Patent Description:
<CIT> discloses a single focal length lens system comprising: at least a front lens unit having positive or negative power, and being fixed in focusing; a focusing lens unit having positive power, and moving along an optical axis in focusing; and a rear lens unit having negative power, and being fixed in focusing, wherein the focusing lens unit includes: a cemented lens of a negative lens and a positive lens; and a positive single lens located on the image side relative to the cemented lens and having an aspheric surface, the rear lens unit is composed of one single lens, and the condition: -<NUM> < fw / fGR < - <NUM> (fw, fGR focal lengths of the entire single focal length lens system, the rear lens unit) is satisfied.

In order to obtain an imaging optical system that is composed of a single focus lens, has high performance, a wide angle of view and a large diameter, and is small in a load of a driving source by an inner focusing method, it is proposed in <CIT> that the imaging optical system is configured by arranging: an aperture stop S; a first lens group <NUM> on the object side of the aperture stop S; and a second lens group <NUM> on the image side of the aperture stop S. The first lens group <NUM> has a negative lens on the most object side, and the second lens group <NUM> is configured by arranging, in order from the object side: a positive 2F lens group 2FG; and a negative 2R lens group 2RG. The 2F lens group is composed of a 2Fa lens group 2FaG and a positive 2Fb lens group 2FbG, and upon focusing, only the 2Fb lens group 2FbG moves in an optical direction, and let M2Fb be a magnification when the 2Fb lens group is focused at infinity, and M2R be a magnification when the 2R lens group is focused at infinity, M2Fb and M2R satisfy a condition: <NUM> < (<NUM> - M2Fb) × M2R < <NUM>.

An imaging lens according to <CIT> includes: a first lens group; a second lens group having positive refractive power; and a third lens group having negative refractive power, which are arranged in order from an object side, wherein the first lens group includes a former lens group having a negative lens in a most object side, a diaphragm, and a rear lens group, and wherein, when focusing is performed, the second lens group is moved in the optical axis direction.

In order to achieve, in an image pickup lens, a small F number, high performance, reduced weight of a focus group, and suppressed aberration change and image angle change at the time of focusing, it is proposed in <CIT> that this image pickup lens is configured by sequentially having, from the object side, a first lens group, a diaphragm, a positive second lens group, which is a focus group, and a third lens group. The first lens group is configured by sequentially having, from the object side, a positive first sub-lens group, which is configured of a negative meniscus lens having the convex surface facing the object side, a positive meniscus lens having the convex surface facing the object side, and a positive lens, and a negative second sub-lens group, which is configured of a negative lens, and a cemented lens configured of negative and positive lenses. The second lens group is configured of a positive single lens or a cemented lens. The third lens group is configured by sequentially having, from the object side, a cemented lens configured of positive and negative lenses, a positive lens, and a negative lens. Predetermined conditional equations relating to a focal point distance and an image-forming magnification of the second lens group are satisfied.

An image pickup lens according to <CIT> includes in order from an object side, a front lens unit having a positive refractive power, one focusing lens unit having a positive refractive power, and a rear lens unit having a negative refractive power, wherein at the time of focusing, the focusing lens moves on an optical axis, and a single lens having a positive refractive power is disposed nearest to an object in the rear lens unit.

A macro lens system according to <CIT> has, from the object side, a first lens group; a second, positive, lens group; and a third, negative, lens group. Focusing from an object at infinity to an object at a close distance is achieved by moving only the second lens group toward the object side along the optical axis while keeping the first and third lens groups stationary, and the conditional formulae B ≤ -<NUM> (B representing the lateral magnification with focus on the closest object) and <NUM> < f2 / f < <NUM> (f2 representing the focal length of the second lens group, f representing the focal length of the macro lens system as a whole with focus on an object at infinity) are fulfilled.

The super-wide lens according to <CIT> substantially consists of a positive first lens group, a positive second lens group, and a negative third lens group, in this order from the object side. The first lens group and the third lens group are fixed and the second lens group moves toward the object side while focusing from an object at infinity to an object at the closest distance. The first lens group substantially consists of a negative first sub lens group, a positive second sub lens group, an aperture stop, and a positive third sub lens group in this order from the object side. Conditional expressions <NUM><f1/f2<<NUM> and <NUM><f2/f<<NUM> (<NUM>) are satisfied, where f1 is the focal length of the first lens group, f2 is the focal length of the second lens group, and f is the focal length of the entire system when focusing on an object at infinity.

