Optical system and image capturing device

Abstract

To provide a compact, light-weight, high-performance optical system which offers an open F-number Fno of 2.0 or brighter and is suitable for compact image capturing systems, and to provide an image capturing device.SOLUTION: An optical system provided herein comprises, in order from the object side, an object-side lens group GF configured to be stationary in an optical axis direction while focusing, a middle group GM consisting of just one or more focusing groups (GN, GP) configured to move in the optical axis direction while focusing, and an image-side lens group GR configured to be stationary in the optical axis direction while focusing, and is configured to move at least one of the focusing groups toward the image side to shift focus from an object at infinity to an object at a finite distance. The image-side lens group GR includes an air lens having negative refractive power. An aperture stop S for determining the axial ray diameter is provided on the object side of the air lens. The optical system satisfies given conditional expressions. Also provided is an image capturing device equipped with such optical system and an image sensor.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical system and an imaging device, and more particularly to an optical system and an imaging device suitable for a small imaging device using a solid-state imaging element or the like. [Background technology]

[0002] BACKGROUND ART Imaging devices using solid-state imaging elements, such as digital still cameras and digital video cameras, have become widespread. As the number of pixels of the solid-state imaging elements used in these imaging devices increases, optical systems are being required to have high resolution performance while maintaining a small size and light weight.

[0003] Furthermore, as solid-state imaging devices become larger, there is a growing demand for greater blurring, which in turn necessitates the need for large-aperture lenses with a shallow depth of field and a maximum aperture of Fno. 2.0 or faster.

[0004] As a large-aperture lens with high resolution performance, for example, an optical system has been proposed that is composed of a first lens group, a second lens group with positive refractive power, and a third lens group with positive refractive power, and has an Fno of 1.8 or brighter (see Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-161646 Summary of the Invention [Problem to be solved by the invention]

[0006] Although the imaging device disclosed in Patent Document 1 achieves high optical performance, the optical system disclosed in Patent Document 1 has a long flange back and the focusing group is heavy, which is undesirable in terms of miniaturizing the entire lens barrel.

[0007] An object of the present invention is to provide an optical system and an imaging device that are small, lightweight, high performance, and have a large aperture that is brighter than Fno. 2.0 at full aperture, suitable for a small imaging system. [Means for solving the problem]

[0008] In order to achieve the above object, an optical system according to the present invention comprises, in order from the object side, an object-side lens group that is fixed in the optical axis direction during focusing, a central group that is made up of only one or more focusing groups that move in the optical axis direction during focusing, and an image-side lens group that is fixed in the optical axis direction during focusing, wherein at least one of the focusing groups moves toward the image side during focusing from an object at infinity to an object at a finite distance, the image-side lens group has an air lens with negative refractive power, an aperture stop for determining the diameter of an axial light beam is located closer to the object than the air lens, a lens that forms the object-side surface of the air lens has negative refractive power, and an image-side lens that forms the air lens has negative refractive power, The following conditional expressions (1), (3), and (4) are satisfied: At least one of the focusing groups must satisfy the following condition (6): It is characterized by: 0.23 <(FB×tanθm) / (f×tanω)< 0.50 (1) 0.50 < Dr / (f×tanω)< 1.80 ···(3) 0.92 < f / Fno / Ds < 1.79 (4) 1.20 < βN < 4.00 (6) however, FB: Air-equivalent length from the surface closest to the image side of the optical system to the image plane θm: the angle of incidence of the axial marginal ray on the image plane at the maximum aperture Fno when the optical system is focused at infinity f: focal length of the optical system when focused at infinity ω: Maximum angle of view of the optical system when focused at infinity Dr: Distance from the image side of the air lens to the image plane Fno: The maximum aperture of the optical system when focused at infinity Ds: diameter of the aperture stop at full aperture Fno when the optical system is focused at infinity βN: Lateral magnification of the focusing group when focused at infinity

[0009] In addition, in order to solve the above problem, the imaging device of the present invention is characterized by comprising the optical system described above and an imaging element that receives an optical image formed by the optical system and converts it into an electrical image signal. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a small, lightweight, high-performance, large-aperture optical system and an imaging device with a maximum aperture of Fno greater than 2.0 that are suitable for small imaging systems. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing an example of a lens configuration of an optical system according to a first embodiment of the present invention. [Figure 2] 3A to 3C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 1 of the present invention. [Figure 3] FIG. 4 is a cross-sectional view showing an example of the lens configuration of an optical system according to a second embodiment of the present invention. [Figure 4] 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 2 of the present invention. [Figure 5] FIG. 10 is a cross-sectional view showing an example of the lens configuration of an optical system according to a third embodiment of the present invention. [Figure 6] 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 3 of the present invention. [Figure 7] FIG. 10 is a cross-sectional view showing an example of the lens configuration of an optical system according to Example 4 of the present invention. [Figure 8] 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 4 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of an optical system and an imaging device according to the present invention will be described.

[0013] 1.Optical system 1-1.Optical configuration of the optical system First, an embodiment of the optical system according to the present invention will be described. The optical system of this embodiment is composed of, in order from the object side, an object-side lens group, a central group, and an image-side lens group.

[0014] In this optical system, focusing from an object at infinity to an object at a finite distance is performed by the central group. Furthermore, the object-side lens group and the image-side lens group are fixed in the optical axis direction during focusing. Lenses located on the object side of a large-diameter lens tend to have large diameters. Furthermore, lenses close to the image plane also tend to have relatively large diameters. For this reason, it is easier to achieve a reduction in the overall size and weight of the optical system, including its lens barrel configuration, by fixing the lens groups that tend to have large diameters in the optical axis direction during focusing and performing focusing with the central group, which has a relatively small diameter.

[0015] In this optical system, the image-side lens group has an air lens with negative refractive power. Therefore, at least two lenses are arranged in the image-side lens group to form the air lens. Furthermore, since the air lens has negative refractive power, the air lens has a convex shape. By having the image-side lens group arranged close to the image plane have an air lens with negative refractive power, it is possible to obtain the effect of diverging light rays, making it easier to reduce the diameter of the image-side lens group.

[0016] In this optical system, the aperture stop that determines the diameter of the axial light beam is located closer to the object than the air lens with negative refractive power in the image-side lens group. This means that the air lens with negative refractive power is located closer to the image side than the aperture stop, and the exit pupil position is closer to the image plane, making it easier to reduce the diameter.

[0017] The optical configuration of the optical system will be described in more detail below.

[0018] (1) Object-side lens group The object-side lens group is positioned closer to the object than the central group and is fixed in the optical axis direction during focusing. The refractive power of the entire object-side lens group may be positive or negative. If the object-side lens group has positive refractive power, the focused light rays are incident on the central group, making it easy to reduce the diameter of the central group. If the object-side lens group has negative refractive power, a diffusion effect occurs on the object side, and the entrance pupil position is on the object side, making it easy to achieve both a wide angle and a small outer diameter.

[0019] Although the specific configuration of the object-side lens group is not particularly limited, it is preferable that the object-side lens group has at least one lens element with negative refractive power whose image-side surface is concave toward the image side, and that the lens element closer to the object than the lens with the strongest refractive power among the negative refractive power elements has a combined positive refractive power. This allows the object-side lens group to be configured as a telephoto system. As a result, it becomes easier to achieve both telephoto and large apertures.

[0020] When a lens having positive refractive power is disposed closest to the object in the object-side lens group, the lens has a condensing effect on the object side of the optical system, which acts to lower the height of light rays. As a result, it becomes easier to reduce the amount of aberration caused by manufacturing errors, and it becomes easier to achieve good imaging performance of the optical system.

[0021] If a lens whose object-side surface faces convexly toward the object is located closest to the object in the object-side lens group, a light-converging effect occurs at the object-side of the optical system, which has the effect of lowering the height of light rays. As a result, it becomes easier to reduce the amount of aberration caused by manufacturing errors, and it becomes easier to achieve good imaging performance in the optical system.

[0022] (2) Central group The central group is disposed between the object-side lens group and the image-side lens group. The refractive power of the entire central group may be either positive or negative. When the entire central group has positive refractive power, the central group has a light-condensing effect, making it easy to increase the aperture of the optical system. When the entire central group has negative refractive power, it is easy to reduce the amount of movement during focusing, which is preferable in terms of size reduction.

