JP5972010B2 - Imaging device - Google Patents

Description

  The present invention relates to an imaging apparatus in which a curved imaging surface is disposed in the vicinity of an image plane of an imaging optical system. In particular, the present invention relates to an imaging apparatus suitable for a digital still camera, a digital video camera, a mobile phone camera, a surveillance camera, and the like.

Several examples of an imaging apparatus using such a curved imaging surface are disclosed.
A spherical lens in which concentric spheres are formed by a spherical shell lens and a spherical lens inside the lens has been proposed (Patent Document 1).
This spherical lens can satisfactorily correct spherical aberration and chromatic aberration. In addition, since it is a point-symmetric configuration, it is easy to widen the angle of view, and it is suitable for an imaging optical system of an imaging apparatus that requires a wide angle and high resolution.
In this spherical lens, an example is disclosed in which an aperture stop is provided on a plane passing through the spherical center of the spherical lens in order to block harmful light such as flare and obtain good imaging performance.

An imaging optical system in which the surface of the electronic imaging element is curved two-dimensionally has been proposed (Patent Document 2).
In order to alleviate the constraints when using an electronic image sensor as an image recording medium, an example in which the light receiving surface of the electronic image sensor is made concave on the object side and the incident angle of the light flux on the electronic image sensor is made closer to the normal angle. It is disclosed.
In recent imaging apparatuses, the pixel size of an electronic imaging element is rapidly reduced, and the imaging optical system is required to have high resolution. Therefore, the imaging optical system has to realize high imaging performance even at a bright F value.
Furthermore, recent imaging apparatuses are required to have a wider angle and a smaller size.

JP 63-081413 A Japanese Patent No. 4628781

The spherical lens of Patent Document 1 has a problem that the aperture stop diameter cannot be changed.
In particular, in an imaging apparatus using an imaging optical system having an F value brighter than F / 2.0, it is desired that the depth of field becomes shallow and the depth of field can be controlled by changing the aperture stop diameter. ing.
However, the spherical lens of Patent Document 1 has a configuration in which the depth of field cannot be controlled.
On the other hand, the imaging device of Patent Document 2 has a problem that the aperture stop diameter can be changed, but the imaging optical system is not a bright and bright F-number.

  In the present invention, in view of the above-described problems, aberrations are corrected well at an F value brighter than F / 2.0, high imaging performance is achieved, and the depth of field can be controlled by changing the aperture stop diameter. An object of the present invention is to provide an imaging apparatus.

An imaging apparatus as one aspect of the present invention is an imaging apparatus having an imaging surface that is concave toward the object side, and an imaging optical system that forms an object image on the imaging surface. And an aperture stop disposed in a fluid medium that fills between the two lenses, and a lens surface in contact with the fluid medium on the object side of the aperture stop is a concave surface The lens surface that is in contact with the fluid medium on the image side of the aperture stop is an aspherical surface in which a peripheral portion is displaced toward the object side with respect to the reference surface when a convex spherical surface is used as the reference surface. When the focal length of the imaging optical system is f_sys, the distance from the exit pupil of the imaging optical system to the imaging surface is d_upp, and the radius of curvature of the imaging surface is R_img, the following equations (1) and (2) It is characterized by satisfying the formula .
0.8 ≦ f_sys / d_pup ≦ 1.5 (1)
0.8 ≦ | R_img | /f_sys≦1.5 (2)
An imaging apparatus as another aspect of the present invention is an imaging apparatus having an imaging surface that is concave toward the object side, and an imaging optical system that forms an object image on the imaging surface. The optical system includes at least two lenses and an aperture stop disposed in a fluid medium that fills between the two lenses, and a lens surface that is in contact with the fluid medium on the object side of the aperture stop is The lens surface that is a concave surface and is in contact with the fluid medium on the image side of the aperture stop is a convex surface, and the fluid medium is a liquid, and satisfies the expressions (1) and (2). And

  According to the present invention, at an F value brighter than F / 2.0, an image pickup apparatus that corrects aberration well, has high imaging performance, and can control the depth of field with a variable aperture stop diameter. Can be realized.

