AG, and a thin film was formed on it to form an infrared cut filter P1.

The imaging optical system 20 shown in the above lens data has a total lens length (distance from the front surface of the first lens L21 to the image plane IM) of 51.54 mm, an F value of 3.5, and a focal length of the entire system of 10.5 mm. , the maximum half angle of view was 47°, and the distortion aberration Da at the maximum half angle of view was -80%. The focal length of the entire system is in the vicinity of the optical axis, and the conversion value of the focal length from the maximum half angle of view is 1.07 mm. was

As can be seen from the field curvature and distortion shown in FIG. 5, the imaging optical system 20 has a wide-angle field of view and intentionally creates a large distortion, but the field distortion is kept small. , and achieves high imaging performance not only in the center of the high-magnification field of view, but also over the entire field of view.

FIG. 6 shows an image of a vehicle 80 m in front of the vehicle captured using the imaging optical system 20. As shown in FIG. Also, FIG. 7 shows an image taken from the same imaging position as the image in FIG. An image captured with a normal wide-angle lens can capture a wide range with a large angle of view, but the image of the vehicle in the center is very small due to the small optical magnification at the center. For this reason, this image is insufficient for use in, for example, object recognition and highly accurate distance measurement. On the other hand, in the image captured by the imaging optical system 20, a wide range is captured with a large angle of view as in the case of a normal wide-angle lens, but the image of the vehicle in the center is large and object recognition and accuracy are high. It was large enough to be used for ranging, and sufficient resolution was obtained. FIG. 8 is an image of a pedestrian at a distance of 10 m in a 45-degree right-front direction together with a forward vehicle captured by the imaging optical system 20 . The imaging optical system 20 was able to capture the pedestrian (the portion surrounded by the circle in the image) with a sufficient size and a resolution capable of reliably recognizing the object.

Further, the inventors have found that when a distance measuring device (stereo camera) based on parallax is constructed using two cameras equipped with the imaging optical system 20 as described above, highly accurate distance measurement of an object 150 m ahead is possible. At the same time, it was confirmed that practical distance measurement was possible within a wide angle of view of 90 degrees or more for both left and right.

[Numerical Example 3]
Table 5 below shows the lens data of the imaging optical system 30 . Also, the aspherical shape of the lens is represented by the above aspherical formula using the conic coefficient k and the aspherical coefficients A ₂ , A ₄ , A ₆ . . . shown in Table 6. FIG. 9 shows curvature of field and distortion aberration of the imaging optical system 30 . The items in Tables 5 and 6, and the curvature of field and distortion in FIG.

"S-TIH14" in the column of "Materials" in Table 5 is a product name of OHARA INC. and is an optical glass. "N-BK7" is a glass material manufactured by SCHOTT AG. As in Example 2, a thin film was formed on "BOROFLOAT (registered trademark)" to form an infrared cut filter P1.

The imaging optical system 30 shown in the above lens data has a total lens length (distance from the front surface of the first lens L31 to the image plane IM) of 51.06 mm, an F value of 3.5, and a focal length of the entire system of 10.5 mm. , the maximum half angle of view was 47°, and the distortion aberration Da at the maximum half angle of view was -80%. The focal length of the entire system is in the vicinity of the optical axis, and the conversion value of the focal length from the maximum half angle of view is 1.07 mm. was

As can be seen from the curvature of field and distortion shown in FIG. 9, the imaging optical system 30 has a wide-angle field of view and intentionally creates large distortion, but the field distortion is kept small. , and achieves high imaging performance not only in the center of the high-magnification field of view, but also over the entire field of view.

10, 20, 30 Imaging optical system Gf Front group Gr Rear group G1 First group G2 Second group ST Aperture L11 to L17, L21 to L28, L31 to L38 Lens

Claims (8)

An imaging optical system comprising, in order from the object side, a front group, a diaphragm, and a rear group having positive refractive power as a whole, and forming an optical image on an image plane ,
The front group has a first group and a second group in order from the object side,
The first group comprises a first lens and a second lens located closer to the image side than the first lens, and the central portion of the first lens has positive refractive power and the peripheral portion has negative refractive power. , the center portion of the second lens has a negative refractive power and the peripheral portion has a positive refractive power, and the light incident on the center portion of the first lens from the center portion of the imaging screen is The central portion of the first lens has a positive refractive power and the central portion of the second lens has a negative refractive power. The peripheral portion of the first lens has a negative refractive power, and the peripheral portion of the second lens has a positive refractive power . The light flux on the surface is narrowed for the parallel light flux incident on the central part of the first lens from the center part of the imaging screen, and for the parallel light flux incident on the peripheral part of the first lens from the peripheral part of the imaging screen. An imaging optical system characterized by having a refractive power that spreads. 2. The imaging optical system according to claim 1, wherein the first lens and the second lens are aspherical lenses. 3. The imaging optical system according to claim 2 , wherein one of four surfaces of said first lens and said second lens is spherical. the first lens is an aspherical lens having an object-side surface that is convex toward the object side and an aspherical surface on the image side;
2. The imaging optical system according to claim 1 , wherein the second lens is an aspherical lens having aspherical surfaces on the object side and the image plane side. 5. The imaging optical system according to claim 1 , wherein the second group of the front group has a negative refractive power. 6. The imaging optical system according to any one of claims 1 to 5 , wherein the F value is 3.5 or less. 7. The imaging optical system according to claim 1 , wherein the distortion aberration Da at the maximum half angle of view satisfies "-95%<Da<-10%". an imaging optical system according to any one of claims 1 to 7 ;
An imaging device, comprising: an imaging element that converts an optical image formed by the imaging optical system into an electrical signal.

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