[0023] The central group is composed of only one or more focusing groups that move along the optical axis during focusing. The central group does not include any lenses (lens groups) that are fixed in the optical axis direction during focusing. Apart from this, the specific configuration of the central group is not particularly limited. The number of focusing groups that make up the central group is not limited. When the central group consists of one focusing group, the number of mechanical parts required to move the focusing group along the optical axis during focusing is reduced, making it easier to achieve low costs, compactness, and lightweight design. When the central group consists of two or more focusing groups, it is possible to reduce aberration fluctuations during focusing, making it easier to achieve high performance. Each focusing group is composed of one or more lenses that move the same distance and direction during focusing.

[0024] The central group has at least one focusing group that moves toward the image when focusing from an object at infinity to an object at a finite distance. If the lateral magnification of the focusing group is βN and the combined lateral magnification of the portion of the focusing group closer to the image is βR, the focusing sensitivity of the focusing group can be expressed as (1-βN2)×βR2. Therefore, if a focusing group moves toward the image when focusing from an object at infinity to an object at a finite distance, the lateral magnification of that focusing group will be greater than 1. Focusing using a focusing group with a lateral magnification greater than 1 enables a telephoto lens to be made larger and more compact, making it easier to realize an optical system with a small telephoto ratio.

[0025] Furthermore, it is preferable that the composite lateral magnification of the lens positioned closer to the image side than the focusing group that moves toward the image side when focusing from an object at infinity to an object at a finite distance is smaller than 1. When the composite lateral magnification of the lens positioned closer to the image side than the focusing group that moves toward the image side is smaller than 1, this has the effect of brightening the image side of the optical system, making it easier to achieve a larger aperture.

[0026] When the central group is made up of multiple focusing groups, it is preferable that they include focusing groups with refractive powers of different signs. That is, rather than being made up of focusing groups all having refractive powers of the same sign, it is preferable that the central group include at least one focusing group with positive refractive power and one focusing group with negative refractive power. Furthermore, when the central group is made up of multiple focusing groups, it is preferable that the central group include not only a focusing group that moves toward the image when focusing from an object at infinity to an object at a finite distance, but also a focusing group that moves toward the object. By configuring the central group in this way with focusing groups with refractive powers of different signs or focusing groups that move in moving directions of different signs, it is possible to cancel out aberrations during focusing while reducing the amount of movement, making it easier to achieve both high performance and compactness.

[0027] It is also preferable that at least one of the focusing groups has a surface closest to the image side that is concave toward the image side. It is also preferable that at least one of the focusing groups has a meniscus shape formed by the surface closest to the object side and the surface closest to the image side. This means that the surface closest to the object side and the surface closest to the image side of at least one of the focusing groups have radii of curvature with the same sign. As long as the signs of the radii of curvature are the same, the signs can be either positive or negative. It is even more preferable that the surface closest to the object side has a convex shape toward the object side and the surface closest to the image side has a concave shape toward the image side. Having one of these shapes can reduce off-axis aberration fluctuations during focusing, making it easier to achieve high performance.

[0028] While there is no limit to the number of lenses in each focusing group constituting the central group, it is preferable that each focusing group (or at least one focusing group) be composed of a single lens unit. Here, a single lens unit refers to a lens unit such as a single lens or a cemented lens in which multiple single lenses are integrated without any air gap. That is, even if a single lens unit has multiple optical surfaces, only the most object-side and most image-side surfaces are in contact with air, and the other surfaces are not in contact with air. Furthermore, in this specification, a single lens may be either a spherical lens or an aspherical lens. Furthermore, an aspherical lens is also considered to include a so-called composite aspherical lens having an aspherical layer attached to its surface. This configuration can minimize various manufacturing errors, such as decentering errors and errors in the spacing between single lenses. This can reduce the degradation of optical performance due to manufacturing errors and minimize performance variations between products. As a result, it becomes easier to achieve high performance. It is even more desirable to configure the focusing group with a single lens, that is, one single lens or one aspherical lens, thereby further minimizing the degradation of optical performance due to manufacturing errors and making it easier to achieve high performance.

[0029] (3) Image-side lens group The image-side lens group is positioned closer to the image than the central group and is fixed in the optical axis direction during focusing. The refractive power of the entire image-side lens group may be positive or negative. If the image-side lens group has positive refractive power, the lens closest to the image side will have a light-condensing effect, making it easier to achieve a large aperture. If the image-side lens group has negative refractive power, a diffusing effect will occur on the image side of the optical system, and the exit pupil position will be on the image side. As a result, it is easier to achieve a small radial size for the image-side lens group.

[0030] The configuration within the image-side lens group is not limited, but it should at least include an air lens with negative refractive power. Having an air lens means that the image-side lens group has at least two lenses, and having an air lens with negative refractive power means that the image-side lens group has a convex air lens. Having an air lens with negative refractive power in the image-side lens group close to the image plane diverges light rays, making it easier to reduce the diameter of the image-side lens group.

[0031] Furthermore, off-axis rays emerging from an air lens with negative refractive power, which has the effect of diverging light rays, tend to be bounced up. In order to suppress off-axis coma, it is preferable that the image-side surface of the air lens be shaped so that its concave surface faces the object side.

[0032] Furthermore, it is preferable that the image-side lens that forms the air lens with negative refractive power has negative refractive power, which makes it easier to correct off-axis coma and field curvature, and makes it easier to achieve high performance.

[0033] Furthermore, it is preferable that the object-side lens forming the air lens with negative refractive power has negative refractive power. By having the object-side lens have negative refractive power, the exit pupil position is located on the image side. As a result, it becomes easier to achieve a small radial size of the image-side lens group.

[0034] Although the configuration of the image-side lens group is not limited, it is preferable that it has at least one lens with positive refractive power. By arranging a lens with a converging effect in the image-side lens group, the effect of brightening the optical system occurs on the image plane side, and the composite F-number on the object side of the image-side lens group can be made darker. As a result, it is possible to reduce the number of lenses on the object side of the image-side lens group, making it easier to achieve both low cost and a large aperture.

[0035] Although the specific configuration of the image-side lens group is not particularly limited, it is preferable that a lens having a positive refractive power, a lens having a negative refractive power, and a lens having a negative refractive power are arranged in this order from the object side. Furthermore, it is even more preferable that an air lens is formed by two lenses having negative refractive power. By having such a configuration in the image-side lens group, it becomes easier to achieve both a large aperture and a compact size.

[0036] (4) Aperture diaphragm The aperture stop of this optical system is located closer to the object than the air lens with negative refractive power provided in the image-side lens group. By locating the aperture stop closer to the object than the air lens with negative refractive power, the air lens with negative refractive power is located closer to the image than the aperture stop, and the exit pupil position is closer to the image plane, making it easier to reduce the diameter.

[0037] It is preferable that the aperture stop of the optical system be fixed in position along the optical axis when focusing. Since the aperture stop of a large-diameter lens has a large diameter, the mechanical components for driving it are even larger. Therefore, by fixing the position along the optical axis when focusing, it becomes easier to reduce the diameter, including the mechanical components.

[0038] The aperture stop of this optical system is preferably located within the object-side lens group. In the case of an interchangeable lens, the lens mount, electronic circuit board, components for focus drive, etc. are all located close to the image plane. Therefore, from the standpoint of efficient layout, including mechanical parts, locating the aperture stop within the object-side lens group makes it easier to achieve compactness.

[0039] 1-2.Conditional Expressions In this optical system, it is preferable to employ the above-mentioned configuration and also satisfy the following conditional expression.

[0040] 1-2-1.Conditional Expression (1) It is preferable that the optical system satisfies the following condition: 0.23 <(FB×tanθm) / (f×tanω)< 0.50 (1) however, FB: Air-equivalent length from the surface closest to the image side of the optical system to the image plane θm: Incident angle of the axial marginal ray to the image plane at the maximum Fno when focused at infinity f: focal length of the optical system when focused at infinity ω: Maximum angle of view of the optical system when focused at infinity

[0041] Conditional formula (1) above defines the ratio of the axial ray height to the image plane height at the surface closest to the image plane at maximum aperture (Fno). Here, θm is the angle between the normal to the image plane and the axial marginal ray, expressed as an absolute value. Increasing the aperture diameter of an optical system with a long flange focal distance results in a higher axial ray height within the image-side lens group. When the axial ray height is high, correcting off-axis performance also affects on-axis performance. Furthermore, if the axial ray height is too low, it becomes difficult to increase the aperture diameter. Therefore, by limiting the axial ray height within the image-side lens group within a certain range, a larger aperture diameter can be achieved more effectively while correcting image plane characteristics. Here, satisfying conditional formula (1) allows for a larger aperture diameter and an optical system with high off-axis performance.

