JP2020038401A - Optical system and imaging apparatus - Google Patents

Abstract

To provide an optical system which achieves a wide half angle of view of 45 degrees or more, while achieving the miniaturization of a lens arranged on the most object side and has a bright F-number of about 1.6.SOLUTION: The optical system comprises, in order from an object side, a first lens which has negative refractive power and whose image side surface is concave, a meniscus second lens which has negative refractive power and is convex to an image side, a third lens which has positive refractive power, a fourth lens which has negative refractive power, and a fifth lens which has positive or negative refractive power.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical system and an imaging apparatus, and particularly to an optical system suitable for an imaging apparatus using a solid-state imaging device such as a digital still camera and a digital video camera, and an imaging apparatus including the optical system.

  In recent years, imaging devices using solid-state imaging devices such as CCDs and CMOSs have become widespread. For example, in addition to an imaging device that can be carried by a user, such as a single-lens reflex camera, a mirrorless single-lens camera, and a digital still camera, a monitoring imaging device, an in-vehicle imaging device, and the like (hereinafter, simply referred to as a “monitoring imaging device”). ), Fixed installation type imaging devices that are installed and fixed to a building or a vehicle body and used for a specific purpose are also becoming widespread. In any of the imaging devices, the performance and miniaturization of the imaging devices have been remarkably advanced, and the optical systems used in these imaging devices are required to have higher performance and smaller size.

  In recent years, the number of pixels in a sensor has been increased, and a high resolution has been required, and an imaging lens which is small and lightweight, has a wide imaging range, and can cope with low illuminance has been demanded. In addition, monitoring imaging devices, security imaging devices, or in-vehicle imaging devices installed indoors and outdoors require reliability that can withstand long-term use, and at the same time, include an actuator that performs focus adjustment from the viewpoint of cost reduction. It is common to use a fixed-focus imaging lens that does not have a fixed focus. In the case of such an imaging lens, it is necessary not only to ensure the performance at room temperature but also to keep the focus position small even when the use environment temperature changes, and to maintain good performance in both high temperature environment and low temperature environment. . As an optical system that satisfies such requirements, for example, there are optical systems described in Patent Documents 1 to 3. Each of these optical systems is known as an imaging lens that has a relatively wide angle of view and is configured to have a high performance and a small size, in which a negative lens is arranged on the object side.

  Patent Literature 1 discloses a wide-angle optical system including five lenses of a negative lens, a negative lens, a positive lens, a negative lens, and a positive lens in order from the object side. The optical system described in Patent Document 1 has good optical performance and a wide angle of view, and further employs an optical system using a plastic lens to reduce the weight, size, and cost of the optical system. ing.

  Patent Literature 2 discloses an optical system including a negative lens, a positive lens, a positive lens, a negative lens, and a positive lens in order from the object side. The optical system described in Patent Literature 2 achieves a bright optical system having a relatively large aperture ratio of Fno 2.0 to Fno 4.0 by entirely using spherical glass lenses, and at the same time, has environmental resistance to fluctuations in the use environment temperature. Is increasing.

  Patent Document 3 discloses an optical system including five lenses of a negative lens, a positive lens, a negative lens, a positive lens, and a positive lens in order from the object side. In the optical system described in Patent Document 3, the relative refractive index change (dN / dT) of the glass material of the lenses disposed before and after the stop is set within a predetermined range, so that the focus can be maintained even when the use environment temperature changes. It has a small variation in position and has good environmental resistance under either high temperature environment or low temperature environment.

JP 2003-307677 A JP 2010-107532 A JP-A-2006-114648

  However, when the optical system disclosed in Patent Document 1 is assumed to be used in a wide temperature environment such as a monitoring imaging device installed indoors and outdoors, the optical system is configured with a change in the operating environment temperature. If the plastic lens deteriorates or deforms, a change in the refractive index or the like occurs, causing a reduction in resolution, and there is a problem that good optical performance cannot be obtained.

  Further, the optical system disclosed in Patent Document 2 employs all spherical glass lenses, achieves a bright optical system having a relatively large aperture ratio, and enhances environmental resistance. However, it is necessary for a monitoring imaging apparatus or the like to make the existence of the imaging apparatus inconspicuous when viewed from a person on the subject side to be observed, and it is also required to reduce the size of the optical system. In particular, it is required to reduce the outer diameter of the lens disposed closest to the subject (object side), but the optical system disclosed in Patent Document 2 cannot sufficiently reduce the size. Further, in order to enhance the visibility in a dark environment, it is necessary to use a lens having a large aperture ratio (bright Fno), but the optical system disclosed in Patent Document 2 cannot ensure sufficient brightness. Further, the half angle of view of the optical system is about 22 degrees, and a sufficient wide angle cannot be achieved. Therefore, when the optical system is applied to a monitoring imaging device or the like, there is a problem that a range in which one monitoring imaging device can capture images is narrow.

  Further, in the optical system disclosed in Patent Document 3, even if the use environment temperature changes, the focus position does not change much, and the optical system has good environmental resistance under any of a high temperature environment and a low temperature environment. If it is composed of "negative lens, positive lens, negative lens, positive lens, positive lens" in order from the object side, the chromatic aberration is reduced because the divergent and convergent light beams from the object side are alternately combined in order. From the viewpoint, good characteristics can be obtained. However, when the lenses are arranged in series and the optical system disclosed in Patent Document 3 is manufactured, the sensitivity of eccentricity is increased, so that it is difficult to adjust the arrangement of the lenses, and the variable magnification optical system is manufactured. In this case, it is difficult to secure stable optical performance, which is not preferable. Further, a bright optical system having a large aperture ratio is required to enhance visibility in a dark environment, but the Fno of the optical system is about 2.0, and sufficient brightness cannot be secured. Further, the half angle of view of the optical system is about 30 degrees, and a sufficient wide angle cannot be achieved. Therefore, similarly to Patent Document 2, there is a problem that the imageable range is narrow in the optical system.

  As can be understood from the above, an object of the present invention is to realize a wide angle of view with a half angle of view of 45 degrees or more while realizing a reduction in the diameter of the lens disposed closest to the object side, and an Fno of 1.6. The purpose is to provide a bright optical system.

    As a result of intensive studies to solve the above-mentioned problems, the inventors have found that the above-mentioned problems can be solved by adopting an optical system having the following configuration.

<Optical system according to the present invention>
The optical system according to the present invention has, in order from the object side, a first lens having a negative refractive power and a concave image side surface, and a meniscus shape having a positive or negative refractive power and convex on the image side. A second lens, a third lens having a positive refractive power and a biconvex shape, a fourth lens having a negative refractive power, and a fifth lens having a positive or negative refractive power; It is also characterized by satisfying the following condition.

