JP7747473B2 - Optical system and imaging device - Google Patents

Description

This invention relates to an optical system and an imaging device, and in particular to an optical system and an imaging device suitable for small imaging devices using solid-state imaging elements, etc.

Photographing devices using solid-state image sensors, such as digital still cameras and digital video cameras, have become widespread. As the solid-state image sensors used in these imaging devices increase in pixel count, optical systems are increasingly required to provide high resolution while remaining small and lightweight.

In addition, as solid-state image sensors become larger, there is a growing demand for greater blur. This means that optical systems require large-aperture lenses with a shallow depth of field and a maximum aperture of Fno. 2.0 or faster.

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 F-number of 1.8 or faster (see Patent Document 1).

JP 2016-161646 A

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 focal length and a heavy focusing group, which makes it undesirable in terms of miniaturizing the entire lens barrel.

The objective of this invention is to provide a compact, high-performance, large-aperture optical system and imaging device with a maximum aperture of Fno. greater than 2.0 that is suitable for small imaging systems.

In order to solve the above problem, the optical system of the present invention is composed of, in order from the object side, an object-side lens group that is fixed in the optical axis direction when focusing, a central group, and an image-side lens group that is fixed in the optical axis direction when focusing, wherein the central group has at least one negative focusing lens group with negative refractive power that moves in the optical axis direction when focusing, and at least one positive focusing lens group with positive refractive power that moves in the optical axis direction when focusing, the image-side lens group has at least one lens with a concave surface facing the object side, an aperture stop for determining the diameter of the axial light beam is positioned on the object side of the lens with the concave surface facing the object side, and is characterized in that the following conditional expression is satisfied.
0.23 < (FB × tan θm) / (f × tan ω) < 0.50 (1)
0.50 < Dr/(f×tanω) < 1.80 (3)
0.60 < f/Fno/Ds < 2.50 (4)
-1.00 < Crn/f < -0.10 (13)
however,
FB: Air-equivalent length from the surface of the optical system closest to the image plane θm: Incident angle of an axial marginal ray to the image plane at the minimum 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 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; Dr: distance from the object side surface of the lens with the concave surface facing the object side to the image plane; Crn: radius of curvature of the object side surface of the lens with the concave surface facing the object side.

Furthermore, in order to solve the above problem, the imaging device according to the present invention is characterized by comprising the optical system described above and an imaging element that receives the optical image formed by the optical system and converts it into an electrical image signal.

The present invention provides a compact, high-performance, large-aperture optical system and imaging device with a maximum aperture of Fno greater than 2.0 that are suitable for small imaging systems.

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. 3A to 3C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 1 of the present invention. FIG. 10 is a cross-sectional view showing an example of a lens configuration of an optical system according to a second embodiment of the present invention. 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 2 of the present invention. FIG. 10 is a cross-sectional view showing an example of a lens configuration of an optical system according to a third embodiment of the present invention. 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 3 of the present invention. FIG. 10 is a cross-sectional view showing an example of a lens configuration of an optical system according to a fourth embodiment of the present invention. 10A to 10C are diagrams showing spherical aberration, astigmatism, and distortion when photographing an object at infinity in Example 4 of the present invention.

The following describes embodiments of the optical system and imaging device according to the present invention.

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, arranged in order from the object side, an object-side lens group, a central lens group, and an image-side lens group.

In this optical system, focusing from an object at infinity to an object at a finite distance is performed by the central group. 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. Lenses close to the image plane also tend to have relatively large diameters. For this reason, fixing 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, makes it easier to reduce the overall size and weight of the optical system, including its lens barrel configuration.

In this optical system, the central group has at least one negative focusing lens group with negative refractive power that moves along the optical axis during focusing, and one positive focusing lens group with positive refractive power that moves along the optical axis during focusing. Configuring the central group to include at least one focusing group with refractive power of different signs makes it possible for aberrations to cancel each other out when focusing from an object at infinity to an object at a finite distance, reducing aberration fluctuations during focusing and facilitating high performance. Furthermore, focusing using multiple lens groups that move along the optical axis during focusing (hereinafter referred to as "focusing groups") makes it possible to reduce the amount of movement of each focusing group during focusing, facilitating the miniaturization of the optical system.

In this optical system, the aperture stop, which determines the diameter of the axial light beam, is positioned closer to the object than the lens in the image-side lens group with its concave surface facing the object. As a result, in the meridional section, the chief ray of off-axis light passes below the optical axis on the object side of the aperture stop and above the optical axis on the image side of the aperture stop. As a result, aberrations tend to cancel each other out on the object and image sides of the aperture stop. Furthermore, because the angle of incidence of off-axis light on the lens with its concave surface facing the object side is not strong, the occurrence of coma aberration can be suppressed. These factors make it easier to achieve high performance. Furthermore, the closer the lens with its concave surface facing the object side is to the image side, the higher the height of off-axis light rays tends to be, resulting in higher off-axis correction capabilities. Therefore, it is preferable to position the lens with its concave surface facing the object side closest to the image side of the optical system.

The optical configuration of this optical system is explained in more detail below.

(1) Object-side lens group The object-side lens group is located 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 easier to reduce the diameter of the central group. Furthermore, if the object-side lens group has negative refractive power, a diffusion effect occurs on the object side, and the entrance pupil position is closer to the object, making it easier to achieve both a wider angle and a smaller outer diameter.

The specific configuration of the object-side lens group is not particularly limited, but it is preferable that it has at least one lens with negative refractive power whose image-side surface faces the image side and that the lens closer to the object than the lens with the strongest refractive power among the negative refractive power lenses has a combined positive refractive power. This allows for a telephoto system configuration within the object-side lens group. As a result, it becomes easier to achieve both telephoto and large apertures.