[0042] On the other hand, if the value of the conditional expression (1) is equal to or greater than the upper limit, the height of the axial ray on the side nearest to the image side becomes too high relative to the height of the image plane, resulting in insufficient correction of curvature of field and coma, which is undesirable from the viewpoint of improving performance.If the value of the conditional expression (1) is equal to or less than the lower limit, the height of the axial ray on the side nearest to the image side becomes too low relative to the height of the image plane, which is undesirable from the viewpoint of increasing the aperture.

[0043] To obtain the above effect, the upper limit of the conditional expression (1) is preferably 0.48, more preferably 0.46, even more preferably 0.44, even more preferably 0.42, and even more preferably 0.40. The lower limit of the conditional expression (1) is preferably 0.24, more preferably 0.25, and even more preferably 0.26.

[0044] 1-2-2.Conditional Expression (2) In the optical system, it is preferable that an air lens having a negative refractive power included in the image-side lens group satisfies the following conditional expression: 0.03 <(Crf+Crr) / (Crf-Crr)< 4.50 ···(2) however, Crf: Radius of curvature of the object side of the air lens Crr: Radius of curvature of the image side of the air lens The sign of the radius of curvature is positive when the vertex of the lens surface (the intersection of the lens surface and the optical axis) is located on the object side of the spherical center of the lens surface, and negative when it is located on the image side.

[0045] The above conditional expression (2) defines the shape of the air lens with negative refractive power included in the image-side lens group. When the shape of the air lens is close to biconvex, conditional expression (2) is close to zero. When conditional expression (2) is positive, the absolute value of the radius of curvature of the object-side surface is greater than the absolute value of the radius of curvature of the image-side surface. When conditional expression (2) is positive, the image-side surface of the air lens is shaped so that its concave surface faces the object side. Here, the air lens with negative refractive power has the effect of diverging light rays toward the image-side lens group. This brings the exit pupil position closer to the image plane. As a result, this is effective in reducing the radial size of the image-side lens group. Furthermore, off-axial light rays emerging from an air lens with negative refractive power tend to be deflected upward. Here, to suppress off-axial coma, it is preferable that the image-side surface of the air lens be shaped so that its concave surface faces the object side. Here, if the shape of the air lens with negative refractive power satisfies conditional expression (2), it is possible to achieve both a compact and high-performance optical system.

[0046] On the other hand, if the value of the above conditional expression (2) is above the upper limit, i.e., if the radius of curvature of the image-side surface of the air lens becomes large, off-axial coma will be undercorrected, which is undesirable from the perspective of high performance. Furthermore, the air lens will have a weaker negative refractive power, which will move the exit pupil farther from the image plane, which is undesirable from the perspective of compactness. If the value of the above conditional expression (2) is below the lower limit, i.e., if the radius of curvature of the object-side surface of the air lens becomes large in the positive direction, the negative refractive power of the object-side surface will become stronger, which will result in overcorrection of off-axial coma and excessive curvature of field, which is undesirable from the perspective of high performance.

[0047] To obtain the above effects, the upper limit of the conditional expression (2) is preferably 4.20, more preferably 3.90, even more preferably 3.50, even more preferably 3.10, and still more preferably 2.80, and the lower limit of the conditional expression (2) is preferably 0.08, more preferably 0.15, even more preferably 0.23, even more preferably 0.31, and still more preferably 0.36.

[0048] 1-2-3.Conditional Expression (3) In the optical system, it is preferable that the image-side surface of an air lens having negative refractive power included in the image-side lens group satisfies the following conditional expression: 0.50 < Dr / (f×tanω)< 1.80 ···(3) however, Dr: Distance from the image side of the air lens to the image plane f: focal length of the optical system when focused at infinity ω: Maximum angle of view of the optical system when focused at infinity

[0049] Conditional expression (3) above defines the distance from the image side of the air lens to the image plane and the height of the image plane. In other words, it defines the distance of the air lens from the image plane. Here, the distance from the surface of the optical system closest to the image plane is the air-equivalent length. If the air lens is farther from the image plane, the effect of bringing the exit pupil position closer to the image plane is reduced, resulting in an increase in diameter and making it difficult to reduce the size in the overall length direction. Furthermore, if the air lens is too close to the image plane, the diameter of the lens closest to the image plane also increases. Therefore, if conditional expression (3) is satisfied, miniaturization can be achieved.

[0050] On the other hand, if the value of conditional expression (3) is above the upper limit, the effect of bringing the exit pupil position closer to the image plane is reduced, resulting in an increase in diameter and an increase in overall length, which is undesirable from the perspective of compactness. Furthermore, the effect of correcting off-axis coma aberration by an air lens with negative refractive power is reduced, which is undesirable from the perspective of high performance. If the value of conditional expression (3) is below the lower limit, the diameter of the lens closest to the image side is increased, which is undesirable from the perspective of compactness.

[0051] To obtain the above effects, the upper limit of the conditional expression (3) is preferably 1.70, more preferably 1.60, even more preferably 1.45, even more preferably 1.30, and still more preferably 1.25, and the lower limit of the conditional expression (3) is preferably 0.60, more preferably 0.65, even more preferably 0.70, even more preferably 0.75, and still more preferably 0.84.

[0052] 1-2-4.Conditional Expression (4) It is preferable that the optical system satisfies the following condition: 0.92 < f / Fno / Ds < 2.50 (4) however, f: focal length of the optical system when focused at infinity Fno: The maximum aperture of the optical system when focused at infinity Ds: Diameter of the aperture stop at maximum Fno when the optical system is focused at infinity

[0053] The above conditional expression (4) defines the ratio of the entrance pupil diameter of the optical system to the diameter of the aperture stop (aperture diameter). By locating the aperture stop at a position that satisfies conditional expression (4), the size of the aperture diameter and the radial size of the optical system are well balanced, thereby achieving both compactness and high performance.

[0054] On the other hand, if the value of the conditional expression (4) is above the upper limit, the refractive power on the object side of the aperture stop becomes too strong, and the amount of aberration generated on the object side of the aperture stop becomes large, which is undesirable from the viewpoint of high performance.If the value of the conditional expression (4) is below the lower limit, the aperture diameter becomes large, which is undesirable from the viewpoint of compactness.

[0055] To obtain the above effects, the upper limit of conditional formula (4) is preferably 2.30, more preferably 2.12, even more preferably 2.02, even more preferably 1.89, and still more preferably 1.79, and the lower limit of conditional formula (4) is preferably 1.00, more preferably 1.10, even more preferably 1.15, even more preferably 1.20, and still more preferably 1.25.

[0056] 1-2-5.Conditional Expression (5) It is preferable that the optical system satisfies the following condition: 0.00 < f / fs < 1.20 (5) however, f: focal length of the optical system when focused at infinity fs: The composite focal length when the lens closest to the aperture stop is focused at infinity

[0057] The above conditional expression (5) defines the ratio between the focal length of the optical system and the composite focal length of the lens on the object side of the aperture stop when focused at infinity. When the composite focal length of the lens on the object side of the aperture stop is infinity, even if the aperture stop is misaligned in the optical axis direction during manufacturing, no Fno error occurs due to position error. On the other hand, when the composite focal length of the lens on the object side of the aperture stop is small, if the aperture stop is misaligned in the optical axis direction during manufacturing, the Fno error due to position error increases. Furthermore, in order to reduce the aperture diameter, it is preferable for the lens on the object side of the aperture stop to have a light-gathering effect. Here, when conditional expression (5) is satisfied, the Fno error due to manufacturing is small, and a compact optical system can be achieved.

[0058] On the other hand, if the value of conditional expression (5) is above the upper limit, the light-gathering effect of the lens located closer to the object than the aperture stop becomes too strong, and if the aperture stop is misaligned in the optical axis direction during manufacturing, the Fno error due to an error in the position of the aperture stop in the optical axis direction will become large, which is undesirable.If the value of conditional expression (5) is below the lower limit, divergent light rays will be incident on the aperture stop, which will result in an increase in the aperture diameter, which is undesirable from the perspective of compactness.