R11 / f <6.0 (1)
Where R11 is the paraxial radius of curvature of the object side surface of the first lens.
f: Focal length of the entire optical system.

<Imaging device according to the present invention>
An imaging apparatus according to the present invention includes the optical system described above, and an imaging device that receives an optical image formed by the optical system and converts the optical image into an electric image signal.

  According to the present invention, it is possible to provide a bright optical system that realizes a wide angle of view with a half angle of view of 45 degrees or more while realizing a small diameter lens closest to the object side and a Fno of about 1.6. Can be.

FIG. 1 is a cross-sectional view illustrating a lens configuration example of an optical system according to a first embodiment of the present invention. FIG. 4 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 1 upon focusing on infinity. FIG. 7 is a cross-sectional view illustrating a lens configuration example of an optical system according to a second embodiment of the present invention. FIG. 10 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 2 upon focusing on infinity. FIG. 10 is a cross-sectional view illustrating a lens configuration example of an optical system according to a third embodiment of the present invention. FIG. 10 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 3 upon focusing on infinity. FIG. 13 is a cross-sectional view illustrating a lens configuration example of an optical system according to a fourth embodiment of the present invention. FIG. 13 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 4 upon focusing on infinity. FIG. 13 is a cross-sectional view illustrating a lens configuration example of an optical system according to a fifth embodiment of the present invention. FIG. 19 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 5 upon focusing on infinity. FIG. 14 is a cross-sectional view illustrating a lens configuration example of an optical system according to a sixth embodiment of the present invention. FIG. 19 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 6 upon focusing on infinity. FIG. 14 is a cross-sectional view illustrating a lens configuration example of an optical system according to Example 7 of the present invention. FIG. 19 is a diagram of spherical aberration, astigmatism, and distortion of the optical system of Example 7 upon focusing on infinity.

  Hereinafter, embodiments of an optical system and an imaging device according to the present invention will be described.

1. Optical system 1-1. Configuration of Optical System The optical system according to the present invention includes a “first lens having a negative refractive power and a concave image side surface”, a “meniscus-shaped second lens convex to the image side”, and a “positive lens. It is basically composed of a “third lens having a refractive power”, a “fourth lens having a negative refractive power”, and a “fifth lens having a positive or negative refractive power”. Hereinafter, two specific configurations included in this concept are as follows. First, the configuration of the optical system according to the present invention will be described, and the contents related to the necessary conditional expressions will be described later.

  The optical system according to the present invention includes, in order from the object side, a “first lens having a negative refractive power and a concave image side surface” and a meniscus shape having a negative refractive power and convex on the image side. The second lens includes a third lens having a positive refractive power, a fourth lens having a negative refractive power, and a fifth lens having a positive or negative refractive power. (Hereinafter, referred to as “first configuration”).

  Further, as an optical system according to the present invention, in order from the object side, a “first lens having a negative refractive power and a concave image side surface” and a “first lens having a positive or negative refractive power and convex to the image side” are used. A second lens having a meniscus shape, a third lens having a positive refractive power and a biconvex shape, a fourth lens having a negative refractive power, and a Having a “fifth lens” and satisfying the following conditional expression (1) (hereinafter, referred to as “second configuration”).

  The optical system according to the present invention described above aims at widening the angle while maintaining the lens diameter of the first lens closest to the object side to a small diameter. In such a case, it is necessary to increase the negative refractive power of the first lens G1, but it is difficult to reduce the diameter of the first lens in order to increase the negative refractive power. Therefore, by adopting a meniscus shape in which the image side surface is convex for the second lens G2, a part of the negative refracting power originally required for the first lens G1 is shared by the second lens G2, which is expected. This enables the wide angle as described above. As a result, it is possible to reduce the diameter of the lens disposed closest to the object side, to exert the effect of making the presence of the imaging device inconspicuous in a monitoring imaging device or the like, and to achieve a wide angle of view with a half angle of view of 45 degrees or more. In addition, a bright image having an Fno of about 1.6 can be obtained. In order to enhance visibility in a dark environment, it is preferable that Fno is brighter than 2.0. When this lens configuration is adopted, the first lens has a concave image side surface, and the second lens has a meniscus shape convex on the image side. Thus, a biconvex air lens is formed. Hereinafter, each lens and conditional expressions will be described.

(1) First Lens As described above, the first lens is not particularly limited as long as it has a negative refractive power and has a concave image side surface. Here, since the image side surface of the first lens is concave, the air lens inevitably formed between the first lens and the second lens having a meniscus shape convex on the image side is formed by the first lens and the second lens. By providing a biconvex air lens between them, it becomes possible to correct spherical aberration and image plane aberration better. Therefore, from the viewpoint that the optical performance of the optical system can be further improved, the image side surface of the first lens is preferably concave.

(2) Second Lens The second lens in the above-described first configuration has a negative refractive power and has a meniscus shape that is convex on the image side. The lens configuration is not particularly limited. When the second lens has this shape, a biconvex air lens is inevitably formed between the first lens and the second lens. At this time, if conditional expression (1) is satisfied, it becomes easier to increase the angle of view and further reduce the diameter of the first lens.

  As the second lens in the above-described second configuration, a lens having a positive or negative refractive power and a convex meniscus shape on the image side is used on condition that the following conditional expression (1) is satisfied. Can be. In the second configuration, when the second lens has a negative refractive power, the same function and effect as the first configuration can be obtained. When the second lens has a positive refractive power, it becomes difficult to share the negative refractive power of the first lens to the second lens. However, as the first lens, a lens having a negative refractive power as strong as possible within a range in which the diameter can be reduced is used as the first lens, and the third lens having a positive refractive power has a biconvex shape. By satisfying (1), the effect of the present invention can be obtained even when the refractive power of the second lens is positive.

  In order to ensure better optical performance, at least one of the object side surface and the image side surface of the second lens described above is preferably an aspheric surface. This is because coma aberration and field curvature can be favorably corrected by providing the second lens with an aspheric surface.

(3) Third Lens Other specific lens configurations of the third lens are not particularly limited as long as they have a positive refractive power. As the third lens, it is preferable to use a biconvex lens having both convex surfaces. The third lens having both convex surfaces can disperse a positive refractive power to the object side surface and the image side surface, and can easily satisfy conditional expression (1) described later, and can suppress deterioration of coma due to eccentricity. It is easy to secure good optical performance.

  It is preferable that at least one of the object side surface and the image side surface of the third lens is an aspheric surface. This is because, by making at least one surface of the third lens aspherical, spherical aberration can be satisfactorily corrected, and better optical performance can be obtained.