<CIT> discloses a projecting lens comprising, when viewed from a screen, a first lens group having positive refracting power and an aspherical lens thereof; a second lens group having positive refracting power, a third lens group having positive refracting power and an aspherical lens, and a fourth lens group having negative refracting power, wherein the following conditions are met assuming that the focal distance of the first lens group is fG1, the focal distance of the third lens group is fG3 and the focal distance of the overall system is f: <NUM>=f/fG1<<NUM>, <NUM>=f/fG3<<NUM>.

<CIT> discloses a single-focus optical system which is configured, in order from the object side to the image side, of a first to third lens groups and in which the first lens group and the third lens group are fixed with respect to a predetermined imaging surface, and the second lens group is moved in the optical axis direction to focus, wherein the first lens group comprises at least one positive lens and at least one negative lens, the second lens group comprises at least one positive lens, the third lens group comprises at least one lens having at least one aspheric surface and having a positive optical power at a peripheral portion thereof, and <NUM>< |Δv1| <<NUM> is satisfied where Δv1 is a maximum value of the Abbe number difference between the positive lens and the negative lens in the first lens group.

PTL <NUM> discloses a wide-angle lens. The wide-angle lens includes a front group located at a fixed position relative to a shooting target and having positive or negative refractive power, a rear group located closer to an imaging surface and having positive refractive power, and an aperture diaphragm fixedly disposed between the front group and the rear group. The rear group is configured with a first partial rear group disposed closer to the aperture diaphragm and slidable along an optical axis line for focusing, and a second partial rear group located at a fixed position relative to the imaging surface.

PTL <NUM> discloses an imaging optical system. The imaging optical system is configured with first lens group G1 having positive refractive power, an aperture diaphragm, second lens group G2 having positive refractive power, and third lens group G3 having negative refractive power in order from an object side. Upon focusing from infinity to a near field, second lens group G2 moves toward an object along an optical axis in the imaging optical system. The second lens group is configured with only cemented lens DB2 and a positive lens having a biconvex shape in order from the object side. Cemented lens DB2 is configured by cementing a negative lens whose concave surface is directed toward the object and a positive lens whose convex surface is directed toward an image. The third lens group is configured with at least one positive lens and at least one negative lens so as to dispose the negative lens closest to the object and the positive lens closest to the image. Furthermore, the imaging optical system is configured to satisfy predetermined conditional expressions.

A fixed focal length imaging optical system according to the present disclosure is defined in claim <NUM>.

The present disclosure can provide a fixed focal length imaging optical system capable of favorably correcting various aberrations, and an imaging device and a camera system each of which uses the fixed focal length imaging optical system.

Hereinafter, exemplary embodiments will be described in detail with reference to the drawings. However, descriptions in more detail than necessary may be omitted. For example, a detailed description of a matter which is already well-known and a repeated description for a substantially identical configuration may be omitted. This is to avoid unnecessarily redundancy in the following description, and to facilitate understanding by those skilled in the art.

The applicant provides the attached drawings and the following description such that those skilled in the art can sufficiently understand the present disclosure, and therefore, they do not intend to restrict the subject matters of claims.

<FIG>, <FIG>, and <FIG> are lens arrangement diagrams of fixed focal length imaging optical systems according to a first exemplary embodiment to a third exemplary embodiment, respectively. Note that each of <FIG>, <FIG>, and <FIG> illustrates the fixed focal length imaging optical system in an infinity focusing state.

Furthermore, an arrow attached to a lens group in each of <FIG>, <FIG>, and <FIG> indicates focusing from the infinity focusing state to a proximity focusing state. Note that, in each of <FIG>, <FIG>, and <FIG>, a reference sign of each lens group is denoted below a position of each lens group, and therefore the arrow indicating focusing is attached to a lower part of the reference sign of each lens group, for convenience.