[0059] To obtain the above effects, the upper limit of conditional formula (5) is preferably 1.10, more preferably 1.05, even more preferably 1.00, even more preferably 0.95, and still more preferably 0.90. The lower limit of conditional formula (5) is preferably 0.05, more preferably 0.10, even more preferably 0.15, and even more preferably 0.19.

[0060] 1-2-6.Conditional Expression (6) It is preferable that at least one of the focusing groups satisfies the following condition: 1.20 < βN < 4.00 (6) however, βN: Lateral magnification of the above focusing group when focused at infinity

[0061] Conditional formula (6) above defines the lateral magnification of the focusing group when focusing at infinity. When the central group is made up of multiple focusing groups, it is preferable that at least one of them satisfy conditional formula (6). If the lateral magnification of the focusing group is βN and the combined lateral magnification of the focusing group toward the image is βR, the focusing sensitivity of the focusing group can be expressed as (1-βN2)×βR2. If the lateral magnification of the focusing group is greater than 1, this means that the focusing group moves toward the image when focusing from an object at infinity to an object at a finite distance. Furthermore, having a lateral magnification greater than 1 makes it possible to extend the focal length and shorten the overall length. Therefore, when the focusing group satisfies conditional formula (6), the lateral magnification of the focusing group falls within an appropriate range, enabling compactness to be achieved. Furthermore, it is preferable that the focusing group that satisfies conditional formula (6) is a lens group that moves toward the image along the optical axis when focusing from an object at infinity to an object at a finite distance.

[0062] On the other hand, if the value of conditional expression (6) is equal to or greater than the upper limit, the lateral magnification of the focusing group when focusing at infinity becomes large, and the composite Fno from the lens closest to the object to the focusing group becomes dark, which is not desirable from the viewpoint of increasing the aperture.On the other hand, if the value of conditional expression (6) is equal to or less than the lower limit, the lateral magnification of the focusing group when focusing at infinity becomes small, which is not desirable from the viewpoint of making it difficult to achieve telephoto and compact size.

[0063] In order to obtain the above effects, the upper limit of the conditional formula (6) is preferably 3.80, more preferably 3.60, even more preferably 3.50, even more preferably 3.40, and still more preferably 3.30. The lower limit of the conditional formula (6) is preferably 1.30, more preferably 1.40, even more preferably 1.50, even more preferably 1.60, and still more preferably 1.70.

[0064] 1-2-7.Conditional Expression (7) It is preferable that at least one of the focusing groups satisfies the following condition: -3.00 < fN / f < -0.30 (7) however, fN: focal length of the above focusing group f: focal length of the optical system

[0065] Conditional formula (7) defines the ratio between the focal length of the focusing group and the focal length of the optical system. When the central group includes multiple focusing groups, it is preferable that at least one of them satisfy conditional formula (7). In this case, the focusing group that satisfies conditional formula (7) can move toward either the image side or the object side when focusing from an object at infinity to a finite distance, and there is no limitation on the direction of movement. When the focusing group has negative refractive power, it is easy to make the lateral magnification of the focusing group greater than 1. Therefore, when the focusing group has negative refractive power, it is preferable that the focusing group move toward the image side when focusing from an object at infinity to a finite distance. When conditional formula (7) is satisfied, the refractive power of the focusing group falls within an appropriate range, suppressing aberration fluctuations during focusing. This allows for good imaging performance to be achieved with a small number of lenses regardless of the distance to the subject, making it easy to achieve a compact, high-performance optical system.

[0066] On the other hand, if the value of conditional expression (7) is above the upper limit, the refractive power of the focusing group becomes strong, making it difficult to achieve good imaging performance with a small number of lenses, which is undesirable in terms of reducing the cost of the optical system. Furthermore, if the number of lenses constituting the focusing group increases, the weight of the focusing group increases, and the mechanical components for driving the focusing group also become larger, making it difficult to achieve a compact and lightweight optical system, which is undesirable. On the other hand, if the value of conditional expression (7) is below the lower limit, the refractive power of the focusing group becomes weak, increasing the amount of movement during focusing, making it difficult to achieve a compact optical system, which is undesirable.

[0067] To obtain the above effect, the upper limit of the conditional expression (7) is preferably −0.40, more preferably −0.50, even more preferably −0.60, even more preferably −0.70, and still more preferably −0.75, and the lower limit of the conditional expression (7) is preferably −2.75, more preferably −2.60, even more preferably −2.35, even more preferably −2.10, and still more preferably −1.95.

[0068] 1-2-8.Conditional Expression (8) It is preferable that the central group has a focusing group having positive refractive power in addition to a focusing group that moves toward the image side when focusing from an object at infinity to an object at a finite distance, and that the following condition be satisfied: 0.40 < fP / f < 10.00 (8) however, fP: focal length of the focusing group having the positive refractive power f: focal length of the optical system

[0069] Conditional expression (8) above defines the ratio of the focal length of another focusing group having positive refractive power to the focal length of the optical system. This focusing group having positive refractive power is a different focusing group from the focusing group that moves toward the image side when focusing from an object at infinity to an object at a finite distance, and the direction of movement when focusing from an object at infinity to an object at a finite distance is not important. By satisfying conditional expression (8), the amount of movement of the focusing group having positive refractive power can be reduced, thereby enabling the optical system to be made more compact in the overall optical length direction. Furthermore, when conditional expression (8) is satisfied, the focal length of the focusing group having positive refractive power, i.e., its refractive power, falls within an appropriate range, suppressing aberration fluctuations associated with changes in the position of the focusing group having positive refractive power. This allows for the realization of good imaging performance regardless of object distance with a smaller number of lenses.

[0070] On the other hand, if the value of conditional expression (8) is equal to or greater than the upper limit, the refractive power of the focusing group having positive refractive power becomes weak. As a result, the amount of movement of the focusing group having positive refractive power during focusing becomes large, making it difficult to reduce the size of the optical system in the overall optical length direction. On the other hand, if the value of conditional expression (8) is equal to or less than the lower limit, the refractive power of the focusing group having positive refractive power becomes too strong, making it difficult to correct spherical aberration and to obtain good imaging performance with a small number of lenses, which is undesirable in terms of high performance and low cost.

[0071] In order to obtain the above effects, the upper limit of conditional formula (8) is preferably 8.00, more preferably 7.00, and even more preferably 6.00, and the lower limit of conditional formula (8) is preferably 0.45, more preferably 0.49, even more preferably 0.52, even more preferably 0.55, and even more preferably 0.59.

[0072] 1-2-9.Conditional Expression (9) It is preferable that the optical system satisfies the following condition: -1.00 < f / fr < 3.00 (9) however, fr: focal length of the image-side lens group f: focal length of the optical system

[0073] The above conditional expression (9) defines the ratio between the focal length of the image-side lens group and the focal length of the optical system. By satisfying conditional expression (9), the focal length of the image-side lens group falls within an optimum range, making it easy to achieve both a large aperture and high performance.

[0074] On the other hand, if the value of the conditional expression (9) is equal to or greater than the upper limit, the positive refractive power of the image-side lens group becomes strong, which makes it easy to increase the aperture diameter, but this leads to an increase in the amount of spherical aberration and coma aberration, which is not desirable from the viewpoint of improving performance.On the other hand, if the value of the conditional expression (9) is equal to or less than the lower limit, the negative refractive power of the image-side lens group becomes strong, which makes the optical system dark, which is not desirable from the viewpoint of increasing the aperture diameter.

[0075] In order to obtain the above effects, the upper limit of conditional formula (9) is preferably 2.30, more preferably 1.80, even more preferably 1.45, even more preferably 1.39, and still more preferably 1.25. The lower limit of conditional formula (9) is preferably -0.90, more preferably -0.80, even more preferably -0.72, even more preferably -0.60, and still more preferably -0.50.

[0076] 1-2-10.Conditional Expression (10) It is preferable that the optical system satisfies the following condition: 0.50 < ff / f < 3.50 (10) however, ff: focal length of the object-side lens group f: focal length of the optical system

[0077] The above conditional expression (10) defines the ratio between the focal length of the object-side lens group and the focal length of the optical system. By satisfying conditional expression (10), the focal length of the object-side lens group falls within an optimum range, making it easy to achieve a compact, high-performance, and large-diameter optical system.