When a lens with positive refractive power is positioned closest to the object in the object-side lens group, it has a focusing effect on 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, making it easier to achieve good imaging performance in the optical system.

When a lens with a convex object-side surface facing the object is placed closest to the object in the object-side lens group, a focusing 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, making it easier to achieve good imaging performance in the optical system.

(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 diameter 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.

The specific configuration of the central group is not particularly limited, except that it must include at least one negative focusing lens group, which is a focusing group with negative refractive power, and one positive focusing lens group, which is a focusing group with positive refractive power. By configuring the central group to include at least one focusing group with refractive powers of different signs, it becomes possible for aberrations to cancel each other out when focusing from an object at infinity to an object at a finite distance, reducing aberration fluctuations during focusing and making it easier to achieve high performance. Furthermore, by using multiple focusing groups for focusing, it becomes possible to reduce the amount of movement of each focusing group during focusing, making it easier to achieve compactness.

It is preferable that the central group include at least one focusing group that moves toward the image side 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 focusing group toward the image side is βR, the focusing sensitivity of the focusing group can be expressed as (1-βN ² )×βR ^2. Therefore, if the focusing group moves toward the image side when focusing from an object at infinity to an object at a finite distance, the lateral magnification of the focusing group will be greater than 1. Focusing using a focusing group with a lateral magnification greater than 1 enables telephoto and compactness to be achieved, making it easier to realize an optical system with a small telephoto ratio.

It is also 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 less than 1. Having the composite lateral magnification of the lens positioned closer to the image side than the focusing group that moves toward the image side be less than 1 has the effect of brightening the image side of the optical system, making it easier to achieve a larger aperture.

A lens group that is fixed in the optical axis direction during focusing may be arranged within the central group. However, since the distance between the object-side lens group and the central group and the distance between the central group and the image-side lens group change during focusing, the lens group closest to the object and the lens group closest to the image are the focusing groups.

It is 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 the shape formed by the surface closest to the object side and the surface closest to the image side is a meniscus shape. In other words, this means that in at least one of the focusing groups, the surface closest to the object side and the surface closest to the image side have radii of curvature with the same sign. As long as the signs of the radii of curvature are the same, the sign 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 makes it possible to reduce off-axis aberration fluctuations during focusing, making it easier to achieve high performance.

While there is no limit to the number of lenses in each focusing 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 an air gap. In other words, even if a single lens unit has multiple optical surfaces, only the object-side and image-side surfaces are in contact with air, and the other surfaces are not. Furthermore, in this specification, a single lens may be either a spherical lens or an aspherical lens. Furthermore, aspherical lenses are also considered to include so-called composite aspherical lenses with an aspherical layer attached to their surface. This configuration minimizes various manufacturing errors, such as decentering errors and errors in the spacing between single lenses. This reduces the degradation of optical performance due to manufacturing errors and reduces performance variation between products. As a result, it becomes easier to achieve high performance. Even more preferably, the focusing group is constructed from a single lens, i.e., one single lens or one aspherical lens, which further reduces the degradation of optical performance caused by manufacturing errors and makes it easier to achieve high performance.

(3) Image-side lens group The image-side lens group is disposed closer to the image side 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 closer to the image side. As a result, it will be easier to achieve a small radial size for the image-side lens group.

The configuration of the image-side lens group is not limited as long as it has at least one lens with a concave surface facing the object side, but it is preferable that it have at least 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 has the effect of diverging light rays, making it easier to reduce the diameter of the image-side lens group.

In addition, 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. Here, in order to suppress off-axis coma, it is preferable that the image-side surface of the air lens has a shape with a concave surface facing the object side. In other words, it is preferable that the image-side surface of the air lens be composed of a lens included in the image-side lens group that has a concave surface facing the object side.

Furthermore, it is preferable that the image-side lens that forms the air lens with negative refractive power also has negative refractive power. Having a negative refractive power for the image-side lens makes it easier to correct off-axis coma and field curvature, making it easier to achieve high performance.

Furthermore, it is preferable that the object-side lens that forms the air lens with negative refractive power also has negative refractive power. When the object-side lens has negative refractive power, the exit pupil position is closer to the image side. As a result, it becomes easier to achieve a compact image-side lens group in the radial direction.

The configuration within the image-side lens group is not limited, but it is preferable for it to have at least one lens with positive refractive power. By arranging a component with a converging effect in the image-side lens group, the effect of brightening the optical system occurs on the image plane side, making it possible to darken the composite F-number on the object side of the image-side lens group. 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.

The specific configuration of the image-side lens group is not particularly limited, but it is preferable that a lens with positive refractive power, a lens with negative refractive power, and a lens with negative refractive power are arranged in that order from the object side. It is even more preferable that an air lens be formed by two lenses with negative refractive power. Having such a configuration in the image-side lens group makes it easier to achieve both a large aperture and compact size. It is also preferable that the lens positioned closest to the image in the image-side lens group, i.e., the lens positioned closest to the image in the optical system, is a lens with a concave surface facing the object side.

(4) Aperture Stop The aperture stop of this optical system is located closer to the object than the lens in the image-side lens group whose concave surface faces the object. This causes the chief ray of off-axis light in the meridional section to pass below the optical axis on the object side of the aperture stop and above the optical axis on the image side of the aperture stop. As a result, aberrations on the object side and image side of the aperture stop tend to cancel each other out, making it easier to achieve high performance.

It is preferable that the aperture stop of this optical system be fixed in position along the optical axis when focusing. Because 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.

The aperture stop of this optical system is preferably located within the object-side lens group. In the case of interchangeable lenses, 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.

1-2. Conditional Expressions In the optical system, it is preferable to employ the above-described configuration and also satisfy the following conditional expressions.