In each diagram, an asterisk "*" attached to a certain surface shows that the surface is aspherical. Further, in each diagram, symbol (+) or symbol (-) attached to a sign of each lens group corresponds to a sign of power of each lens group. That is, symbol (+) indicates positive power, and symbol (-) indicates negative power. In each of the drawings, a straight line drawn at a rightmost side indicates a position of image surface S (corresponding to a surface at which an imaging element to be described later is disposed and that faces an object). Note that aspect ratios of <FIG>, <FIG>, <FIG>, and <FIG> are the same.

A fixed focal length imaging optical system according to the first exemplary embodiment will be described below with reference to <FIG>.

<FIG> is a lens arrangement diagram illustrating an infinity focusing state of the fixed focal length imaging optical system according to the first exemplary embodiment.

The fixed focal length imaging optical system according to the first exemplary embodiment includes first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from an object side toward an image side. Note that the object side corresponds to a side of first lens group G1, and the image side corresponds to a side of image surface S.

First lens group G1 includes first A lens group G1A having negative power, aperture diaphragm A, and first B lens group G1B having positive power in order from the object side toward the image side.

First A lens group G1A includes first lens element L1 having positive power and second lens element L2 having negative power in order from the object side toward the image side.

First B lens group G1B includes third lens element L3 having positive power, fourth lens element L4 having negative power, fifth lens element L5 having negative power, and sixth lens element L6 having positive power in order from the object side toward the image side. Third lens element L3 and fourth lens element L4 configure a cemented lens that is bonded using, for example, an adhesive such as an ultraviolet curing resin.

Second lens group G2 is configured with seventh lens element L7 having positive power.

Third lens group G3 is configured with eighth lens element L8 having negative power.

Each lens element will be described below.

First, the lens elements in first A lens group G1A will be described below. First lens element L1 is a meniscus lens having a convex surface on the object side. Second lens element L2 is a meniscus lens having a convex surface on the object side. Both surfaces of second lens element L2 are aspherical.

Next, the lens elements in first B lens group G1B will be described below. Third lens element L3 is a meniscus lens having a convex surface on the image side. Fourth lens element L4 is a meniscus lens having a convex surface on the image side. Fifth lens element L5 is a biconcave lens. Sixth lens element L6 is a biconvex lens.

Next, the lens element in second lens group G2 will be described below. Seventh lens element L7 is a meniscus lens having a convex surface on the image side. Both surfaces of seventh lens element L7 are aspherical.

Next, the lens element in third lens group G3 will be described below. Eighth lens element L8 is a meniscus lens having a convex surface on the image side.

In the fixed focal length imaging optical system according to the present disclosure, upon focusing from the infinity focusing state to the proximity focusing state, first lens group G1 and third lens group G3 do not move, and second lens group G2 moves toward the object along an optical axis. In other words, in the fixed focal length imaging optical system, upon focusing, intervals between first lens group G1, second lens group G2, and third lens group G3 vary.

The fixed focal length imaging optical system according to the present disclosure is configured and operates as described above.

A fixed focal length imaging optical system according to the second exemplary embodiment will be described below with reference to <FIG>.

<FIG> is a lens arrangement diagram illustrating an infinity focusing state of the fixed focal length imaging optical system according to the second exemplary embodiment.

The fixed focal length imaging optical system according to the second exemplary embodiment includes first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from an object side toward an image side.

First, the lens elements in first A lens group G1A will be described below. First lens element L1 is a meniscus lens having a convex surface on the object side. Second lens element L2 is a meniscus lens having a convex surface on the object side.

Next, the lens elements in first B lens group G1B will be described below. Third lens element L3 is a meniscus lens having a convex surface on the image side. The image-side surface of third lens element L3 is aspherical. Fourth lens element L4 is a meniscus lens having a convex surface on the image side. Fifth lens element L5 is a biconcave lens. Sixth lens element L6 is a biconvex lens.

Next, the lens element in third lens group G3 will be described below. Eighth lens element L8 is a biconcave lens.

A fixed focal length imaging optical system according to the third exemplary embodiment will be described below with reference to <FIG>.