[0078] On the other hand, if the value of the conditional expression (10) is equal to or greater than the upper limit, that is, the focal length of the object-side lens group becomes too long relative to the focal length of the optical system, making it difficult to reduce the size of the optical system in the overall optical length direction.If the value of the conditional expression (10) is equal to or less than the lower limit, that is, the focal length of the object-side lens group becomes too short relative to the focal length of the optical system, making it difficult to correct spherical aberration, axial chromatic aberration, and coma, which is undesirable in terms of improving performance.

[0079] In order to obtain the above effects, the upper limit of conditional formula (10) is preferably 2.90, more preferably 2.60, even more preferably 2.30, even more preferably 1.900, and still more preferably 1.50. The lower limit of conditional formula (10) is preferably 0.55, more preferably 0.60, even more preferably 0.65, even more preferably 0.70, and still more preferably 0.75.

[0080] 1-2-11.Conditional Expression (11) It is preferable that the optical system satisfies the following condition: 0.30 < CrL1f / f (11) however, CrL1f: Radius of curvature of the surface closest to the object in the optical system f: focal length of the optical system

[0081] The above conditional expression (11) defines the ratio between the radius of curvature of the surface closest to the object in the optical system and the focal length of the optical system. The surface closest to the object in the optical system is a convex or flat surface facing the object side. By satisfying conditional expression (11), the surface closest to the object in the optical system is a convex or flat surface facing the object side, making it easier to reduce the occurrence of coma and distortion, and achieving an optical system with high imaging performance.

[0082] Furthermore, in order to more effectively prevent a light ray incident on the optical system from being reflected on the image plane, being re-reflected by the surface closest to the object in the optical system, and then reaching the image plane, it is preferable to set an upper limit to the above conditional expression (11). By satisfying conditional expression (11)', the object-side radius of curvature of the surface closest to the object falls within an optimum range, thereby reducing the occurrence of coma and effectively suppressing the occurrence of ghost light. 0.30 < CrL1f / f < 2000.00 ···(11)' however, CrL1f: Radius of curvature of the surface closest to the object in the optical system f: focal length of the optical system

[0083] On the other hand, if the value of conditional expression (11)' is equal to or greater than the upper limit, the radius of curvature of the surface closest to the object in the optical system approaches a flat surface, resulting in a conjugate relationship in which light rays reflected on the image plane are re-reflected by the surface closest to the object in the optical system and re-imaged on the image plane, making it difficult to effectively suppress the occurrence of ghosts.On the other hand, if the value of conditional expression (11) or (11)' is equal to or less than the lower limit, the radius of curvature of the surface closest to the object in the optical system becomes too small, increasing coma aberration, which is undesirable in terms of improving performance.

[0084] In order to obtain the above effects, the upper limit of the above conditional formula (11)' is preferably 1000.00, more preferably 200.00, even more preferably 100.00, still more preferably 10.00, and even more preferably 3.00. The lower limit of the above conditional formula (11) or (11)' is preferably 0.35, more preferably 0.40, even more preferably 0.45, still more preferably 0.50, and even more preferably 0.55.

[0085] 1-2-12.Conditional Expression (12) It is preferable that the image-side lens group of the optical system has at least one lens having positive refractive power, and that the following conditional expression be satisfied: 1.73 < Ndrp < 2.50 (12) however, Ndrp: The refractive index at the d-line of the lens with the above positive refractive power

[0086] The above conditional expression (12) defines the refractive index of the lens with positive refractive power included in the lens closest to the image side of the optical system. By arranging a lens with a converging effect in the image-side lens group, the effect of brightening the optical system occurs on the image plane side, and the composite Fno on the object side of the image-side lens group can be made darker. As a result, it is possible to reduce the number of lenses on the object side of the image-side lens group, making it easier to achieve both low cost and a large aperture. By satisfying the above conditional expression (12), the refractive index of the lens with positive refractive power included in the image-side lens falls within an optimal range, thereby achieving low cost and a large aperture.

[0087] On the other hand, if the value of conditional expression (12) is equal to or greater than the upper limit, the lens having positive refractive power will be made of an expensive material, which is undesirable as it will make it difficult to reduce costs. On the other hand, if the value of conditional expression (12) is equal to or less than the lower limit, this will lead to a need to reduce the radius of curvature in order to increase the refractive power of the lens having positive refractive power, which will make it difficult to correct spherical aberration and coma, which is undesirable. This will also lead to an increase in the number of lenses, which will make it difficult to reduce costs, which is undesirable.

[0088] In order to obtain the above effects, the upper limit of conditional expression (12) is preferably 2.20, more preferably 2.11, even more preferably 2.06, and even more preferably 2.01. The lower limit of conditional expression (12) is preferably 1.76, more preferably 1.78, even more preferably 1.80, and even more preferably 1.82.

[0089] 1-2-13.Conditional Expression (13) It is preferable that the optical system satisfies the following condition: -1.25 < Crr / f < -0.10 (13) however, Crr: Radius of curvature of the image side of the air lens f: focal length of the optical system

[0090] The above conditional expression (13) defines the ratio of the radius of curvature of the image-side surface of the air lens included in the image-side lens group to the focal length of the optical system. By satisfying the above conditional expression (13), the angle of incidence of light rays incident on the lens on the image side of the air lens becomes optimal, making it easier to reduce the occurrence of coma aberration, and achieving an optical system with high imaging performance.

[0091] On the other hand, if the value of the above conditional expression (13) is equal to or greater than the upper limit, that is, the radius of curvature of the image-side surface of the air lens becomes too small relative to the focal length of the optical system, causing overcorrection of coma and making it difficult to achieve high performance.If the value of the above conditional expression (13) is equal to or less than the lower limit, that is, the radius of curvature of the image-side surface of the air lens becomes too large relative to the focal length of the optical system, making it difficult to correct coma, which is not preferable in terms of improving high performance.

[0092] To obtain the above effect, the upper limit of conditional expression (13) is preferably −0.15, more preferably −0.18, even more preferably −0.21, even more preferably −0.24, and still more preferably −0.28. The lower limit of conditional expression (13) is preferably −1.15, more preferably −1.10, even more preferably −1.05, even more preferably −1.00, and still more preferably −0.95.

[0093] 2. Imaging device Next, an imaging device according to the present invention will be described. The imaging device according to the present invention is characterized by comprising the optical system according to the present invention described above and an imaging element that receives an optical image formed by the optical system and converts it into an electrical image signal.

[0094] Here, there are no particular limitations on the imaging element, and solid-state imaging elements such as a CCD (Charge Coupled Device) sensor and a CMOS (Complementary Metal Oxide Semiconductor) sensor can also be used. The imaging device according to the present invention is suitable for imaging devices using these solid-state imaging elements, such as digital cameras, video cameras, surveillance cameras, and vehicle-mounted cameras. Furthermore, the imaging device may be a fixed-lens imaging device in which the lens is fixed to the housing, or an interchangeable-lens imaging device such as a single-lens reflex camera or a mirrorless single-lens camera.

[0095] Next, the present invention will be described in detail with reference to examples. However, the present invention is not limited to the following examples. In each lens cross-sectional view, the left side is the object side and the right side is the image side. [Example]

[0096] (1) Lens configuration of the optical system FIG. 1 is a cross-sectional view showing the configuration of an optical system according to a first embodiment of the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a central lens group GM, and an image-side lens group GR. The object-side lens group GF is composed of, in order from the object side, a meniscus-shaped first lens L1 having positive refractive power and a convex object-side surface facing the object side; a meniscus-shaped second lens L2 having positive refractive power and a convex object-side surface facing the object side; a biconcave third lens L3 having negative refractive power and a concave object-side and image-side surfaces; an aperture stop S; and a meniscus-shaped fourth lens L4 having positive refractive power and a convex object-side surface facing the object side. The central lens group GM is composed of a biconcave fifth lens L5 having negative refractive power and a concave object-side and image-side surfaces; and a biconvex sixth lens L6 having positive refractive power and a convex object-side and image-side surfaces. The image-side lens group GR is composed of, in order from the object side, a cemented lens formed by cementing together a seventh lens L7 having negative refractive power, an eighth lens L8 having positive refractive power, and a ninth lens L9 having negative refractive power, and a meniscus-shaped tenth lens L10 having negative refractive power and whose object-side surface is concave toward the object side.