(4) Fourth Lens and Fifth Lens As described above, other specific lens configurations of the fourth lens are not particularly limited as long as they have negative refractive power. However, it is preferable that the fourth lens has a meniscus shape convex on the object side. This is because the field curvature can be corrected well and good optical performance can be secured.

  Other specific lens configurations of the fifth lens are not particularly limited as long as they have positive or negative refractive power. Whether the positive or negative refracting power is selected as the refracting power of the fifth lens is determined in consideration of the aberration of the light beam having the refracting power from the first lens to the fourth lens, and satisfies a conditional expression described later. Can be arbitrarily selected. However, in order to ensure more stable optical performance, it is preferable that the fifth lens be configured to have a positive refractive power. Also, like the fourth lens, it is preferable that the fifth lens has a meniscus shape that is convex toward the object side. This is because the field curvature can be corrected well and good optical performance can be secured.

  It is also preferable that the fourth lens and the fifth lens described above are cemented and used as a cemented lens. Adjustment of the lens arrangement when manufacturing the optical system is facilitated, and the sensitivity to eccentricity can be reduced. This is because, by using a cemented lens, a stronger combined refractive power can be obtained as compared with the refractive power when not used as a cemented lens, which can contribute to the improvement of the optical performance of the optical system according to the present invention.

(5) Lens Glass Material It is preferable that all of the first to fifth lenses constituting the optical system according to the present invention are glass lenses. Compared with plastic, it has higher thermal stability and less expansion and contraction due to temperature fluctuation. Therefore, by using glass lenses for all lenses, it is possible to satisfactorily suppress the change in the focal position even when the use environment temperature changes.

(6) Stop In the optical system according to the present invention, the position of the stop (aperture stop) is not particularly limited. For example, it can be arranged between the second lens and the third lens, between the third lens and the fourth lens, and the like. However, the closer the stop is to the image plane of the optical system, the greater the angle of incidence of the imaging light with respect to the image plane, and the more difficult it is to appropriately enter the photodiode arranged in the image sensor. As a result, it is difficult to secure an appropriate exposure, and therefore, sensitivity unevenness (shading unevenness) and peripheral coloring are generated, which is not preferable. Therefore, it is preferable to dispose an aperture between the second lens and the third lens.

1-2. Next, conditions that the optical system according to the present invention should satisfy, or conditions that it is preferable to satisfy, will be described.

1-2-1. Conditional expression (1)
In the optical system according to the present invention, in order from the object side, "a first lens having a negative refractive power and a concave image side surface" and "a meniscus having a positive or negative refractive power and convex on the image side." A second lens having a positive refractive power, a third lens having a positive refractive power and a biconvex shape, a fourth lens having a negative refractive power, and a fourth lens having a positive or negative refractive power. When “5 lenses” are satisfied, the following conditional expression (1) is required to be satisfied.

R11 / f <6.0 (1)
Here, R11 is the paraxial radius of curvature of the object side surface of the first lens.
f: Focal length of the entire optical system.

  The conditional expression (1) defines the ratio of the paraxial radius of curvature R11 of the object side surface of the first lens to the focal length f of the entire optical system. The numerical range of conditional expression (1) is defined for the following reasons. When the value of R11 / f in the conditional expression (1) is 6.0 or more, it means that the paraxial radius of curvature R11 of the object side surface of the first lens increases. In such a case, the paraxial radius of curvature of the image side surface of the first lens becomes small, and when the light reflected on the image surface enters the image side surface of the first lens, the reflected light is likely to re-enter the image surface, Ghost images are more likely to occur. Therefore, from the viewpoint of ensuring good optical performance, the range of the conditional expression (1) is defined.

  In the above conditional expression (1), only the upper limit is defined, but it is considered unnecessary to define the lower limit from the viewpoint of those skilled in the art. However, if the lower limit is determined, it is empirically 3. When the value of R11 / f is less than 3, the paraxial radius of curvature R11 on the object side surface of the first lens becomes excessively small, and it is possible to avoid the above-described ghost image formation. And it becomes difficult to secure the imaging performance. For this reason, it is more preferable that 3 <R11 / f <6.

  Furthermore, in order to reduce the curvature of field and to avoid formation of a ghost image, it is preferable that the lower limit of conditional expression (1) is 3.5, more preferably 4.0. The upper limit of conditional expression (1) is preferably 5.8, more preferably 5.7.

1-2-2. Conditional expression (2)
The following conditional expression (2) defines the half angle of view w of the entire optical system in the optical system according to the present invention.

45 ° <w <90 ° (2)
Here, w is a half angle of view of the entire optical system.

  The reason why the half angle of view of the entire optical system is defined in this way is that the optical system according to the present invention uses surface reflection ghost generated near the center of the image plane when a certain wide-angle lens is used. This is to avoid occurrence. If the half angle of view w of the entire optical system is 45 ° or less, the effect of widening the angle of view to 45 ° or more in the present invention is undesirably lost. On the other hand, the reason why the half angle of view of the entire optical system is less than 90 ° is that it is difficult to secure a half angle of view of 90 ° or more in a commonly used wide-angle lens. For the reasons described above, the range of the half angle of view w of the entire optical system is determined.

  The range of the half angle of view w in the conditional expression (2) is set so that the occurrence of the surface reflection ghost occurring near the center of the image plane can be more reliably avoided and the wide angle can be reliably achieved. The lower limit of (2) is preferably 50 °, more preferably 55 °. The upper limit of conditional expression (2) is preferably 80 °, more preferably 75 °, and even more preferably 70 °.

1-2-3. Conditional expression (3) and conditional expression (4)
It is preferable that the optical system according to the present invention satisfies the following conditional expressions (3) and (4) simultaneously.

Conditional expression (3): Conditional expression (3) is as follows.
1.0 <| f1 / f | <3.0 (3)
Here, f1: the focal length of the first lens.
f: Focal length of the entire optical system.

  This conditional expression (3) is an expression that defines the ratio between the focal length f of the entire optical system and the focal length f1 of the first lens. When conditional expression (3) is 3.0 or more, it means that the negative refractive power of the first lens is weak, and it is difficult to reduce the diameter of the first lens closest to the object. On the other hand, if the negative refracting power of the first lens is increased so that the conditional expression (3) becomes 1.0 or less, it is difficult to correct the curvature of field, which leads to deterioration of performance, which is not preferable. Therefore, conditional expression (3) is defined as described above.

  It is preferable that the diameter of the first lens disposed closest to the object side is surely reduced, and the upper limit value of conditional expression (3) is 2.5 as a range in which correction of field curvature is sure. , 2.3 and more preferably 2.2. The lower limit of conditional expression (3) is preferably 1.2, more preferably 1.3, and more preferably 1.5.