1-2-1. Conditional Expression (1)
It is preferable that the optical system satisfies the following condition:
0.23 < (FB × tan θm) / (f × tan ω) < 0.50 ... (1)
however,
FB: Air-equivalent length from the surface of the optical system closest to the image plane to the image plane θm: Incident angle of an axial marginal ray to the image plane at open 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

The above conditional expression (1) defines the ratio of the axial ray height to the height of the image plane at the surface closest to the image plane at maximum F-number. Here, θm is the angle between the normal to the image plane and the axial marginal ray, and is expressed as an absolute value. Increasing the aperture of an optical system with a long flange focal distance results in a higher axial ray height within the image-side lens group. If the axial ray height is high, correcting off-axis performance will also affect on-axis performance. Furthermore, if the axial ray height is too low, it becomes difficult to increase the aperture. Therefore, by defining the axial ray height within the image-side lens group within a certain range, it is possible to achieve a larger aperture while more effectively correcting image plane characteristics. Here, if the above conditional expression (1) is satisfied, a larger aperture can be achieved, and an optical system with high off-axis performance can be achieved.

On the other hand, if the value of conditional expression (1) above exceeds the upper limit, the height of the axial ray on the surface furthest to the image side becomes too high relative to the image plane height, resulting in insufficient correction of field curvature and coma, which is undesirable from the perspective of high performance. If the value of conditional expression (1) above falls below the lower limit, the height of the axial ray on the surface furthest to the image side becomes too low relative to the image plane height, which is undesirable from the perspective of large apertures.

To achieve the above effect, the upper limit of the above conditional formula (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. Furthermore, the lower limit of the above conditional formula (1) is preferably 0.24, more preferably 0.25, and even more preferably 0.26.

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:
-0.50 <(Crf+Crr)/(Crf-Crr)< 4.50...(2)
however,
Crf: radius of curvature of the object side surface of the air lens Crr: radius of curvature of the image side surface 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.

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) approaches zero. When conditional expression (2) is positive, the absolute value of the radius of curvature on the object side is greater than the absolute value of the radius of curvature on the image side. Furthermore, 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, an air lens with negative refractive power has the effect of diverging light rays toward the image-side lens group. This shifts the exit pupil position closer to the image side. As a result, this is effective in reducing the radial size of the image-side lens group. Furthermore, off-axis light rays emerging from an air lens with negative refractive power tend to be deflected upward. To suppress off-axial coma, it is preferable for the image-side surface of the air lens to 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.

On the other hand, if the value of conditional expression (2) above exceeds 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 negative refractive power of the air lens will become weaker, which will move the exit pupil farther from the image plane, which is undesirable from the perspective of compactness. If the value of conditional expression (2) above falls 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.

To achieve the above effect, the upper limit of the above conditional formula (2) is preferably 4.20, more preferably 3.90, even more preferably 3.50, even more preferably 3.10, and even more preferably 2.80. Furthermore, the lower limit of the above conditional formula (2) is preferably -0.40, more preferably -0.20, even more preferably -0.10, even more preferably 0.03, and even more preferably 0.15.

1-2-3. Conditional Expression (3)
It is preferable that the optical system satisfies the following condition:
0.50 < Dr/(f×tanω) < 1.80 (3)
however,
Dr: distance from the object side of the lens with the concave surface facing the object side 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

The above conditional expression (3) defines the distance from the object-side surface of the lens with a concave surface facing the object side, included in the image-side lens group of the optical system, to the image plane and the image plane height. Here, the distance from the surface closest to the image side of the optical system to the image plane is the air-equivalent length. Because the lens with a concave surface facing the object side is positioned closer to the image side than the aperture stop, the chief ray of off-axis light passes above the optical axis in the meridional cross section. If the lens with a concave surface facing the object side is farther from the image plane, the height of off-axis light rays decreases, reducing the effect of off-axis performance correction. This makes it difficult to achieve high performance, increases the flange back distance, and makes it difficult to reduce the size in the overall length direction. Furthermore, if the lens with a concave surface facing the object side is too close to the image plane, the diameter of the lens with a concave surface facing the object side increases. Therefore, satisfying conditional expression (3) makes it possible to achieve both compactness and high performance.

On the other hand, if the numerical value of the above conditional expression (3) is above the upper limit, the effect of correcting coma aberration will be reduced, making it difficult to improve performance and also resulting in an increase in size in the overall length direction, which is undesirable from the perspectives of high performance and compactness. If the numerical value of the above conditional expression (3) is below the lower limit, the diameter of the lens with its concave surface facing the object side will increase, which is undesirable from the perspective of compactness.

To achieve the above effect, the upper limit of the above conditional formula (3) is preferably 1.70, more preferably 1.60, even more preferably 1.45, even more preferably 1.30, and even more preferably 1.25. Furthermore, the lower limit of the above conditional formula (3) is preferably 0.60, more preferably 0.65, even more preferably 0.70, even more preferably 0.75, and even more preferably 0.84.

1-2-4. Conditional Expression (4)
It is preferable that the optical system satisfies the following condition:
0.60 < f/Fno/Ds < 2.50 (4)
however,
f: focal length of the optical system when focused at infinity Fno: minimum Fno of the optical system when focused at infinity
Ds: diameter of the aperture stop at the minimum Fno when the optical system is focused at infinity

The above conditional expression (4) defines the ratio between the entrance pupil diameter of the optical system and the diameter of the aperture stop (aperture diameter). By positioning the aperture stop at a position that satisfies conditional expression (4), a good balance is achieved between the aperture diameter and the radial size of the optical system, achieving both compactness and high performance.