[0097] Here, the fifth lens L5 included in the central group GM is a focusing group GN that moves toward the image side in the optical axis direction when focusing from an object at infinity to an object at a finite distance. The image-side surface of the fifth lens L5 has a concave shape toward the image side. The sixth lens L6 included in the central group GM is a focusing group GP that has positive refractive power and is separate from the focusing group GN. The sixth lens L6, which is a focusing group GP with positive refractive power, moves toward the object side when focusing from an object at infinity to an object at a finite distance. The third lens L3 corresponds to a lens included in the object-side lens group GF that has a concave shape toward the image side and has negative refractive power. The lenses located on the object side of the third lens L3 are the first lens L1 and the second lens L2, which, when combined, have positive refractive power. The eighth lens L8 corresponds to a lens included in the image-side lens group GR that has positive refractive power.

[0098] Note that "IMG" in the diagram indicates the image plane. This is the imaging surface of a solid-state imaging element such as a CCD sensor or CMOS sensor as described above. Light incident from the object side of the optical system is focused on the image plane. The solid-state imaging element converts the received optical image into an electrical image signal. An image processing unit (image processing processor, etc.) provided in the imaging device, etc., generates a digital image corresponding to the image of the subject based on the electrical image signal output from the imaging element. This digital image can be recorded on a recording medium such as an HDD (Hard Disk Device), memory card, optical disk, or magnetic tape. Note that the image plane may also be the film surface of a silver halide film.

[0099] Additionally, "CG" in the figure indicates the image plane, which is an optical block. The optical block CG corresponds to an optical filter, a face plate, a crystal low-pass filter, an infrared cut filter, etc. These symbols (IMG, CG) indicate the same things in the drawings shown in other embodiments, so explanations will be omitted below.

[0100] (2) Numerical examples A numerical example will be described that applies specific numerical values ​​of the optical system adopted in Example 1. Table 1 shows lens data of the imaging lens. In Table 1, "surface number" is the number of the lens surface counted from the object side, "r" is the radius of curvature (mm) of the lens surface (however, a surface where the value of r is INF indicates that the surface is flat), "d" is the distance (mm) on the optical axis between the ith lens surface (i is a natural number) and the (i+1)th lens surface counted from the object side, "Nd" is the refractive index for the d-line (wavelength λ=587.56 nm), "νd" is the Abbe number for the d-line, and "h" is the effective radius (mm).

[0101] Table 2 shows various data about the optical system. Specifically, it shows the focal length (mm), F-number (F-number), half angle of view (°), image height (mm), total lens length (mm), and back focus (BF (in air)) (mm) of the imaging lens. Here, the total lens length is the distance on the optical axis from the object-side surface of the first lens to the image plane. Also, the back focus is the air-equivalent value of the distance on the optical axis from the image-side surface of the nth lens, which is positioned closest to the image, to the image plane.

[0102] Table 3 shows the variable distance data for this optical system. D0 is the distance from the subject to the surface closest to the object.

[0103] Table 4 shows the focal lengths of the lenses that make up the optical system.

[0104] Table 5 shows the focal length of each lens group that makes up the optical system.

[0105] Moreover, the values ​​of each conditional expression of the optical system are shown in Table 21. The matters relating to these tables are the same as those of the tables shown in the other examples, and therefore, explanations thereof will be omitted below.

[0106] FIG. 2 shows a longitudinal aberration diagram of the optical system when focused at infinity. The longitudinal aberration diagram in FIG. 2 shows, from left to right, spherical aberration (mm), astigmatism (mm), and distortion (%). In the diagram showing spherical aberration, the vertical axis represents the maximum aperture F-number (Fno). The solid line represents spherical aberration at the d-line (wavelength 587.56 nm), the dotted line represents spherical aberration at the C-line (wavelength 656.27 nm), and the dash-dot line represents spherical aberration at the g-line (wavelength 435.84 nm). In the diagram showing astigmatism, the vertical axis represents the half angle of view (°). The solid line represents the sagittal direction (S) at the d-line (wavelength 587.56 nm), and the dotted line represents the meridional direction (T) at the d-line. In the diagrams showing distortion, the vertical axis represents the half angle of view (°) and the distortion (%) at the d-line (wavelength 587.56 nm) is shown. The matters relating to these longitudinal aberration diagrams are the same as those shown in the longitudinal aberration diagrams of other examples, and therefore will not be described below.

[0107] [Table 1] Surface number rd Nd vd h 1 62.0586 8.980 1.80420 46.50 26.900 2 893.0019 0.336 26.061 3 47.5037 4.330 1.72916 54.67 23.205 4 82.0672 6.440 22.531 5 -12299.2665 1.776 1.78472 25.72 20.709 6 39.2187 9.680 18.840 7 S INF 2.653 18.400 8 59.4538 4.616 1.80420 46.50 18.221 9 564.0375 D9 17.901 10 -2792.5999 1.220 1.48749 70.44 16.774 11 35.6836 D11 15.734 12 77.4054 5.588 1.72916 54.67 15.000 13 -76.7059 D13 15.433 14 -82.8462 1.492 1.78472 25.72 15.874 15 40.9008 13.464 1.98113 32.69 16.989 16 -36.2936 1.538 1.60026 45.25 17.512 17 -185.3912 3.979 17.484 18 -45.4182 1.335 1.85883 30.00 17.395 19 -124.0495 19.603 17.932 20 INF 2.500 1.51633 64.15 21.280 21 INF 1.000 21.542

[0108] [Table 2] Focal length 75.830 F-number 1.458 Half angle of view 16.269 Image height 21.630 Lens length 111.780 BF(in air) 22.252

[0109] [Table 3] Variable Interval Data D0 INF 2519.137 1083.967 664.211 D9 2.573 5.073 9.084 13.783 D11 16.342 13.501 9.352 4.406 D13 2.336 2.678 2.816 3.063

[0110] [Table 4] Lens Surface Number Focal Length L1 1-2 82.534 L2 3-4 146.926 L3 5-6 -49.816 L4 8-9 82.305 L5 10-11 -72.265 L6 12-13 53.658 L7L8L9 14-17 103.934 L10 18-19 -84.089

[0111] [Table 5] Group Surface number Focal length GF 1-9 80.875 GM 10-13 105.923 GR 14-19 -442.481 GN 10-11 -72.265 GP 12-13 53.658 [Example]

[0112] (1) Lens configuration of the optical system FIG. 3 is a cross-sectional view showing the configuration of an optical system according to a second embodiment of the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a central lens group GM, and an image-side lens group GR. The object-side lens group GF is composed of, in order from the object side, a meniscus-shaped first lens L1 having positive refractive power and a convex object-side surface facing the object side, a cemented lens formed by cementing a biconvex second lens L2 having positive refractive power and a biconvex third lens L3 having negative refractive power, a meniscus-shaped fourth lens L4 having positive refractive power and a convex object-side surface facing the object side, a cemented lens formed by cementing a biconcave fifth lens L5 having negative refractive power and a biconcave sixth lens L6 having positive refractive power and a biconvex sixth lens L6, and an aperture stop S. The central lens group GM is composed of a meniscus-shaped seventh lens L7 having negative refractive power and a concave image-side surface facing the image side, and an eighth lens L8 having positive refractive power and a convex object-side surface facing the object side. The image-side lens group GR is composed of, in order from the object side, a cemented lens formed by cementing together a ninth lens L9 having positive refractive power and a tenth lens L10 having negative refractive power, and an eleventh lens L11 having negative refractive power and a meniscus shape whose object-side surface is concave toward the object side.

[0113] Here, the seventh lens L7 included in the central group GM is a focusing group GN that moves toward the image side in the optical axis direction when focusing from an object at infinity to an object at a finite distance. The image-side surface of the seventh lens L7 has a concave shape toward the image side. The eighth lens L8 included in the central group GM is a focusing group GP that has positive refractive power and is separate from the focusing group GN. The eighth lens L8, which is a focusing group GP with positive refractive power, moves toward the object side when focusing from an object at infinity to an object at a finite distance. The third lens L3 corresponds to a lens included in the object-side lens group GF that has a concave shape toward the image side and has negative refractive power. The lenses located on the object side of the third lens L3 are the first lens L1 and the second lens L2, which, when combined, have positive refractive power. The ninth lens L9 corresponds to a lens included in the image-side lens group GR that has positive refractive power.