Conditional expression (4): Conditional expression (4) is as follows.
_{0.01 <d 1-2 /f<1.5 ··· (4} )
Where f is the focal length of the entire optical system.
d _1-2 : Air interval on the optical axis between the first lens and the second lens.

The conditional expression (4) includes an air gap d _1-2 on the optical axis between the first lens and the second lens, and defines a ratio between the focal length f1 of the first lens, the present inventors Is a formula that defines the distance on the optical axis between the first lens and the second lens with respect to the focal length of the entire optical system according to the above. Here, the interval on the optical axis between the first lens and the second lens means the surface interval of the air lens formed between the first lens and the second lens. By satisfying conditional expression (4), various aberrations can be satisfactorily corrected by the air lens formed between the first lens and the second lens, and an optical system having good optical performance can be obtained. Further, by satisfying the conditional expression (4), the surface distance of the air lens falls within an appropriate range, and the optical system can be made compact. If d _1-2 / f in conditional expression (4) is 0.01 or less, it becomes difficult to obtain the aberration correction effect of the air lens formed between the first lens and the second lens. In this case, it is particularly difficult to correct the curvature of field, and good optical performance cannot be obtained. On the other hand, when d _1-2 / f in conditional expression (4) is 1.5 or more, the surface distance of the air lens becomes too large, so that the entire length of the optical system becomes longer, and the size of the optical system becomes smaller. It becomes difficult. Therefore, the conditional expression (4) is defined as described above.

  The upper limit of conditional expression (4) is preferably set to 1.2 so that the surface interval of the air lens is set to a more appropriate range and the optical system can be compactly configured. , 1.0 is more preferable. The lower limit of conditional expression (4) is preferably 0.2, and more preferably 0.5.

1-2-4. Conditional expression (5)
Conditional expression (5) is an expression that defines the ratio between the focal length of the second lens and the focal length of the first lens. By satisfying the following conditional expression (5), better optical performance can be obtained in the optical system according to the present invention, and further downsizing of the optical system can be easily achieved.

5 <f2 / f1 <100 (5)
Here, f1: the focal length of the first lens.
f2: a focal length of the second lens.

  In the conditional expression (5), the case where the value of f2 / f1 is 100 or more means that the negative refractive power of the first lens is excessively strong and that the negative refractive power of the second lens is excessively weak. Can be assumed. In the former case, a bright image cannot be obtained, and it is difficult to obtain good optical performance, which is not preferable. In the latter case, the dispersion effect of the negative refractive power assumed between the first lens and the second lens is weakened. In order to ensure the performance of a wide angle of view, the lens diameter of the first lens is reduced. It is not preferable because the diameter cannot be reduced. On the other hand, the case where the value of f2 / f1 is 5 or less is defined as the case where the negative refractive power of the first lens is excessively weak and the case where the negative refractive power of the second lens is excessively strong. Can be assumed. In the former case, if the performance of a wide angle of view is to be ensured, it is not preferable because the lens diameter of the first lens cannot be reduced. In the latter case, a bright image cannot be obtained, and it is difficult to obtain good optical performance, which is not preferable. Therefore, conditional expression (5) is defined as described above.

  The negative refracting power of the first lens and the negative refracting power of the second lens are in a well-balanced range, and are defined as a range in which a brighter image can be stably obtained. Is preferably 50, and more preferably 35. The lower limit value of conditional expression (5) is preferably 10 and more preferably 15.

1-2-5. Conditional expression (6), conditional expression (7), conditional expression (8)
It is preferable that the optical system according to the present invention satisfies the following conditional expressions (6), (7) and (8) at the same time.

Conditional expression (6): Conditional expression (6) is an expression defining the ratio of the focal length of the entire optical system to the focal length of the third lens. By satisfying conditional expression (6), the positive refracting power of the third lens falls within an appropriate range, and better optical performance can be obtained. Further, it is easy to further reduce the size of the optical system. Conditional expression (6) is as follows.

1.0 <f3 / f <3.0 (6)
Here, f3 is a focal length of the third lens.
f: Focal length of the entire optical system.

  If the value of f3 / f in conditional expression (6) is 1.0 or less, the refractive power of the third lens becomes too strong. In this case, it is difficult to correct coma aberration and curvature of field, and it is difficult to ensure good optical performance. Further, since the sensitivity to eccentricity increases, it is necessary to assemble the lens with high accuracy, and the productivity is reduced. On the other hand, when the value of f3 / f in conditional expression (6) is 3.0 or more, the convergence effect of the incident light beam on the third lens is weakened, and the overall length of the optical system is increased. For this reason, it is not preferable in reducing the size of the optical system. Therefore, conditional expression (6) is defined as described above.

  In order to make the refractive power of the third lens more appropriate, to ensure better optical performance, and to ensure a range of eccentric sensitivity that does not adversely affect productivity, conditional expression (6) must be satisfied. The upper limit is preferably 2.7, more preferably 2.5, more preferably 2.3, and even more preferably 2.0. The lower limit value of conditional expression (6) is preferably 1.2, and more preferably 1.3.

Conditional expression (7): Conditional expression (7) is an expression defining the Abbe number of the third lens with respect to the d-line. By satisfying conditional expression (7), it becomes easy for the optical system according to the present invention to obtain good optical performance. Further, a glass material satisfying the conditional expression (7) often has a negative temperature coefficient of refractive index. If the third lens is formed using a glass material having a negative temperature coefficient of refractive index, it is possible to suppress a change in the focusing position of the optical system due to a change in the ambient temperature. Therefore, it is possible to make the optical system more suitable as an optical system of the installation type imaging device which is often used outdoors or the like. This will be described later. Conditional expression (7) is as follows.

νd3> 40.0 (7)
νd3: Abbe number of the third lens with respect to d-line (587.56 nm).

  If the numerical value of the conditional expression (7) is 40.0 or less, the axial chromatic aberration and the chromatic aberration of magnification deteriorate, and the optical performance of the optical system according to the present invention is undesirably reduced. In addition, many glass materials having a numerical value of conditional expression (7) of 40.0 or less have a positive temperature coefficient of refractive index, and suppress the above-described change in the focus position of the optical system due to the change in the ambient temperature. It becomes difficult. Therefore, conditional expression (7) is defined as described above.

  In the conditional expression (7), only the lower limit is specified, but it is considered unnecessary to specify the upper limit from the viewpoint of those skilled in the art. However, if an upper limit value is determined, it is empirically 90.0.

  Then, from the viewpoint of suppressing deterioration of longitudinal chromatic aberration and lateral chromatic aberration and ensuring better optical performance, more preferably 40.0 <νd3 <90.0, and still more preferably 60.0 <νd3 <. 90.0.