FIG. 6 illustrates a configuration example of an imaging apparatus according to Reference Example 1 of the present invention. FIG. 6 is a longitudinal aberration diagram of the imaging optical system in Reference Example 1 of the present invention. FIG. 5 is a lateral aberration diagram of the imaging optical system in Reference Example 1 of the present invention. FIG. 6 illustrates a configuration example of an imaging apparatus according to Reference Example 2 of the present invention. FIG. 6 is a longitudinal aberration diagram of the imaging optical system in Reference Example 2 of the present invention. The lateral aberration figure of the imaging optical system in the reference example 2 of this invention. FIG. 6 is a diagram illustrating a configuration example of an imaging apparatus according to a third embodiment of the present invention. FIG. 6 is a diagram illustrating an aspherical amount of an imaging optical system used in an imaging apparatus according to Embodiment 3 of the present invention. FIG. 6 is a diagram illustrating an aspherical amount of an imaging optical system used in an imaging apparatus according to Embodiment 3 of the present invention. FIG. 12 is a longitudinal aberration diagram of the imaging optical system according to the third embodiment of the present invention. FIG. 10 is a lateral aberration diagram of the imaging optical system in Example 3 of the present invention. FIG. 6 is a diagram illustrating a configuration example of an imaging apparatus according to a fourth embodiment of the present invention. FIG. 6 is a diagram illustrating an aspheric amount of an imaging optical system used in an imaging apparatus according to Embodiment 4 of the present invention. FIG. 6 is a diagram illustrating an aspheric amount of an imaging optical system used in an imaging apparatus according to Embodiment 4 of the present invention. FIG. 10 is a longitudinal aberration diagram of the imaging optical system according to Example 4 of the present invention. FIG. 10 is a lateral aberration diagram of the imaging optical system according to Example 4 of the present invention. The figure which shows the imaging relationship at the time of arrange | positioning the object plane of the imaging device in embodiment of this invention in a finite distance. FIG. 6 is a diagram illustrating a relationship between a focus position and an imaging surface shape during focus adjustment of the imaging apparatus according to the embodiment of the invention.

A configuration example of the imaging device according to the embodiment of the present invention will be described.
First, the overall configuration will be described.
The imaging apparatus of the present embodiment includes an imaging optical system having a plurality of lenses, and has an imaging surface curved with a concave surface facing the object side in the vicinity of the image plane of the imaging optical system. The Petzval image plane can be corrected.
Furthermore, by bringing the imaging optical system closer to a point-symmetric optical system, the occurrence of off-axis aberrations such as coma, astigmatism, distortion, and lateral chromatic aberration is suppressed, and the correction target aberrations are spherical aberration, axial chromatic aberration, etc. Limited to axial aberrations only.
Making the imaging optical system closer to a point-symmetric optical system restricts the lens shape and reduces the degree of freedom in optical design, but it has the advantage that the aberration to be corrected can be limited only to axial aberration. Is more important.
As a result, it is possible to realize an imaging apparatus having a bright and high imaging performance over a wide angle of view.

Therefore, in the imaging apparatus of the present invention, the focal length of the imaging optical system is set to be approximately equal to the distance from the exit pupil of the imaging optical system to the imaging surface, and the radius of curvature of the imaging surface is set to be approximately equal to the focal length of the imaging optical system. By doing so, the imaging optical system is brought close to a point-symmetric optical system.
In particular, it is important that the optical system on the image side is closer to point symmetry than the aperture stop of the imaging optical system.
By setting the focal length of the imaging optical system to be approximately equal to the distance from the exit pupil of the imaging optical system to the imaging surface, the image-side principal point and the exit pupil of the imaging optical system can be aligned at substantially the same position.
This is because the incident height of the viewing angle light beam is lowered, so that the viewing angle light beam can be handled in the same way as the axial light beam, and the optical system on the image side is closer to point symmetry than the aperture stop of the imaging optical system. be able to. Specifically, as described below, it may be configured to satisfy the expression (1) (the conditional expression of the circled number 1 shown in the following table).
Thereby, it is possible to satisfactorily correct off-axis aberrations such as coma, astigmatism, distortion, and lateral chromatic aberration over a wide angle of view.
In addition, by setting the radius of curvature of the imaging surface to be approximately equal to the focal length of the imaging optical system, it is possible to favorably correct curvature of field.
Specifically, as described below, it may be configured to satisfy the expression (2) (the conditional expression of the circled number 2 shown in the following table).
Astigmatism can be satisfactorily corrected by the configuration of the imaging optical system, so that the curvature of field can be limited to only the Petzval image surface, and the Petzval image surface can be corrected by curving the imaging surface, so that all off-axis aberrations can be corrected. Generation can be suppressed. That is, the remaining aberration can be limited to axial aberration.