On the other hand, if the numerical value of the above 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 perspective of high performance. If the numerical value of the above conditional expression (4) is below the lower limit, the aperture diameter becomes large, which is undesirable from the perspective of compactness.

To achieve the above effect, the upper limit of the above conditional formula (4) is preferably 2.30, more preferably 2.12, even more preferably 2.02, even more preferably 1.89, and even more preferably 1.79. Furthermore, the lower limit of the above conditional formula (4) is preferably 0.70, more preferably 0.90, even more preferably 1.00, even more preferably 1.10, and even more preferably 1.20.

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: composite focal length of the lens located on the object side of the aperture stop when focused at infinity

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. If the composite focal length of the lens on the object side of the aperture stop is infinity, no F-number error due to positional error will occur even if the aperture stop is misaligned in the optical axis direction during manufacturing. On the other hand, if the composite focal length of the lens on the object side of the aperture stop is small, and the aperture stop is misaligned in the optical axis direction during manufacturing, the F-number error due to positional error will increase. 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, if conditional expression (5) is satisfied, the F-number error due to manufacturing will be small, and a compact optical system can be achieved.

On the other hand, if the numerical value of the above conditional expression (5) is above the upper limit, the light-gathering effect of the lens on the object side of the aperture stop will be too strong, and if the aperture stop is misaligned in the optical axis direction during manufacturing, the F-number error due to an error in the position of the aperture stop in the optical axis direction will increase, which is undesirable. If the numerical value of the above 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.

To achieve the above effect, the upper limit of the above conditional formula (5) is preferably 1.10, more preferably 1.05, even more preferably 1.00, even more preferably 0.95, and even more preferably 0.90. Furthermore, the lower limit of the above conditional formula (5) is preferably 0.05, more preferably 0.10, even more preferably 0.15, and even more preferably 0.19.

1-2-6. Conditional Expression (6)
It is preferable that the optical system satisfies the following condition:
1.20 < βN < 4.00 (6)
however,
βN: lateral magnification of the negative focusing lens group when focusing at infinity

The above conditional expression (6) defines the lateral magnification of the negative focusing lens group when focusing at infinity. If the lateral magnification of the focusing lens group is βN and the combined lateral magnification on the image side of the focusing lens group is βR, the focusing sensitivity of the focusing lens group can be expressed as (1-βN ² )×βR ^2. If the lateral magnification of the focusing lens group is greater than 1, this means that the focusing lens 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 increase the focal length and shorten the overall length. When the negative focusing lens group satisfies conditional expression (6), the lateral magnification of the negative focusing lens group falls within an appropriate range, allowing for compactness.

On the other hand, if the numerical value of the above conditional expression (6) is above the upper limit, the lateral magnification of the negative focusing lens group when focusing at infinity becomes large, and the composite F-number from the lens closest to the object to the negative focusing lens group becomes dark, which is undesirable in terms of increasing the aperture. On the other hand, if the numerical value of the above conditional expression (6) is below the lower limit, the lateral magnification of the negative focusing lens group when focusing at infinity becomes small, which is undesirable in terms of making it difficult to achieve telephoto and compactness.

To achieve the above effect, the upper limit of the above conditional formula (6) is preferably 3.80, more preferably 3.60, even more preferably 3.50, even more preferably 3.40, and even more preferably 3.30. Furthermore, the lower limit of the above conditional formula (6) is preferably 1.30, more preferably 1.40, even more preferably 1.50, even more preferably 1.60, and even more preferably 1.70.

1-2-7. Conditional Expression (7)
In the optical system, it is preferable that the negative focusing lens group satisfies the following condition:
-3.00 < fN/f < -0.30 (7)
however,
fN: focal length of the negative focusing lens group f: focal length of the optical system

The above conditional expression (7) defines the ratio between the focal length of the negative focusing lens group and the focal length of the optical system. There are no restrictions on the direction in which this negative focusing lens group moves when focusing from an object at infinity to an object at a finite distance. If a lens group with negative refractive power is used as the focusing group, it is easy to make the lateral magnification greater than 1, so it is preferable that this negative focusing lens group move toward the image side when focusing from an object at infinity to an object at a finite distance. When conditional expression (7) is satisfied, the refractive power of the negative focusing lens group falls within an appropriate range, suppressing aberration fluctuations during focusing and enabling good imaging performance to be obtained with a small number of lenses regardless of the distance to the subject, making it easy to achieve a compact, high-performance optical system.

On the other hand, if the numerical value of the above conditional expression (7) is above the upper limit, the refractive power of the negative focusing lens group will be 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, the weight of the negative focusing lens group will increase, and the mechanical components for driving the negative focusing lens group will also become larger, making it difficult to achieve compactness and weight reduction, which is undesirable. On the other hand, if the numerical value of the above conditional expression (7) is below the lower limit, the refractive power of the negative focusing lens group will be weak, increasing the amount of movement during focusing, making it difficult to achieve compactness of the optical system, which is undesirable.

To achieve the above effect, the upper limit of the above conditional formula (7) is preferably -0.40, more preferably -0.50, even more preferably -0.60, even more preferably -0.70, and even more preferably -0.75. Furthermore, the lower limit of the above conditional formula (7) is preferably -2.75, more preferably -2.60, even more preferably -2.35, even more preferably -2.10, and even more preferably -1.95.

1-2-8. Conditional Expression (8)
In the optical system, it is preferable that the focusing lens group satisfies the following condition:
0.40 < fP/f < 10.00 (8)
however,
fP: focal length of the lens group with correct focus f: focal length of the optical system

The above conditional expression (8) defines the ratio between the focal length of the focus lens group and the focal length of the optical system. By satisfying conditional expression (8), the amount of movement of the focus lens group can be reduced, allowing for the miniaturization of the optical system along the overall optical length. Furthermore, when conditional expression (8) is satisfied, the focal length of the focus lens group, i.e., the refractive power, falls within an appropriate range, making it possible to suppress aberration fluctuations that accompany changes in the position of the focus lens group during focusing, and achieving good imaging performance regardless of object distance with a smaller number of lenses.