Conditional expression (8): Conditional expression (8) defines the relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at the d-line (587.56 nm) under a 20 ° C. environment of the third lens. . The lens expands and contracts due to a change in environmental temperature. As a result, the refractive index of the lens changes, the focal length in the optical system fluctuates, and the resolution tends to decrease. Therefore, in the optical system according to the present invention, in order to maintain high resolution in a wide range of environmental temperatures from low to high temperatures, the third lens which is considered to have the greatest influence on the refractive index change of the lens in high and low temperature environments is used. Conditional expression (8) is defined as an object. Conditional expression (8) is as follows.

dn3 / dT <6.0 × 10 ⁻⁶ / K (8)
dn3 / dT: a relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at a d-line (587.56 nm) under a 20 ° C. environment of the third lens.

When the value of dn3 / dT in conditional expression (8) is equal to or greater than 6.0 × 10 ⁻⁶ / K, the d-line of the material forming the third lens, which is the positive lens closest to the stop, is located. When the relative refractive index change due to the temperature becomes too small, the temperature characteristic is deteriorated such that the focal length changes in a high-temperature environment and a low-temperature environment, and the image resolution is undesirably reduced. Therefore, conditional expression (8) is defined as described above. The relative refractive index change due to temperature is defined as the temperature change of the refractive index in air at the same temperature as the lens material. Hereinafter, the same applies.

In the conditional expression (8), only the upper limit is defined, but it is considered unnecessary to define the lower limit from the viewpoint of those skilled in the art. However, if the lower limit is determined, it is empirically −8.0 × 10 ⁻⁶ / K. Then, from the viewpoint of suppressing deterioration of longitudinal chromatic aberration and lateral chromatic aberration and ensuring better optical performance, more preferably dn3 / dT <0.0, and still more preferably −6.0 × 10 ^−6. /K<(dn3/dT)<0.0.

1-2-6. Conditional expression (9)
Conditional expression (9) is defined as “the relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at the d-line (587.56 nm) under the 20 ° C. environment of the third lens” and “the 20 ° C. environment of the second lens. Below, it is an equation that defines the ratio to the relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at the d-line (587.56 nm). In the case of the optical system according to the present application, a third lens is provided as a positive lens closest to the stop, and a second lens is provided as a negative lens closest to the stop. By satisfying conditional expression (9), higher resolution can be obtained even in a high-temperature atmosphere or a low-temperature atmosphere. Conditional expression (9) is as follows.

−5.0 <(dn3 / dT) / (dn2 / dT) <20.0 (9)
Here, dn3 / dT is a relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at d-line (587.56 nm) in a 20 ° C. environment of the third lens.
dn2 / dT: a relative refractive index temperature coefficient (1 × 10 ⁻⁶ / K) at d-line (587.56 nm) under a 20 ° C. environment of the second lens.

  As for the value of (dn3 / dT) / (dn2 / dT) in the conditional expression (9), each value of dn3 / dT and dn2 / dT is a positive value as long as the value satisfies the range of the conditional expression (9). It can be a negative number. However, in the optical system of the present invention, when the second lens has a negative refractive power, the third lens has a positive refractive power, and the relative refractive index temperature coefficients are different between positive and negative, Temperature characteristics tend to deteriorate. On the other hand, the temperature coefficient of relative refractive index (dn3 / dT) of the third lens having a positive refractive power has a negative value according to the conditional expression (8). In such a case, it is more preferable that the relative refractive index temperature coefficient (dn3 / dT) of the second lens is also a negative value in order to suppress the fluctuation of the temperature characteristic. On the other hand, when the value of the conditional expression (9) is 20.0 or more, if the difference between the relative refractive index temperature coefficient of the third lens and the relative refractive index temperature coefficient of the second lens becomes too large, the temperature characteristic will be reduced. Deteriorates, which is not preferable. Therefore, conditional expression (9) is defined as described above.

  In the conditional expression (9), it is most preferable that both the “relative refractive index temperature coefficient of the third lens” and the “relative refractive index temperature coefficient of the second lens” have negative values. In order to obtain extremely stable and high resolution even in a high-temperature atmosphere or a low-temperature atmosphere, it is more preferable that -5 <(dn3 / dT) / (dn2 / dT) <15, and still more preferable that 0 <(dn3 / DT) / (dn2 / dT) <5.

1-2-7. Conditional expression (10)
Conditional expression (10) is an expression that defines the ratio of the focal length of the entire optical system to the focal length of the fourth lens having negative refractive power. By satisfying conditional expression (10), better optical performance can be imparted to the optical system according to the present invention, and further downsizing of the optical system can be easily achieved. Conditional expression (10) is as follows.

1.0 <| f4 / f | <3.5 (10)
Here, f4 is a focal length of the fourth lens.
f: Focal length of the entire optical system.

  If the numerical value of | f4 / f | in the conditional expression (10) is 1.0 or less, the refractive power of the fourth lens becomes too strong, and it becomes difficult to correct the field curvature. Therefore, it becomes difficult to obtain good optical performance. On the other hand, when the value of | f4 / f | in conditional expression (10) is 3.5 or more, the refractive power of the fourth lens is weak, and the diverging effect of the incident light beam on the fourth lens is weak. Therefore, in order to secure a desired image height, it is necessary to lengthen the entire length of the optical system, and it is difficult to reduce the size of the optical system. Therefore, conditional expression (10) is defined as described above.

  In conditional expression (10), the upper limit of conditional expression (10) is preferably 3.0 in order to obtain extremely stable and high resolution even in a high-temperature atmosphere or a low-temperature atmosphere. It is preferably 8, and more preferably 2.5. The lower limit of conditional expression (10) is preferably 1.2, more preferably 1.5, and still more preferably 1.8.

1-2-7. Conditional expression (11), conditional expression (12)
In the optical system according to the present invention, assuming that the fourth lens and the fifth lens are cemented, it is preferable that conditional expressions (11) and (12) described below are simultaneously satisfied.

Conditional expression (11): Conditional expression (11) is an expression that defines a ratio between a combined focal length when the fourth lens and the fifth lens are cemented and a focal length of the entire optical system. . By satisfying conditional expression (11), better optical performance can be imparted to the optical system according to the present invention, and further downsizing of the optical system can be easily achieved. Conditional expression (11) is as follows.

f45 / f <200 (11)
Here, f45 is a combined focal length of the fourth lens and the fifth lens.
f: Focal length of the entire optical system.

  In conditional expression (11), if the value of f45 / f is not smaller than 200, the combined refractive power of the cemented lens will be weak, and the significance of joining the fourth lens and the fifth lens will be lost. It is not preferable because it cannot contribute to the improvement of the optical performance of the optical system according to the present invention. Therefore, conditional expression (11) is defined as described above. In the above conditional expression (11), only the upper limit is specified, but from the viewpoint of those skilled in the art, it is not necessary to specify the lower limit.