<FIG> is a lens arrangement diagram illustrating an infinity focusing state of the fixed focal length imaging optical system according to the third exemplary embodiment.

The fixed focal length imaging optical system according to the third exemplary embodiment includes first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from an object side toward an image side.

First, the lens elements in first A lens group G1A will be described below. First lens element L1 is a meniscus lens having a convex surface on the object side. Second lens element L2 is a meniscus lens having a convex surface on the object side. The object-side surface of second lens element L2 is aspherical.

Conditions that can satisfy the configurations of the fixed focal length imaging optical systems of the first to third exemplary embodiments will be described below.

A plurality of possible conditions are defined to the fixed focal length imaging optical system of each exemplary embodiment. In this case, the configuration of the fixed focal length imaging optical system satisfying all the conditions is most effective.

Alternatively, by satisfying an individual condition as follows, a fixed focal length imaging optical system exhibiting an effect corresponding to this condition can be obtained.

The fixed focal length imaging optical system according to each of the first to third exemplary embodiments includes, for example, first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from the object side toward the image side. In the fixed focal length imaging optical system, upon focusing, second lens group G2 moves along an optical axis, and first lens group G1 and third lens group G3 do not move.

This enables focusing without changing a total length of the fixed focal length imaging optical system.

Desirably the fixed focal length imaging optical system according to the present disclosure satisfies the following conditions (<NUM>), (<NUM>), and (<NUM>). <MAT> <MAT> <MAT>.

Herein, TL is a total optical length (a distance from a lens surface closest to the object to an image surface), TG is a thickness of the optical system (a sum of thicknesses of the lens elements configuring the fixed focal length imaging optical system), Y' is a maximum imaged height of the imaging surface, and FL is a focal length for the infinity.

In other words, the condition (<NUM>) defines a relationship between the maximum imaged height of the imaging surface and the total optical length (the distance from the lens surface closest to the object to the image surface).

When TL/Y' is less than or equal to a lower limit value (<NUM>) in the condition (<NUM>), the total optical length becomes excessively short, thereby hindering sufficient aberration correction. In contrast, when TL/Y' is more than or equal to an upper limit value (<NUM>) in the condition (<NUM>), the total optical length becomes long, thereby hindering achievement of downsizing.

In addition, the condition (<NUM>) defines a relationship between the total optical length (the distance from the lens surface closest to the object to the image surface) and the thickness of the optical system (the sum of thicknesses of the lens elements configuring the fixed focal length imaging optical system).

When TG/TL is less than or equal to a lower limit value (<NUM>) in the condition (<NUM>), the lens thickness becomes excessively thin, thereby hindering correction of various aberrations, especially coma aberration. In contrast, when TG/TL is more than or equal to an upper limit value (<NUM>) in the condition (<NUM>), a sufficient back focus cannot be ensured. This hinders disposition of optical elements provided for a camera, the optical elements including an infrared (IR) cut filter and a low-pass filter (LPF), in the fixed focal length imaging optical system.

In addition, the condition (<NUM>) defines a relationship between the focal length for the infinity and the total optical length (the distance from the lens surface closest to the object to the image surface).

When TL/FL is less than or equal to a lower limit value (<NUM>) in the condition (<NUM>), the total optical length becomes excessively short, thereby hindering sufficient correction of various aberrations, especially coma aberration and image surface curvature. In contrast, when TL/FL is more than or equal to an upper limit value (<NUM>) in the condition (<NUM>), the total optical length becomes excessively long, thereby hindering achievement of downsizing.

At this time, it is more preferable to satisfy any one of the following conditions (1a), (1b), (2a), (2b), (3a), and (3b) within each range of the conditions (<NUM>), (<NUM>), and (<NUM>). <MAT> <MAT> <MAT> <MAT> <MAT> <MAT>.

Satisfying any one of these conditions further improves the above-described effects.

In addition, it is more preferable to satisfy any one of the following conditions (1c), (1d), (2c), (2d), (3c), and (3d) within each range of the conditions (<NUM>), (<NUM>), and (<NUM>). <MAT> <MAT> <MAT> <MAT> <MAT> <MAT>.