In contrast, if the value of conditional expression (8) above exceeds the upper limit, the refractive power of the focusing lens group will be weak. As a result, the amount of movement of the focusing lens group during focusing will be large, making it difficult to reduce the size of the optical system along the overall optical length. On the other hand, if the value of conditional expression (8) above is below the lower limit, the refractive power of the focusing lens group during focusing will be too strong, making it difficult to correct spherical aberration and to achieve good imaging performance with a small number of lenses, which is undesirable from the perspectives of high performance and low cost.

To achieve the above effect, the upper limit of conditional formula (8) is preferably 8.00, more preferably 7.00, and even more preferably 6.00. 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.

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

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 the optimal range, making it easier to achieve both a large aperture and high performance.

On the other hand, if the numerical value of the above conditional expression (9) is above the upper limit, the positive refractive power of the image-side lens group becomes strong, making it easier to increase the aperture, but the amount of spherical aberration and coma aberration generated increases, which is undesirable from the perspective of improving performance. On the other hand, if the numerical value of the above conditional expression (9) is below the lower limit, the negative refractive power of the image-side lens group becomes strong, making the optical system dark, which is undesirable from the perspective of increasing the aperture.

To achieve the above effect, the upper limit of the above conditional formula (9) is preferably 2.30, more preferably 1.80, even more preferably 1.45, even more preferably 1.39, and even more preferably 1.25. Furthermore, the lower limit of the above conditional formula (9) is preferably -0.90, more preferably -0.80, even more preferably -0.72, even more preferably -0.60, and even more preferably -0.50.

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

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 optimal range, making it easier to achieve a compact, high-performance, and large-aperture optical system.

On the other hand, if the numerical value of the above conditional expression (10) is above the upper limit, that is, the focal length of the object-side lens group becomes too large relative to the focal length of the optical system, making it difficult to miniaturize the optical system in the overall optical length direction. If the numerical value of the above conditional expression (10) is below the lower limit, that is, the focal length of the object-side lens group becomes too small 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.

To achieve the above effect, the upper limit of the above conditional formula (10) is preferably 2.90, more preferably 2.60, even more preferably 2.30, even more preferably 1.900, and even more preferably 1.50. Furthermore, the lower limit of the above conditional formula (10) is preferably 0.55, more preferably 0.60, even more preferably 0.65, even more preferably 0.70, and even more preferably 0.75.

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

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 convex or flat toward the object side. By satisfying conditional expression (11), the surface closest to the object in the optical system is convex or flat toward the object side, making it easier to reduce the occurrence of coma and distortion, and achieving an optical system with high imaging performance.

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 the following conditional expression (11)', the object-side radius of curvature of the surface closest to the object falls within an optimum range, thereby making it possible to reduce the occurrence of coma aberration and effectively suppress the occurrence of ghost light.
0.30 < CrL1f/f <2000.00...(11)'

In contrast, if the value of conditional expression (11)' above exceeds 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)' above falls below 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.

To achieve the above effect, the upper limit of conditional formula (11)' is preferably 1000.00, more preferably 200.00, even more preferably 100.00, even more preferably 10.00, and even more preferably 3.00. Furthermore, the lower limit of conditional formula (11) or conditional formula (11)' is preferably 0.35, more preferably 0.40, even more preferably 0.45, even more preferably 0.50, and even more preferably 0.55.

1-2-12. Conditional Expression (12)
It is preferable that the image-side lens unit 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: refractive index at d line of the lens having the positive refractive power

The above conditional expression (12) defines the refractive index of the lens with positive refractive power included in the image-side lens group. By arranging a component with a converging effect in the image-side lens group, the effect of brightening the optical system occurs on the image plane side, making it possible to darken the composite Fno on the object side of the image-side lens group. 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 conditional expression (12), the refractive index of the lens with positive refractive power included in the image-side lens falls within the optimal range, achieving low cost and a large aperture.

On the other hand, if the value of conditional expression (12) above exceeds 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) above is below the lower limit, this will lead to the radius of curvature being reduced 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 aberration, which is undesirable. It will also lead to an increase in the number of lenses, which will make it difficult to reduce costs, which is undesirable.

To achieve the above effect, the upper limit of conditional formula (12) is preferably 2.20, more preferably 2.11, even more preferably 2.06, and even more preferably 2.01. Furthermore, the lower limit of conditional formula (12) is preferably 1.76, more preferably 1.78, even more preferably 1.80, and even more preferably 1.82.

1-2-13. Conditional Expression (13)
It is preferable that the optical system satisfies the following condition:
-1.00 < Crn/f < -0.10 (13)
however,
Crn: Radius of curvature of the object-side surface of a lens with a concave surface facing the object side f: Focal length of the optical system

The above conditional expression (13) defines the ratio between the radius of curvature of the object-side surface of a lens included in the image-side lens group with its concave surface facing the object side and the focal length of the optical system. By satisfying conditional expression (13), the angle of incidence of light rays incident on the lens with its concave surface facing the object side is optimized, making it easier to reduce the occurrence of coma aberration and achieving an optical system with high imaging performance.

On the other hand, if the value of conditional expression (13) above exceeds the upper limit, that is, the radius of curvature of the object-side surface of the lens with a concave surface facing the object side becomes too small for the focal length of the optical system, resulting in over-correction of coma and making it difficult to achieve high performance. If the value of conditional expression (13) above falls below the lower limit, that is, the radius of curvature of the object-side surface of the lens with a concave surface facing the object side becomes too large for the focal length of the optical system, making it difficult to correct coma, which is undesirable in terms of improving high performance.