  Then, in the optical system according to the present invention, from the viewpoint of providing better optical performance and facilitating further miniaturization of the optical system, -200 <f45 / f <0, and more preferably -150 <. f45 / f <0.

Conditional expression (12): Conditional expression (12) is defined as “the average linear expansion coefficient of the glass material used for the fourth lens from −30 ° C. to 70 ° C.” and “from -30 ° C. of the glass material used for the fifth lens. It is an equation that defines the ratio with the “average coefficient of linear expansion at 70 ° C.”. It is preferable that all of the first to fifth lenses constituting the optical system according to the present invention are glass lenses. When the fourth lens and the fifth lens are joined, an organic component (adhesive) is interposed between the fourth lens and the fifth lens. The organic component has a higher linear expansion coefficient than glass, and its volume is apt to change due to a change in the use environment temperature. By satisfying conditional expression (12), a stable joint state between the fourth lens and the fifth lens can be obtained even when the use environment temperature changes. Conditional expression (12) is as follows.

| Α4-α5 | <50 × 10 ⁻⁷ / K (12)
Where α4 is the average linear expansion coefficient (1 × 10 ⁻⁷ / K) of the glass material used for the fourth lens from −30 ° C. to 70 ° C.
α5: Average coefficient of linear expansion (1 × 10 ⁻⁷ / K) at −30 ° C. to 70 ° C. of the glass material used for the fifth lens.

The difference between the average linear expansion coefficient of the glass material used for the fourth lens at −30 ° C. to 70 ° C. and the average linear expansion coefficient of the glass material used for the fifth lens at −30 ° C. to 70 ° C. is 50 × 10 ^{−. If it is 7} or more, the possibility of peeling of the joint due to expansion and contraction behavior due to a temperature change in an assumed usage environment of -30 ° C to 70 ° C is not preferable. Therefore, conditional expression (12) is defined as described above.

2. Imaging device Next, an imaging device according to the present invention will be described. An imaging apparatus according to the present invention includes the optical system according to the present invention, and an imaging device that receives an optical image formed by the optical system and converts the optical image into an electric image signal.

  Here, the image sensor is not particularly limited, and a solid-state image sensor such as a CCD sensor (Charge Coupled Device) or a CMOS sensor (Complementary Metal Oxide Semiconductor) can be used. The imaging device according to the present invention is suitable for an imaging device using these solid-state imaging devices such as a digital camera and a video camera. Further, the imaging device may be a lens-fixed imaging device in which a lens is fixed to a housing, or may be an interchangeable lens imaging device such as a single-lens reflex camera or a mirrorless single-lens camera. Of course.

  In particular, the imaging device according to the present invention is suitable for an installation-fixed imaging device that is fixed to a building or a vehicle body and used for a specific purpose, such as a monitoring imaging device. An imaging device for these uses is required to make the existence of the imaging device inconspicuous from the subject side. The optical system according to the present invention realizes a reduction in the diameter of the lens disposed closest to the object side, and simultaneously realizes a wide angle of view at a half angle of view of 45 degrees or more, and obtains a bright image with an Fno of about 1.6. Can be. Therefore, the monitoring imaging device including the optical system according to the present invention has an inconspicuous appearance, but can capture a wide range, and can obtain a clear image even in a low light amount such as at night. Will be provided.

  Next, the present invention will be specifically described with reference to examples. However, the present invention is not limited to the following embodiments. The optical system of each embodiment described below is an imaging optical system used for an imaging device (optical device) such as a digital camera, a video camera, and a silver halide film camera. It can be preferably applied to an imaging device. Further, in each lens cross-sectional view, the left side is the object side and the right side is the image plane side in the drawing.

(1) Configuration of Optical System The optical system of the first embodiment is shown in FIG. As can be understood from FIG. 1, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a negative refractive power and a convex surface on the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, the fifth lens G5 having a positive refractive power has a meniscus shape convex on the object side. Further, the fourth lens G4 and the fifth lens G5 in the first embodiment are cemented lenses, and their combined refractive power is negative.

  In the first embodiment, the aperture stop SP is disposed between the second lens G2 and the third lens G3. The aperture stop SP limits the diameter (light amount) of a light beam incident from the object side to the image plane IP side. An optical block G is arranged between the fifth lens G5 and the image plane IP. The optical block G corresponds to an optical filter, a face plate, a crystal low-pass filter, an infrared cut filter, and the like.

  When an imaging apparatus is configured using the optical system according to the first embodiment, the image plane IP corresponds to an imaging plane of a solid-state imaging device. As the solid-state imaging device, a photoelectric conversion device such as the above-described CCD sensor and CMOS sensor can be used. In the imaging apparatus, light incident from the object side of the imaging lens of the present embodiment finally forms an image on the imaging surface of the solid-state imaging device. Then, the light received by the solid-state imaging device is photoelectrically converted and output as an electric signal, thereby generating a digital image corresponding to the image of the subject. The digital image can be recorded on a recording medium such as a hard disk device (HDD), a memory card, an optical disk, or a magnetic tape. When the imaging device is a silver halide film camera, the image plane IP corresponds to the film plane. When the imaging device is a silver halide film camera, the image plane IP corresponds to a film plane. Note that the notation of SP, IP, G, and the like is the same in each lens cross-sectional view shown in the following examples, and therefore, description thereof will be omitted below.

(2) Numerical Examples Numerical examples in which specific numerical values of the optical system adopted in the first embodiment are applied will be described. Table 1 shows lens data of the optical system. In Table 1 (1A), “surface number” is the number of the lens surface counted from the object side to the image surface side, and “r (mm)” is the radius of curvature of the lens surface (however, the value of r is ∞). The surface indicates that the surface is a flat surface.), “D” is the distance on the optical axis between the lens surface of the i-th (i is a natural number) from the object side and the (i + 1) -th lens surface, “Nd” indicates the refractive index for the d-line (wavelength λ = 587.56 nm), and “νd” indicates the Abbe number for the d-line. However, when the lens surface is an aspherical surface, “*” is added before the surface number in the table below. In the case of an aspherical surface, the column of “r” indicates the paraxial radius of curvature. “INF” in the table represents infinity.

  Table 1 (1B) shows various data of the optical system. Specifically, the focal length (mm), F number (Fno value), half angle of view (w / °), image height (mm), total lens length (mm), back focus (BF / mm) of the optical system Is shown. Here, the total length of the lens is defined as a back focus from the object side surface of the first lens to the distance on the optical axis from the lens closest to the image side in the subsequent lens group, here, the image side surface of the fifth lens. This is the added value. The back focus is a value obtained by converting the distance from the image side surface of the fifth lens G5 to the paraxial image surface into air.