In addition, in the fixed focal length imaging optical system according to the present disclosure, first lens group G1 desirably includes first A lens group G1A having negative power, aperture diaphragm A, and first B lens group G1B having positive power in order from the object side toward the image side.

This configuration allows first lens group G1 to be a retrofocus type configured with first A lens group G1A having negative power and first B lens group G1B having positive power. The fixed focal length imaging optical system is therefore configured to be suitable for a wide-angle lens.

In addition, in the fixed focal length imaging optical system according to the present disclosure, first A lens group G1A desirably includes first lens element L1 having positive power and second lens element L2 having negative power in order from the object side toward the image side.

This allows first lens element L1 to have a convex surface on the object side, while allowing first A lens group G1A to have negative power. Accordingly, a total lens length can be made shorter in the fixed focal length imaging optical system.

Desirably the fixed focal length imaging optical system of the present disclosure satisfies the following condition (<NUM>).

Herein, β2G is lateral magnification of second lens group G2, and β3G is lateral magnification of third lens group G3.

In other words, the condition (<NUM>) defines a relationship between the lateral magnification of second lens group G2 and the lateral magnification of third lens group G3.

When (<NUM> - β2G<NUM>)β3G<NUM> is less than or equal to a lower limit value (<NUM>) in the condition (<NUM>), a movement amount of the focus lens becomes large. Accordingly, sufficient group intervals are needed for focusing, in the fixed focal length imaging optical system. As a result, a total length of the lens barrel is needed to be long. In contrast, when (<NUM> - β2G<NUM>)β3G<NUM> is more than or equal to an upper limit value (<NUM>) in the condition (<NUM>), an image-surface focus movement amount becomes large relative to the movement amount of the focus lens. Accordingly, highly accurate focusing control is needed. With this configuration, even a slight error in focusing control easily produces focusing blur.

At this time, it is more preferable to satisfy any one of the following conditions (4a) and (4b) within a range of the condition (<NUM>). <MAT> <MAT>.

Further, it is more preferable to satisfy any one of the following conditions (4c) and (4d) within the range of the condition (<NUM>). <MAT> <MAT>.

In the fixed focal length imaging optical system of the present disclosure, desirably second lens group G2 includes a single lens element having positive power and the following condition (<NUM>) is satisfied.

Herein, Nd2G is a refractive index of second lens group G2 at a d-line.

The condition (<NUM>) defines the refractive index of a positive lens element at the d-line, the positive lens element being included in second lens group G2.

When Nd2G is less than or equal to a lower limit value (<NUM>) of the condition (<NUM>), image surface curvature and astigmatism is difficult to correct. In contrast, when Nd2G is more than or equal to an upper limit value (<NUM>) in the condition (<NUM>), a glass material in this case has large chromatic dispersion, thereby hindering correction of chromatic aberration.

At this time, it is more preferable to satisfy any one of the following conditions (5a) and (5b) within a range of the condition (<NUM>). <MAT> <MAT>.

Further, it is more preferable to satisfy any one of the following conditions (5c) and (5d) within the range of the condition (<NUM>). <MAT> <MAT>.

An imaging device applied with the fixed focal length imaging optical system of the first exemplary embodiment will be described below with reference to <FIG>. As the imaging device, a digital camera will be exemplified and described.

<FIG> is a schematic configuration diagram illustrating the digital camera applied with the fixed focal length imaging optical system of the first exemplary embodiment.

Note that the imaging device may be applied with the fixed focal length imaging optical system of any one of the second and third exemplary embodiments.

Digital camera <NUM> exemplifying the imaging device is configured with, for example, housing <NUM>, imaging element <NUM>, fixed focal length imaging optical system <NUM>, and lens barrel <NUM>.

Fixed focal length imaging optical system <NUM> includes first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from an object side toward an image side. First lens group G1 includes aperture diaphragm A.

Lens barrel <NUM> holds the lens groups of fixed focal length imaging optical system <NUM> and aperture diaphragm A.

Imaging element <NUM> is disposed at a position of image surface S in the fixed focal length imaging optical system according to the first exemplary embodiment as illustrated in <FIG>.