To achieve the above effect, the upper limit of the above conditional expression (13) is preferably -0.15, more preferably -0.18, even more preferably -0.21, even more preferably -0.24, and even more preferably -0.28. Furthermore, the lower limit of the above conditional expression (13) is preferably -0.98, more preferably -0.96, even more preferably -0.94, and even more preferably -0.92.

2. Imaging Device Next, we will explain the imaging device according to the present invention. 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.

Here, there are no particular limitations on the imaging element, and solid-state imaging elements such as CCD (Charge Coupled Device) sensors and CMOS (Complementary Metal Oxide Semiconductor) sensors can also be used. The imaging device of the present invention is suitable for imaging devices that use 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, of course, an interchangeable-lens imaging device such as a single-lens reflex camera or a mirrorless single-lens camera.

Next, the present invention will be explained in detail using examples. However, the present invention is not limited to the following examples. Also, in each lens cross-sectional view, the left side is the object side and the right side is the image side as you face the drawing.

(1) Lens Configuration of Optical System FIG. 1 is a cross-sectional view showing the configuration of an optical system according to Example 1 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.

Here, the fifth lens L5 included in the central group GM corresponds to the negative focusing lens group GN. The image side surface of the fifth lens L5 has a concave shape facing the image side. The fifth lens L5, which is part of the negative focusing lens group GN, 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 sixth lens L6 included in the central group GM corresponds to the positive focusing lens group GP. The sixth lens L6, which is part of the positive focusing lens group GP, moves toward the object side in the optical axis direction 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 surface facing the image side and has negative refractive power. The lenses located 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. The tenth lens L10 corresponds to a lens with a concave surface facing the object side.

Note that "IMG" in the diagram indicates the image plane. This is the imaging surface of a solid-state imaging element, such as the CCD sensor or CMOS sensor mentioned 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 processor, etc.) provided in the imaging device generates a digital image corresponding to the subject's image 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 disc, or magnetic tape. Note that the image plane may also be the film surface of a silver halide film.

Furthermore, "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.

(2) Numerical Examples Numerical Examples will be described below, which apply specific numerical values of the optical system employed in Example 1. Table 1 shows lens data for 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 for which 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).

Table 2 shows various data for 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. Additionally, 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.

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.

Table 4 shows the focal lengths of each lens that make up the optical system.

Table 5 shows the focal lengths of each lens group that makes up the optical system.

Table 21 also shows the values of each conditional expression for this optical system. The details regarding these tables are the same as those for the tables shown in the other examples, so explanations will be omitted below.

Figure 2 shows the longitudinal aberration diagram of this optical system when focused at infinity. The longitudinal aberration diagram in Figure 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). 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 image height (mm). The solid line represents the sagittal direction at the d-line (wavelength 587.56 nm), and the dotted line represents the meridional direction at the d-line. In the diagram showing distortion, the vertical axis represents image height (mm) and distortion (%) at the d-line (wavelength 587.56 nm). These matters regarding longitudinal aberration diagrams are the same as those shown in the other examples, so explanations will be omitted below.

[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

[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

[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

[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

[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

(1) Lens Configuration of Optical System Figure 3 is a cross-sectional view showing the configuration of an optical system according to Example 2 of the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a center group GM, and an image-side lens group GR. The front 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 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 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 group GM is composed of a seventh lens L7 having negative refractive power and a meniscus shape with its image-side surface concave toward the image side, and an eighth lens L8 having positive refractive power and a meniscus shape with its object-side surface convex toward the object side. The image-side lens group GR is composed of, in order from the object side, a cemented lens formed by cementing 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 with its object-side surface concave toward the object side.

Here, the seventh lens L7 included in the central group GM corresponds to the negative focusing lens group GN. The image side surface of the seventh lens L7 has a concave shape facing the image side. The seventh lens L7, which is part of the negative focusing lens group GN, 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 eighth lens L8 included in the central group GM corresponds to the positive focusing lens group GP. The eighth lens L8, which is part of the positive focusing lens group GP, moves toward the object side in the optical axis direction 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 surface facing the image side and has negative refractive power. The lenses located 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. The eleventh lens L11 corresponds to a lens with a concave surface facing the object side.

(2) Numerical Examples Next, we will explain numerical examples that apply specific numerical values of the optical system adopted in Example 2. 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. Also, Figure 4 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[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

[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

[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

[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

[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

(1) Lens Configuration of Optical System Figure 5 is a cross-sectional view showing the configuration of an optical system according to Example 3 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 surface and an image-side surface, 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 surface and an concave object-side surface and an image-side surface, 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.

Here, the fifth lens L5 included in the central group GM corresponds to the negative focusing lens group GN. The image side surface of the fifth lens L5 has a concave shape facing the image side. The fifth lens L5, which is part of the negative focusing lens group GN, 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 sixth lens L6 included in the central group GM corresponds to the positive focusing lens group GP. The sixth lens L6, which is part of the positive focusing lens group GP, moves toward the object side in the optical axis direction 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 surface facing the image side and has negative refractive power. The lenses located 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. The ninth lens L9 corresponds to a lens with a concave surface facing the object side.