[0114] (2) Numerical examples Next, a description will be given of a numerical example in which the specific numerical values ​​of the optical system employed in Example 2 are applied. Tables 6 to 10 show the lens data of the optical system, various data of the optical system, variable interval data, focal length of each lens, and focal length of each lens group. FIG. 4 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[0115] [Table 6] Surface number rd Nd vd h 1 66.4384 5.022 1.80610 33.27 26.320 2 155.4249 0.840 25.885 3 59.0388 10.043 1.49700 81.61 24.987 4 -96.4986 1.500 1.69895 30.05 24.613 5 37.2272 1.651 22.070 6 42.0739 7.104 1.78800 47.49 22.172 7 114.0819 6.910 21.628 8 -61.0198 1.797 1.74950 35.04 21.449 9 206.6899 5.942 2.00100 29.13 21.847 10 -70.7550 4.500 21.896 11 S INF D11 19.880 12 82.8478 0.900 1.48749 70.44 18.500 13 33.7450 D13 17.512 14 35.6623 5.798 1.49700 81.61 16.441 15 204.3133 D15 15.880 16 104.0545 10.273 1.83481 42.72 18.023 17 -37.5664 1.874 1.48749 70.44 17.979 18 652.9120 5.774 16.992 19 -34.9682 1.426 1.92286 20.88 16.721 20 -221.7201 14.996 17.500 21 INF 2.500 1.51633 64.15 21.096 22 INF 1.000 21.468

[0116] [Table 7] Focal length 75.563 F-number 1.443 Half angle of view 16.366 Image height 21.630 Lens length 112.239 BF(in air) 17.645

[0117] [Table 8] Variable Interval Data D0 INF 2416.705 1083.767 872.818 D11 2.566 4.650 4.923 5.626 D13 11.504 7.503 3.251 1.193 D15 8.318 10.235 14.214 15.569

[0118] [Table 9] Lens Surface Number Focal Length L1 1-2 140.418 L2L3 3-5 -89.569 L4 6-7 81.066 L5L6 8-10 267.418 L7 12-13 -117.499 L8 14-15 85.948 L9L10 16-18 61.898 L11 19-20 -45.152

[0119] [Table 10] Group Surface number Focal length GF 1-10 97.486 GM 11-15 241.436 GR 16-20 -398.279 GN 12-13 -117.499 GP 14-15 85.948 [Example]

[0120] (1) Lens configuration of the optical system FIG. 5 is a cross-sectional view showing the configuration of an optical system according to a third embodiment of the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a central lens group GM, and an image-side lens group GR. The object-side lens group GF is composed of, in order from the object side, a meniscus-shaped first lens L1 having positive refractive power and a convex object-side surface facing the object side; a meniscus-shaped second lens L2 having positive refractive power and a convex object-side surface facing the object side; a biconcave third lens L3 having negative refractive power and a concave object-side and image-side surfaces; an aperture stop S; and a meniscus-shaped fourth lens L4 having positive refractive power and a convex object-side surface facing the object side. The central lens group GM is composed of a meniscus-shaped fifth lens L5 having negative refractive power and a biconcave object-side and image-side surfaces facing the object side; and a meniscus-shaped sixth lens L6 having positive refractive power and a convex image-side surface facing the image side. The image-side lens group GR is composed of, in order from the object side, a cemented lens formed by cementing together a seventh lens L7 having positive refractive power and an eighth lens L8 having negative refractive power, and a meniscus-shaped ninth lens L9 having negative refractive power and whose object-side surface is concave toward the object side.

[0121] Here, the fifth lens L5 included in the central group GM is a focusing group GN that moves toward the image side in the optical axis direction when focusing from an object at infinity to an object at a finite distance. The image-side surface of the fifth lens L5 has a concave shape facing the image side. The sixth lens L6 included in the central group GM is a focusing group GP that has positive refractive power and is separate from the focusing group GN. The sixth lens L6, which is a focusing group GP with positive refractive power, moves toward the object side when focusing from an object at infinity to an object at a finite distance. The third lens L3 corresponds to a lens included in the object-side lens group GF that has a concave shape facing the image side and has negative refractive power. The lenses located on the object side of the third lens L3 are the first lens L1 and the second lens L2, which, when combined, have positive refractive power. The seventh lens L7 corresponds to a lens included in the image-side lens group GR that has positive refractive power.

[0122] (2) Numerical examples Next, a description will be given of a numerical example in which the specific numerical values ​​of the optical system employed in Example 3 are applied. Tables 11 to 15 show the lens data of the optical system, various data of the optical system, variable interval data, focal length of each lens, and focal length of each lens group. Also, Fig. 6 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[0123] [Table 11] Surface number rd Nd vd h 1 59.9541 8.370 1.90201 42.58 26.400 2 685.6447 0.636 25.742 3 51.3779 5.000 1.74412 58.51 23.184 4 79.8054 6.296 21.978 5 -3878.5326 1.800 1.84666 23.78 20.072 6 36.9083 9.008 18.199 7 S INF 2.464 17.950 8 51.0456 5.019 1.80420 46.50 18.225 9 1038.2992 D9 18.091 10 -333.6868 1.130 1.48749 70.44 16.500 11 35.6367 D11 16.155 12 -379.5118 4.481 1.59349 67.00 17.000 13 -149.7005 D13 18.181 14 66.4021 13.160 1.99802 40.61 22.339 15 -46.7195 1.740 1.48890 31.50 22.361 16 -190.9183 4.203 21.164 17 -53.3536 1.800 1.84666 23.78 20.739 18 -1853.6962 19.532 20.880 19 INF 2.500 1.51633 64.15 21.560 20 INF 1.000 21.617

[0124] [Table 12] Focal length 76.090 F-number 1.461 Half angle of view 16.175 Image height 21.630 Lens total length 114.314 BF(in air) 22.181

[0125] [Table 13] Variable Interval Data D0 INF 2395.240 1081.545 640.062 D9 2.772 5.535 9.083 13.831 D11 20.981 17.886 13.863 7.717 D13 2.421 2.754 3.228 4.626

[0126] [Table 14] Lens Surface Number Focal Length L1 1-2 72.377 L2 3-4 180.302 L3 5-6 -43.173 L4 8-9 66.605 L5 10-11 -65.983 L6 12-13 413.541 L7L8 14-16 36.926 L9 17-18 -64.914

[0127] [Table 15] Group Surface number Focal length GF 1-9 76.595 GM 10-13 -84.769 GR 14-18 64.976 GN 10-11 -65.983 GP 12-13 413.541 [Example]

[0128] (1) Lens configuration of the optical system FIG. 7 is a cross-sectional view showing the configuration of an optical system according to a fourth embodiment of the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a central lens group GM, and an image-side lens group GR. The object-side lens group GF is composed of, in order from the object side, a meniscus first lens L1 having positive refractive power and a convex object-side surface facing the object side; a meniscus second lens L2 having positive refractive power and a convex object-side surface facing the object side; a biconcave third lens L3 having negative refractive power and a concave object-side surface and an image-side surface; an aperture stop S; and a meniscus fourth lens L4 having positive refractive power and a convex object-side surface facing the object side. The central lens group GM is composed of a biconcave fifth lens L5 having negative refractive power and a concave object-side surface and an image-side surface facing the object side. The image-side lens group GR is composed of, in order from the object side, a sixth lens L6 which has positive refractive power and is biconvex in shape with both the object-side and image-side surfaces being convex, a cemented lens formed by cementing together a seventh lens L7 which has negative refractive power, an eighth lens L8 which has positive refractive power, and a ninth lens L9 which has negative refractive power, and a meniscus-shaped tenth lens L10 which has negative refractive power and is concave on the object-side surface facing the object side.

[0129] Here, the fifth lens L5 included in the central group GM is a focusing group GN that moves toward the image side in the optical axis direction when focusing from an object at infinity to an object at a finite distance. The image side surface of the fifth lens L5 has a concave shape toward the image side. The third lens L3 corresponds to a lens included in the object-side lens group GF that has a concave shape toward the image side and has negative refractive power, and the lenses located on the object side of the third lens L3 are the first lens L1 and the second lens L2, which have positive refractive power when combined. The eighth lens L8 corresponds to a lens included in the image-side lens group GR that has positive refractive power.