Furthermore, for example, an actuator and a lens frame included in housing <NUM> are disposed in fixed focal length imaging optical system <NUM>. The actuator and the lens frame configure second lens group G2 in a movable manner upon focusing.

This configuration can achieve digital camera <NUM> capable of favorably correcting various aberrations.

In the above description, the fixed focal length imaging optical system according to the first exemplary embodiment is applied to the digital camera by way of example. However, the present disclosure is not limited to this example. For example, the fixed focal length imaging optical system of the present disclosure may be applied to a surveillance camera or a smartphone.

A camera system applied with the fixed focal length imaging optical system according to the first exemplary embodiment will be described below with reference to <FIG>.

<FIG> is a schematic configuration diagram illustrating the camera system applied with the fixed focal length imaging optical system according to the first exemplary embodiment.

Note that the camera system may be applied with the fixed focal length imaging optical system of any one of the second and third exemplary embodiments.

Camera system <NUM> includes, for example, camera body <NUM> and interchangeable lens device <NUM> detachably connected to camera body <NUM>.

Camera body <NUM> includes, for example, imaging element <NUM>, monitor <NUM>, a memory (not illustrated), camera mount <NUM>, and finder <NUM>. Imaging element <NUM> receives an optical image formed by the fixed focal length imaging optical system in interchangeable lens device <NUM>, and converts the optical image into an electrical image signal. Monitor <NUM> displays the image signal converted by imaging element <NUM>. The memory stores the image signal.

Interchangeable lens device <NUM> includes, for example, first lens group G1 having positive power, second lens group G2 having positive power, and third lens group G3 having negative power in order from an object side toward an image side. First lens group G1 includes aperture diaphragm A.

Lens barrel <NUM> includes the lens groups of fixed focal length imaging optical system <NUM> and lens mount <NUM> holding aperture diaphragm A. Lens mount <NUM> is connected to camera mount <NUM> of camera body <NUM>.

Camera mount <NUM> and lens mount <NUM> are physically connected to each other. Further, camera mount <NUM> and lens mount <NUM> cause a controller (not illustrated) in camera body <NUM> and a controller (not illustrated) in interchangeable lens device <NUM> to be electrically connected to each other. In other words, camera mount <NUM> and lens mount <NUM> function as interfaces enabling mutual transmission and reception of signals.

Fixed focal length imaging optical system <NUM> is configured with camera body <NUM> and the lens groups held by lens barrel <NUM>. For example, an actuator and a lens frame to be controlled by the controller in interchangeable lens device <NUM> are disposed in fixed focal length imaging optical system <NUM>. The actuator and the lens frame configure second lens group G2 in a movable manner upon focusing.

The technique disclosed in the present application has been described above with the first to third exemplary embodiments as examples.

However, the technique in the present disclosure is not limited to the first to third exemplary embodiments, and can also be applied to exemplary embodiments in which changes, replacements, additions, omissions, and the like are made.

An example in which each lens group in the fixed focal length imaging optical systems according to the first to third exemplary embodiments is configured only with refractive lens elements each of which deflects incident light beams by refraction has been described above. However, the present disclosure is not limited to this example. Note that the refractive lens element means a lens element in which deflection occurs at an interface between media having different refractive indexes from each other.

The lens group may be configured with, for example, a diffraction type lens element that deflects the incident light beams by diffraction, or a refraction-diffraction hybrid type lens element that deflects the incident light beams by combining the refraction and the diffraction. Alternatively, the lens group may be configured with, for example, a refractive index distribution type lens element that deflects the incident light beams through a refractive index distribution in the medium. In particular, in the refraction-diffraction hybrid type lens element, a diffraction structure is preferably formed at the interface between the media having different refractive indexes from each other. This improves wavelength dependence of diffraction efficiency of the refraction-diffraction hybrid type lens element. Those lens elements can achieve the camera system that is excellent in various aberrations.

Numerical examples specifically performed in the configuration of the fixed focal length imaging optical system of any one of the first to third exemplary embodiments will be described below with reference to <FIG>, <FIG>, and <FIG>.