(2) Numerical Examples Next, numerical examples will be described that apply the specific numerical values of the optical system employed in Example 3. Tables 11 to 15 show the lens data of the optical system, various data of the optical system, variable interval data, the focal length of each lens, and the focal length of each lens group. Also, FIG. 6 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[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

[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

[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

[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

[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

(1) Lens Configuration of Optical System Figure 7 is a lens cross-sectional view showing the configuration of an optical system of Example 4 according to the present invention. This optical system is composed of, in order from the object side, an object-side lens group GF, a center 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 first lens L1 having positive refractive power and a meniscus shape with its object-side surface convex toward the object side, a second lens L2 having positive refractive power and a meniscus shape with its object-side surface convex toward the object side, a third lens L3 having negative refractive power and a meniscus shape with its object-side surface convex toward the object side, an aperture stop S, and a fourth lens L4 having positive refractive power and a meniscus shape with its object-side surface convex toward the object side. The central group GM is composed of a fifth lens L5 having negative refractive power and a meniscus shape with its image-side surface concave toward the image side, a sixth lens L6 having positive refractive power and a meniscus shape with its object-side surface convex toward the object side, and a seventh lens L7 having positive refractive power and a biconvex shape with its object-side and image-side surfaces both convex. The image-side lens group GR is composed of, in order from the object side, an eighth lens L8 having negative refractive power, a ninth lens L9 having positive refractive power, and a tenth lens L10 having negative refractive power cemented together, and an eleventh lens L11 having negative refractive power and a meniscus shape with its object-side surface concave toward the object side.

Here, the fifth lens L5 included in the central group GM corresponds to the negative focusing lens group GN. The image side surface of the fifth lens L5 has a concave shape facing the image side. The fifth lens L5, which is part of the negative focusing lens group GN, 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 sixth lens L6 included in the central group GM does not move in the optical axis direction when focusing from an object at infinity to an object at a finite distance. The seventh lens L7 included in the central group GM corresponds to the positive focusing lens group GP. The seventh lens L7, which is part of the positive focusing lens group GP, moves toward the object side in the optical axis direction 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 ninth lens L9 corresponds to a lens having positive refractive power included in the image-side lens group GR. The tenth lens L10 corresponds to a lens with a concave surface facing the object side.

(2) Numerical Examples Next, numerical examples will be described that apply the specific numerical values of the optical system employed in Example 4. Tables 16 to 20 show the lens data of the optical system, various data of the optical system, aspherical data, the focal length of each lens, and the focal length of each lens group. Also, FIG. 8 shows longitudinal aberration diagrams of the optical system when focused at infinity.

[Table 16]
Surface number rd Nd vd h
1 60.8231 8.898 1.80420 46.50 26.900
2 723.4945 0.320 26.089
3 48.0053 4.280 1.72916 54.67 23.281
4 84.5273 6.320 22.600
5 12646.5964 1.757 1.78472 25.72 20.755
6 39.2354 9.660 18.869
7 S INF 2.550 18.400
8 62.2575 4.563 1.80420 46.50 18.206
9 553.0732 D9 17.879
10 159196.6360 1.200 1.48749 70.44 16.758
11 36.1336 D11 15.751
12 77.4716 1.298 1.72916 54.67 15.104
13 90.4790 D13 15.018
14 82.9791 4.730 1.69680 55.46 15.000
15 -89.8273 D15 15.358
16 -94.7217 1.352 1.78472 25.72 15.916
17 38.8992 12.870 1.99427 32.31 17.061
18 -36.7128 1.453 1.57307 41.58 17.497
19 -210.0840 4.086 17.397
20 -43.2484 1.335 1.85883 30.00 17.303
21 -120.3514 19.553 17.870
22 INF 2.500 1.51633 64.15 21.269
23 INF 1.000 21.535

[Table 17]
Focal length 75.729
F-number 1.457
Half angle of view 16.394
Image height 21.630
Lens length 111.931
BF (in air) 22.202

[Table 18]
Variable Interval Data
D0 INF 2583.498 1082.824 664.019
D9 2.548 5.073 9.337 14.058
D11 15.344 12.820 8.556 3.834
D13 2.089 1.676 1.496 1.065
D15 2.226 2.638 2.818 3.249

[Table 19]
Lens Surface Number Focal Length
L1 1-2 82.083
L2 3-4 145.198
L3 5-6 -50.158
L4 8-9 86.875
L5 10-11 -74.139
L6 12-13 709.220
L7 14-15 62.606
L8L9L10 16-19 85.719
L11 20-21 -79.237

[Table 20]
Group Surface number Focal length
GF 1-9 82.466
GM 10-15 123.629
GR 16-21 -1609.540
GN 10-11 -74.139
GP 14-15 62.606

[Table 21]
Example 1 Example 2 Example 3 Example 4
(1) (FB × tanθm)/(f × tanω) 0.367 0.294 0.365 0.364
(2) (Crf+Crr)/(Crf-Crr) 1.649 0.898 1.776 1.518
(3) Dr/(f×tanω) 1.066 0.859 1.087 1.056
(4) f/Fno/Ds 1.413 1.317 1.451 1.412
(5) f/fs 0.315 0.775 0.224 0.328
(6) βN 3.035 2.215 3.174 3.019
(7) fN/f -0.953 -1.555 -0.867 -0.979
(8) fP/f 0.708 1.137 5.435 0.827
(9) f/fr -0.171 -0.190 1.171 -0.047
(10) ff/f 1.067 1.290 1.007 1.089
(11) CrL1f/f 0.818 0.879 0.788 0.803
(12) Ndrp 1.981 1.835 1.998 1.994
(13) Crn/f -0.599 -0.463 -0.701 -0.571

Example 1 Example 2 Example 3 Example 4
FB 22.252 17.645 22.181 22.202
θm 20.067 20.271 19.983 20.073
f 75.830 75.563 76.090 75.729
ω 16.269 16.366 16.175 16.394
Crf -185.391 652.912 -190.918 -210.084
Crr -45.418 -34.968 -53.354 -43.248
Dr 23.587 19.071 23.981 23.537
Fno 1.458 1.443 1.461 1.457
Ds 36.800 39.760 35.900 36.800
fs 241.015 97.486 339.381 230.373
fN -72.265 -117.500 -65.983 -74.139
fP 53.658 85.948 413.541 62.606
fr -442.480 -398.279 64.976 -1609.54
ff 80.8747 97.486 76.595 82.466
CrL1f 62.059 66.438 59.954 60.823
Crn -45.418 -34.968 -53.354 -43.248

The present invention provides a compact, high-performance, large-aperture optical system and imaging device with a maximum aperture of Fno greater than 2.0, suitable for use in small imaging systems.