[0130] (2) Numerical examples Next, we will explain numerical examples that apply the specific numerical values ​​of the optical system adopted in Example 4. Tables 16 to 20 show the lens data of the optical system, various data of the optical system, variable interval data, focal length of each lens, and focal length of each lens group. Also, Fig. 8 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[0131] [Table 16] Surface number rd Nd vd h 1 61.5589 8.873 1.80420 46.50 26.900 2 785.0877 0.215 26.094 3 47.9893 4.340 1.72916 54.67 23.327 4 83.4360 6.581 22.647 5 -6985.5575 1.730 1.78472 25.72 20.739 6 39.2274 9.610 18.876 7 S INF 2.670 18.450 8 60.8077 4.590 1.80420 46.50 18.295 9 997.7238 D9 17.998 10 -1052.0018 1.131 1.48749 70.44 16.799 11 35.4148 D11 15.753 12 77.5735 5.508 1.72916 54.67 15.000 13 -79.5596 2.447 15.437 14 -81.5322 1.634 1.78472 25.72 15.808 15 40.2990 13.290 1.98001 32.71 17.011 16 -36.4388 1.630 1.57522 43.85 17.537 17 -204.3736 4.252 17.502 18 -44.1137 1.233 1.85883 30.00 17.418 19 -107.0057 19.587 17.957 20 INF 2.500 1.51633 64.15 21.280 21 INF 1.000 21.538

[0132] [Table 17] Focal length 75.744 F-number 1.455 Half angle of view 16.317 Image height 21.630 Lens length 111.825 BF(in air) 22.236

[0133] [Table 18] Variable Interval Data D0 INF 2813.666 1080.509 663.967 D9 2.681 5.240 9.667 14.551 D11 16.323 13.764 9.338 4.454

[0134] [Table 19] Lens Surface Number Focal Length L1 1-2 82.608 L2 3-4 147.312 L3 5-6 -49.705 L4 8-9 80.345 L5 10-11 -70.257 L6 12-13 54.674 L7L8L9 14-17 102.745 L10 18-19 -88.192

[0135] [Table 20] Group Surface number Focal length GF 1-9 79.895 GM 10-11 -70.257 GR 12-19 87.949 GN 10-11 -70.257

[0136] [Table 21] Example 1 Example 2 Example 3 Example 4 (1) (FB×tanθm) / (f×tanω) 0.367 0.294 0.365 0.367 (2) (Crf+Crr) / (Crf-Crr) 1.649 0.898 1.776 1.551 (3) Dr / (f×tanω) 1.066 0.859 1.087 1.058 (4) f / Fno / Ds 1.413 1.317 1.451 1.411 (5) f / fs 0.315 0.775 0.224 0.310 (6) βN 3.035 2.215 3.174 3.145 (7) fN / f -0.953 -1.555 -0.867 -0.928 (8) fP / f 0.708 1.137 5.435 - (9) f / fr -0.171 -0.190 1.171 1.306 (10)ff / f 1.067 1.290 1.007 1.055 (11)CrL1f / f 0.818 0.879 0.788 0.813 (12)Ndrp 1.981 1.835 1.998 1.980 (13)Crr / f -0.599 -0.463 -0.701 -0.582 Example 1 Example 2 Example 3 Example 4 FB 22.252 17.645 22.181 22.236 θm 20.067 20.271 19.983 20.120 f 75.830 75.563 76.090 75.744 ω 16.269 16.366 16.175 16.317 Crf -185.391 652.912 -190.918 -204.374 Crr -45.418 -34.968 -53.354 -44.114 Dr 23.587 19.071 23.981 23.469 Fno 1.458 1.443 1.461 1.455 Ds 36.800 39.760 35.900 36.900 fs 241.015 97.486 339.381 244.026 fN -72.265 -117.500 -65.983 -70.257 fP 53.658 85.948 413.541 - fr -442.480 -398.279 64.976 58.001 ff 80.8747 97.486 76.595 79.895 CrL1f 62.059 66.438 59.954 61.559 [Industrial Applicability]

[0137] According to the present invention, it is possible to provide a small, lightweight, high-performance, large-aperture optical system and an imaging device with a maximum aperture of Fno greater than 2.0 that are suitable for small imaging systems. [Explanation of symbols]

[0138] GF: Object-side lens group GM... Central group GR: Image-side lens group GN: A lens group that moves along the optical axis when focusing GP: A focusing group with positive refractive power that moves along the optical axis during focusing. L1: First lens L2: Second lens L3: Third lens L4: Fourth lens L5: Fifth lens element L6: Sixth lens element L7: Seventh lens element L8: Eighth lens element L9: 9th lens L10: 10th lens S Aperture stop CG ··· Optical Block IMG... Image plane

Claims

1. the optical system is composed of, in order from the object side, an object-side lens group that is fixed in the optical axis direction during focusing, a central group that is made up of only one or more focusing groups that move in the optical axis direction during focusing, and an image-side lens group that is fixed in the optical axis direction during focusing, wherein at least one of the focusing groups moves toward the image side during focusing from an object at infinity to an object at a finite distance, the image-side lens group has an air lens with negative refractive power, an aperture stop for determining the diameter of an axial light beam is arranged closer to the object than the air lens, a lens that forms the object-side surface of the air lens has negative refractive power, and the image-side lens that forms the air lens has negative refractive power; The following conditional expressions (1), (3), and (4) are satisfied: At least one of the focusing groups must satisfy the following condition (6): Characteristic optical system. 0.23 < (FB × tan θm) / (f × tan ω) < 0.50 (1) 0.50 < Dr / (f×tanω) < 1.80 (3) 0.92 < f / Fno / Ds < 1.79 (4) 1.20 < βN < 4.00 (6) however, FB: Air-equivalent length from the surface of the optical system closest to the image plane θm: the angle of incidence of an axial marginal ray on the image plane at the maximum Fno when the optical system is focused at infinity f: focal length of the optical system when focused at infinity ω: Maximum angle of view of the optical system when focused at infinity Dr: distance from the image side of the air lens to the image plane Fno: Open Fno when the optical system is focused at infinity Ds: diameter of the aperture stop at open Fno when the optical system is focused at infinity βN: lateral magnification of the focusing group when focused at infinity

2. 2. The optical system according to claim 1, wherein the following condition is satisfied: 0.00 < f / fs < 1.20 (5) however, fs: composite focal length of the lens positioned closer to the object than the aperture stop when focused at infinity

3. 3. The optical system according to claim 1, wherein at least one of the focusing groups satisfies the following condition: 1<T<1 / 2; -3.00 < fN / f < -0.30 (7) however, fN: focal length of the focusing group

4. 4. The optical system according to claim 1, wherein in at least one of the focusing groups, the surface closest to the image side has a shape concave toward the image side.

5. 5. The optical system according to claim 1, wherein the central group includes a focusing group having positive refractive power that satisfies the following conditional expression in addition to the focusing group that moves toward the image side when focusing from an object at infinity to an object at a finite distance: 0.40 < fP / f < 10.00 (8) however, fP: focal length of the focusing group having positive refractive power

6. 6. The optical system according to claim 1, wherein the following condition is satisfied: -1.00 < f / fr < 3.00...(9) however, fr: focal length of the image-side lens group

7. 7. The optical system according to claim 1, wherein the following condition is satisfied: 0.50 < ff / f < 3.50 (10) however, ff: focal length of the object-side lens group

8. 8. The optical system according to claim 1, wherein the object-side lens group has at least one lens having a concave surface facing the image side and negative refractive power, and the object-side lens group has positive refractive power on a side closer to the object than a lens having the strongest refractive power among the lenses having negative refractive power.

9. 9. The optical system according to claim 1, wherein the following condition is satisfied: 0.30 < CrL1f / f (11) however, CrL1f: radius of curvature of the surface closest to the object in the optical system

10. 10. The optical system according to claim 1, wherein the lens closest to the object side has positive refractive power.

11. 11. The optical system according to claim 1, wherein the image-side lens group has at least one lens having a positive refractive power that satisfies the following condition: 1.0<f<1.

0. 1.73 < Ndrp < 2.50 (12) however, Ndrp: refractive index at d line of the lens having positive refractive power

12. 12. The optical system according to claim 1, wherein the following condition is satisfied: -1.25 < Crr / f < -0.10 (13)

13. 13. The optical system according to claim 1, wherein a lens having a convex object-side surface facing the object side is disposed closest to the object side of the object-side lens group.

14. 14. An imaging device comprising: the imaging lens according to claim 1; and an imaging element that receives an optical image formed by the imaging lens and converts the optical image into an electrical image signal.

Images