Note that in each numerical example, a unit of the length is "mm", and a unit of the view angle is "°" in tables. In each numerical example, r is a radius of curvature, d is an interplanar spacing, nd is a refractive index at the d-line, and ν d (also written as vd) is an Abbe number at the d-line. Further, in each numerical example, the surfaces marked with * are aspherical. The aspherical shape is defined by the following mathematical formula.

Herein, Z is a distance from a point on the aspherical surface having height h from the optical axis to a tangential plane at a peak of the aspherical surface, h is a height from the optical axis, r is a radius of curvature at the peak, κ is a conic constant, and An is an aspherical coefficient of n-th order.

<FIG>, <FIG>, and <FIG> are longitudinal aberration diagrams of fixed focal length imaging optical systems in the infinity focusing state according to the first to third numerical examples corresponding to the first to third exemplary embodiments, respectively.

In each longitudinal aberration diagram, spherical aberration (SA) (mm), astigmatism (AST) (mm), and distortion (DIS) (%) are illustrated in order from the left.

In the view of the SA, a vertical axis indicates an F number (denoted by "F"), a solid line indicates a characteristic for a d-line, a short broken line indicates a characteristic for an F-line, and a long broken line indicates a characteristic for a C-line.

In the view of the AST, a vertical axis indicates an imaged height (denoted by "H"), the solid line indicates a characteristic for a sagittal plane (denoted by "s"), and the broken line indicates a characteristic for a meridional plane (denoted by "m").

In the view of the DIS, a vertical axis indicates an imaged height (denoted by "H").

The first numerical example of the fixed focal length imaging optical system corresponding to the first exemplary embodiment in <FIG> will be described below. Specifically, as the first numerical example of the fixed focal length imaging optical system, the surface data is indicated in (Table <NUM>), the aspherical data is indicated in (Table <NUM>), and various pieces of data in the infinity focusing state are indicated in (Table 3A to Table 3B).

The second numerical example of the fixed focal length imaging optical system corresponding to the second exemplary embodiment in <FIG> will be described below. Specifically, as the second numerical example of the fixed focal length imaging optical system, the surface data is indicated in (Table <NUM>), the aspherical data is indicated in (Table <NUM>), and various pieces of data in the infinity focusing state are indicated in (Table 6A to Table 6B).

The third numerical example of the fixed focal length imaging optical system corresponding to the second exemplary embodiment in <FIG> will be described below. Specifically, as the third numerical example of the fixed focal length imaging optical system, the surface data is indicated in (Table <NUM>), the aspherical data is indicated in (Table <NUM>), and various pieces of data in the infinity focusing state are indicated in (Table 9A to Table 9B).

As described above, the fixed focal length imaging optical system of any one of the first to third exemplary embodiments was specifically implemented in the first to third numerical examples, respectively.

(Table <NUM>) below illustrates values corresponding to the above conditions (<NUM>) to (<NUM>) in each numerical example.

As illustrated in (Table <NUM>), it is shown that the fixed focal length imaging optical system implemented in each numerical example satisfies the above conditions (<NUM>) to (<NUM>).

As described above, a fixed focal length imaging optical system that is excellent in various aberrations, and an imaging device and a camera system each of which includes the fixed focal length imaging optical system can be provided.

Claim 1:
A fixed focal length imaging optical system (<NUM>) consisting of, in order from an object side toward an image side:
a first lens group (G1) having positive power;
a second lens group (G2) having positive power; and
a third lens group (G3) having negative power, wherein
the second lens group (G2) includes a single lens element having positive power, and
upon focusing, the second lens group (G2) moves along an optical axis, and the first lens group (G1) and the third lens group (G3) do not move
wherein the first lens group (G1) consists of, in order from the object side toward the image side,
- a first A lens group (G1A) having negative power, the first A lens group (G1A) consisting of, in order from the object side toward the image side, a first lens element (L1) having positive power, and a second lens element (L2) having negative power,
- aperture diaphragm (A), and
- a first B lens group (G1B) having positive power,
wherein the second lens group (G2) consists of a single lens element (L7) having positive power, and
a condition (<NUM>) shown below is satisfied, <MAT>
where Nd2G is a refractive index of the second lens group (G2) at a d-line.