GF: Object-side lens group GM: Central group GR: Image-side lens group GN: Negative focusing lens group GP: Positive focusing lens group L1: First lens L2: Second lens L3: Third lens L4: Fourth lens
L5: 5th lens L6: 6th lens L7: 7th lens L8: 8th lens L9: 9th lens L10: 10th lens L11: 11th lens S: Aperture stop CG: Optical block IMG: Image plane

Claims (13)

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, and an image-side lens group that is fixed in the optical axis direction during focusing, the central group having at least one negative focusing lens group that has negative refractive power and that moves in the optical axis direction during focusing, and one positive focusing lens group that has positive refractive power and that moves in the optical axis direction during focusing, the image-side lens group having a lens with a concave surface facing the object side and an air lens with negative refractive power , an aperture stop for determining the diameter of an axial light beam is located on the object side of the lens with the concave surface facing the object side,
An optical system characterized by satisfying the following conditional expression:
0.23 < (FB × tan θm) / (f × tan ω) < 0.50 (1)
0.50 < Dr/(f×tanω) < 1.80 (3)
0.60 < f/Fno/Ds < 2.50 (4)
-1.00 < Crn/f < -0.10 (13)
1.20 < βN < 4.00 (6)
-0.50 <(Crf+Crr)/(Crf-Crr)<2.80...(2)
however,
FB: Air-equivalent length from the surface of the optical system closest to the image plane θm: Incident angle of an axial marginal ray to the image plane at the minimum 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 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; Dr: distance from the object side surface of the lens with the concave surface facing the object side to the image plane; Crn: radius of curvature of the object side surface of the lens with the concave surface facing the object side.
βN: lateral magnification of the negative focusing lens group when focusing at infinity
Crf: radius of curvature of the object side surface of the air lens
Crr: radius of curvature of the image side surface of the air lens 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, and an image-side lens group that is fixed in the optical axis direction during focusing, the central group having at least one negative focusing lens group that has negative refractive power and moves in the optical axis direction during focusing, and one positive focusing lens group that has positive refractive power and moves in the optical axis direction during focusing, the image-side lens group having a lens with a concave surface facing the object side, an aperture stop for determining the diameter of an axial light beam is arranged on the object side of the lens with the concave surface facing the object side, the lens arranged on the object side of the lens with the concave surface facing the object side has negative refractive power, and the lens closest to the image side is a lens with a concave surface facing the object side,
An optical system characterized by satisfying the following conditional expression:
0.23 < (FB × tan θm) / (f × tan ω) < 0.50 (1)
0.50 < Dr/(f×tanω) < 1.80 (3)
0.60 < f/Fno/Ds < 2.50 (4)
-1.00 < Crn/f < -0.10 (13)
however,
FB: Air-equivalent length from the surface of the optical system closest to the image plane θm: Incident angle of an axial marginal ray to the image plane at the minimum 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 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; Dr: distance from the object side surface of the lens with the concave surface facing the object side to the image plane; Crn: radius of curvature of the object side surface of the lens with the concave surface facing the object side. 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, and an image-side lens group that is fixed in the optical axis direction during focusing, the central group having at least one negative focusing lens group that has negative refractive power and that moves in the optical axis direction during focusing, and at least one positive focusing lens group that has positive refractive power and that moves in the optical axis direction during focusing, the image-side lens group having a lens with a concave surface facing the object side, and an aperture stop for determining the diameter of an axial light beam is located on the object side of the lens with the concave surface facing the object side,
An optical system characterized by satisfying the following conditional expression:
0.23 < (FB × tan θm) / (f × tan ω) < 0.50 (1)
0.50 < Dr/(f×tanω) < 1.80 (3)
0.60 < f/Fno/Ds < 2.02 ...(4)
-1.00 < Crn/f < -0.10 (13)
0.15 < f/fs < 1.05 (5)
however,
FB: Air-equivalent length from the surface of the optical system closest to the image plane θm: Incident angle of an axial marginal ray to the image plane at the minimum 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 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; Dr: distance from the object side surface of the lens with the concave surface facing the object side to the image plane; Crn: radius of curvature of the object side surface of the lens with the concave surface facing the object side.
fs: composite focal length when the lens positioned on the object side of the aperture stop is focused at infinity 4. The optical system according to claim 1 , wherein the following condition is satisfied:
-3.00 < fN/f < -0.30 (7)
however,
fN: focal length of the negative focusing lens group 5. The optical system according to claim 1 , wherein the surface of the negative focusing lens group closest to the image side has a shape concave toward the image side. 6. The optical system according to claim 1 , wherein the following condition is satisfied:
0.40 < fP/f < 10.00 (8)
however,
fP: focal length of the in-focus lens group 7. 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 8. 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 9. 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. 10. 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 11. The optical system according to claim 1, wherein the lens closest to the object side has positive refractive power. 12. The optical system according to claim 1 , wherein the image-side lens group includes at least one lens having a positive refractive power that satisfies the following condition: 1.0 <1.0 <1.0.
1.73 < Ndrp < 2.50 (12)
however,
Ndrp: refractive index at d line of the lens having positive refractive power 13. 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.