Table 1 (1C) shows the aspherical surface coefficients when the shape of the aspherical surface (*) shown in Table 1 (1A) is defined by the following equation. The aspherical coefficient can be expressed by the following aspherical expression, using the displacement in the optical axis direction at a position at a height h from the optical axis as a surface vertex reference. In Table 1 (1C), “ ^Ea ” means “× 10 ^−a ”.

  Table 1 (1E) shows the focal length (f1) of the first lens, the focal length (f2) of the second lens, the focal length (f3) of the third lens, and the focal length of the third lens that constitute the optical system used in the first embodiment. The focal length of the four lenses (f4), the focal length of the fifth lens (f5), and the combined focal length of the fourth and fifth lenses (f45) are shown.

z = ch ² / [1+ {1- (1 + k) c ² h ² } ^1/2 ] + A4h ⁴ + A6h ⁶ + A8h ⁸ + A10h ¹⁰・ ・ ・
Here, c is the curvature (1 / r), h is the height from the optical axis, k is the cone coefficient (conic constant), and A4, A6, A8, A10... Are the aspherical coefficients of each order. The notation “E ± m” (m represents an integer) in the numerical values of the aspheric coefficient and the conic constant means “× 10 ^{± m} ”.

Table 1 (1D) shows the relative refractive index temperature coefficient (unit: 1 × 10 ⁻⁶ / K) at d-line (587.56 nm) under a 20 ° C. environment of the glass material used for the second lens and the third lens. The average linear expansion coefficient (unit: 1 × 10 ⁻⁷ / K) at −30 ° C. to 70 ° C. of the glass material used for the fourth lens and the fifth lens is described.

  Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. Matters relating to each of these tables are the same in each of the tables shown in the other embodiments, and therefore, redundant description is omitted below.

  FIG. 2 is a longitudinal aberration diagram of the optical system according to the first embodiment when focused on infinity. The longitudinal aberration diagram shown in FIG. 2 shows spherical aberration (SA / mm), astigmatism (AST / mm), and distortion (DIS /%) in order from the left side in the drawing.

  The vertical axis of the spherical aberration diagram represents the F number (Fno). In addition, the spherical aberration at the d-line (wavelength 587.56 nm) is shown by a solid line, the spherical aberration at the C line (wavelength 656.27 nm) is shown by a long dashed line, and the spherical aberration at the g-line (435.84 nm) is shown by a short dashed line.

  The vertical axis of the astigmatism diagram represents the image height (y). The graph also shows the astigmatism of the sagittal ray ΔS (solid line) and the meridional ray ΔM (dashed line) at the d-line (wavelength: 587.56 nm).

  The vertical axis of the distortion diagram represents the image height (y). Further, the distortion at the d-line (wavelength: 587.56 nm) is shown by a solid line. The matters relating to these longitudinal aberration diagrams are the same in the longitudinal aberration diagrams shown in the other examples, and therefore, the following repeated description will be omitted.

(1) Configuration of Optical System FIG. 3 shows an optical system according to the second embodiment. As can be understood from FIG. 3, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a negative refractive power and a convex surface on the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, a double-sided convex lens is used as the fifth lens G5 having a positive refractive power. Further, the fourth lens G4 and the fifth lens G5 in Example 2 are cemented lenses, and their combined refractive power is negative. In the second embodiment, as in the first embodiment, the aperture stop SP is disposed between the second lens G2 and the third lens G3.

(2) Numerical Embodiment A numerical embodiment in which specific numerical values of the optical system adopted in the second embodiment are applied will be described. Hereinafter, since it is the same as that of the first embodiment, redundant description is omitted as much as possible. Table 2 (2A) shows lens data of the optical system, Table 2 (2B) shows various data of the optical system, and Table 2 (2C) shows aspherical aspheric surfaces shown in Table 2 (2A). Table 2 (2D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 2 (2E) lists the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 4 is a longitudinal aberration diagram when the optical system is focused on infinity.

(1) Configuration of Optical System FIG. 5 shows an optical system according to the third embodiment. As can be understood from FIG. 5, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a negative refractive power and convex toward the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, the fifth lens G5 having a positive refractive power has a meniscus shape convex on the object side. The fourth lens G4 and the fifth lens G5 in the third embodiment are cemented lenses, and the combined refractive power is positive. In the third embodiment, as in the first embodiment, the aperture stop SP is disposed between the second lens G2 and the third lens G3.

(2) Numerical Embodiment A numerical embodiment in which specific numerical values of the optical system adopted in the third embodiment are applied will be described. Hereinafter, since it is the same as that of the first embodiment, redundant description is omitted as much as possible. Table 3 (3A) shows lens data of the optical system, Table 3 (3B) shows various data of the optical system, and Table 3 (3C) shows an aspherical aspheric surface shown in Table 3 (3A). Table 3 (3D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 3 (3E) lists the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 6 is a longitudinal aberration diagram when the optical system is focused on infinity.

(1) Configuration of Optical System FIG. 7 shows an optical system according to the fourth embodiment. As can be understood from FIG. 7, the optical system has a negative refractive power in order from the object side and sequentially from the object side, and has a first lens G1 having a concave image side surface and a negative refractive power. A second lens G2 having a meniscus shape convex on the image side, a third lens G3 having a positive refractive power, a fourth lens G4 having a negative refractive power, and a fifth lens G5 having a positive refractive power. It is composed of In this case, a double-sided convex lens is used as the fifth lens G5 having a positive refractive power. Further, the fourth lens G4 and the fifth lens G5 in the fourth embodiment are used without being joined, and the combined refractive power is positive. In the fourth embodiment, as in the first embodiment, the aperture stop SP is disposed between the second lens G2 and the third lens G3.

(2) Numerical Embodiment A numerical embodiment in which specific numerical values of the optical system employed in the fourth embodiment are applied will be described. Hereinafter, since it is the same as that of the first embodiment, redundant description is omitted as much as possible. Table 4 (4A) shows lens data of the optical system, Table 4 (4B) shows various data of the optical system, and Table 4 (4C) shows aspherical aspheric surfaces shown in Table 4 (4A). Table 4 (4D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 4 (4E) shows the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, which are the same as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 8 is a longitudinal aberration diagram when the optical system is focused on infinity.

(1) Configuration of Optical System FIG. 9 shows an optical system according to the fifth embodiment. As can be understood from FIG. 9, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a positive refractive power and a convex surface on the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, a double-sided convex lens is used as the fifth lens G5 having a positive refractive power. In addition, the fourth lens G4 and the fifth lens G5 in the fifth embodiment are used without being joined, and the combined refractive power is positive. In the fourth embodiment, as in the first embodiment, the aperture stop SP is disposed between the second lens G2 and the third lens G3.

(2) Numerical Embodiment A numerical embodiment in which specific numerical values of the optical system adopted in the fifth embodiment are applied will be described. Hereinafter, since it is the same as that of the first embodiment, redundant description is omitted as much as possible. Table 5 (5A) shows lens data of the optical system, Table 5 (5B) shows various data of the optical system, and Table 5 (5C) shows aspherical aspheric surfaces shown in Table 5 (5A). Table 5 (5D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 5 (5E) lists the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 10 is a longitudinal aberration diagram when the optical system is focused on infinity.

(1) Configuration of Optical System FIG. 11 shows an optical system according to the sixth embodiment. As can be understood from FIG. 11, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a negative refractive power and a convex surface on the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, the fifth lens G5 having a positive refractive power has a meniscus shape convex on the object side. In addition, the fourth lens G4 and the fifth lens G5 in the sixth embodiment are cemented lenses, and their combined refractive power is negative.

(2) Numerical Examples Next, numerical examples using specific numerical values of the optical system will be described. Table 6 (6A) shows lens data of the optical system, Table 6 (6B) shows various data of the optical system, and Table 6 (6C) shows aspherical aspheric surfaces shown in Table 6 (6A). Table 6 (6D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 6 (6E) lists the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, which are the same as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 12 shows a longitudinal aberration diagram when the optical system is focused on infinity.

(1) Configuration of Optical System FIG. 13 shows an optical system according to the seventh embodiment. As can be understood from FIG. 13, the optical system has, in order from the object side, a first lens G1 having a negative refractive power and a concave image side surface, and a first lens G1 having a negative refractive power and convex toward the image side. The second lens G2 has a meniscus shape, the third lens G3 has a positive refractive power, the fourth lens G4 has a negative refractive power, and the fifth lens G5 has a positive refractive power. . In this case, the fifth lens G5 having a positive refractive power has a meniscus shape convex on the object side. Further, the fourth lens G4 and the fifth lens G5 in the seventh embodiment are cemented lenses, and their combined refractive power is negative.

(2) Numerical Examples Next, numerical examples using specific numerical values of the optical system will be described. Table 7 (7A) shows lens data of the optical system, Table 7 (7B) shows various data of the optical system, and Table 7 (7C) shows an aspherical aspheric surface shown in Table 7 (7A). Table 7 (7D) shows the relative refractive index temperature coefficient of the glass material used for the second lens and the third lens, and the average linear expansion coefficient of the glass material used for the fourth lens and the fifth lens. Table 7 (7E) shows the focal lengths (f1 to f5) of the first to fifth lenses and the combined focal length (f45) of the fourth and fifth lenses, which are the same as in Example 1. . Table 8 shows numerical values of the conditional expressions (1) to (13) of the optical system. FIG. 14 is a longitudinal aberration diagram when the optical system is focused on infinity.

  The optical system according to the present invention enables observation and imaging of a wide range by widening the angle of view to a half angle of view of 45 degrees or more while reducing the diameter of the lens closest to the object side, and has an Fno of about 1.6. To obtain a bright image. Therefore, a monitoring imaging device or the like employing this optical system can exert an effect of making the presence of the imaging device inconspicuous when viewed from the side of a subject to be observed.

G1 to G5 First to fifth lenses SP Aperture G Optical block IP Image plane

Claims (14)

In order from the object side, a first lens having a negative refractive power and a concave image side surface, a second meniscus-shaped lens having a positive or negative refractive power and convex on the image side, and a positive refractive power A third lens having a biconvex shape, a fourth lens having a negative refractive power, and a fifth lens having a positive or negative refractive power, and satisfying the following conditions: Optical system.
R11 / f <6.0 (1)
Where R11 is the paraxial radius of curvature of the object side surface of the first lens.
f: Focal length of the entire optical system. The optical system according to claim 1, wherein the following conditional expression is satisfied.
45 ° <w <90 ° (2)
Here, w is a half angle of view of the entire optical system. The optical system according to claim 1, wherein the following conditional expression is satisfied.
1.0 <| f1 / f | <3.0 (3)
_{0.01 <d 1-2 /f<1.5 ··· (4} )
However, f1: the first lens focal length d _1-2: an air space on the optical axis between the first lens and the second lens. 4. The optical system according to claim 1, wherein the second lens has a negative refractive power and satisfies the following conditional expression. 5.
5 <f2 / f1 <100 (5)
Here, f1: focal length of the first lens f2: focal length of the second lens The optical system according to claim 1, wherein the following conditional expression is satisfied.
1.0 <f3 / f <3.0 (6)
νd3> 40.0 (7)
(Dn3 / dT) <6.0 × 10 ⁻⁶ / K (8)
Where f3: focal length of the third lens. Νd3: Abbe number dn3 / dT for d-line (587.56 nm) of the third lens: d-line (587.56 nm) in a 20 ° C. environment of the third lens. Temperature coefficient of relative refractive index (1 × 10 ⁻⁶ / K)
It is. The optical system according to claim 5, wherein the following conditional expression is satisfied.
−5.0 <(dn3 / dT) / (dn2 / dT) <20.0 (9)
Where dn2 / dT: temperature coefficient of relative refractive index (1 × 10 ⁻⁶ / K) at d-line (587.56 nm) in a 20 ° C. environment of the second lens.
It is. The optical system according to claim 1, wherein the following conditional expression is satisfied.
1.0 <| f4 / f | <3.5 (10)
Here, f4 is a focal length of the fourth lens.   The optical system according to any one of claims 1 to 7, wherein the second lens has at least one aspheric surface.   The optical system according to claim 1, wherein the third lens has at least one aspheric surface.   The optical system according to any one of claims 1 to 9, wherein the fourth lens has a meniscus shape convex toward the object side, and the fifth lens has a meniscus shape convex toward the object side.   The optical system according to claim 1, wherein each of the first to fifth lenses is a glass lens. The optical system according to any one of claims 1 to 11, wherein the fourth lens and the fifth lens are cemented and satisfy the following conditional expression.
f45 / f <200 (11)
| Α4-α5 | <50 × 10 ⁻⁷ / K (12)
Where f45 is a composite focal length of the fourth lens and the fifth lens.
α4: Average linear expansion coefficient of glass material used for the fourth lens at −30 ° C. to 70 ° C. (1 × 10 ⁻⁷ / K)
α5: Average linear expansion coefficient of glass material used for the fifth lens at −30 ° C. to 70 ° C. (1 × 10 ⁻⁷ / K)
It is.   The optical system according to claim 1, wherein the fifth lens has a positive refractive power.   An imaging apparatus comprising: the optical system according to any one of claims 1 to 13; and an imaging device that receives an optical image formed by the optical system and converts the optical image into an electric image signal. .

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