Furthermore, the present invention is configured such that the aperture diameter can be changed in a wide-angle and bright F-number imaging optical system when adopting a configuration that satisfies the equations (1) and (2) as follows. Yes.
That is, in an imaging apparatus having an imaging optical system and an imaging surface curved with a concave surface facing the object side in the vicinity of the image plane of the imaging optical system,
The imaging optical system has at least two lenses, a space between the two lenses is filled with a fluid medium, and an aperture stop is disposed in the fluid medium,
The shape of the lens surface on the object side of the aperture stop and in contact with the fluid medium is a concave surface, and the shape of the lens surface on the image side of the aperture stop and in contact with the fluid medium is a convex surface,
The focal length of the imaging optical system and the distance from the exit pupil of the imaging optical system to the imaging surface are set to be substantially equal, and the radius of curvature of the imaging surface is from the exit pupil of the imaging optical system to the imaging surface. Is set to be approximately equal to the distance.
At this time, when the focal length of the imaging optical system is f_sys, the distance from the exit pupil of the imaging optical system to the imaging surface is d_upp, and the radius of curvature of the imaging surface is R_img, the following equations (1) and (2) Can be configured to satisfy.
0.8 ≦ f_sys / d_pup ≦ 1.5 (1)
0.8 ≦ | R_img | /f_sys≦1.5 (2)
In addition, the imaging surface in the imaging device here is a curved electronic imaging device or an optical transmission unit having a curved incident surface.
As the curved electronic imaging device, for example, an electronic imaging device formed on a shape-variable substrate, or a small planar electronic imaging device arranged in an array shape to have a concave shape can be considered.
The light transmission means may be, for example, an image plate configured by bundling optical fibers into a plate shape, and one end is processed into a concave shape and the other end is processed into a flat surface.
Then, it is possible to adopt a configuration in which an incident surface of the optical transmission means is curved with the concave surface facing the object side as an imaging surface, and a flat emission surface is connected to an electronic imaging device to form an imaging unit.

Next, the aperture stop will be described in more detail.
The aperture stop of this embodiment is characterized in that the stop diameter can be made variable.
In the spherical lens of Patent Document 1, the aperture of the aperture stop is filled with optical glass, and the aperture diameter cannot be changed.
Therefore, in the present invention, an aperture stop is disposed in the fluid medium so that the aperture diameter can be varied.
Here, the fluid medium is a gas such as air or a liquid such as water or oil. In general, the fluid medium has a lower refractive index than optical materials such as optical glass and optical plastic, and the refractive index difference between the fluid medium and the lens surface is a factor that degrades optical performance.
For example, a case where a spherical lens is filled with a fluid medium around an aperture stop will be described.
When the spherical lens is divided into two at the position of the aperture stop and the fluid medium is filled between them, the axial light beam is not so much affected, but the field angle light beam is largely refracted on the lens surface in contact with the fluid medium.
In a spherical lens, aberration is corrected by point symmetry. However, if a field angle light beam is largely refracted at the interface between the lens surface and the fluid medium, the point symmetry of the optical path is lost and large aberration is generated, which becomes a problem.
More specifically, coma, astigmatism, curvature of field, and lateral chromatic aberration are greatly generated due to large refraction at the lens surface.

Further, when the fluid medium region is a spherical surface centered on the aperture stop, the point symmetry is maintained even in the view angle light flux.
However, since the distance from the aperture stop is short, the radius of curvature of the spherical surface is tight, and a part of the light beam at the angle of view is totally reflected, causing a problem of vignetting.
That is, a bright F-number imaging optical system cannot be realized.
Therefore, in the present embodiment, the shape of the lens surface that is closer to the object side than the aperture stop and is in contact with the fluid medium is a concave surface, and the refraction on the lens surface is weakened to reduce the occurrence of aberrations.
Further, the shape of the lens surface located on the image side of the aperture stop and in contact with the fluid medium is a convex surface, and the power applied by the concave surface is canceled. As a result, the angle of view light flux in the rear group is returned to the angle of the front group, and the rear group has a configuration close to a point-symmetric optical system, and aberrations are corrected well.

As described above, by using the configuration of the present embodiment, it is possible to provide an imaging apparatus capable of changing the aperture diameter in a wide-angle and bright F-number imaging optical system.
In particular, it has a great effect on an imaging optical system having an F value brighter than F / 2.0.
In addition, since the imaging optical system having a bright F value has a narrow depth of field, it is possible to shoot with the background blurred while having a short focal length. For example, even a compact camera can blur the background.
Furthermore, since an imaging optical system with a bright F value can set the exposure time in proportion to the square of the F value, camera shake, subject blur, and shot noise can be significantly reduced, resulting in a high-quality image. An imaging device can be provided.

Next, the effect | action which improves peripheral light quantity fall is demonstrated.
In a general imaging optical system, it is known that the peripheral light amount ratio is reduced according to the cos ω4 power law with respect to the field angle (incident angle) ω. Therefore, the peripheral part of the photographed image becomes very dark and a beautiful image cannot be obtained.
Some recent digital cameras and digital video cameras digitally correct peripheral light loss by significantly increasing the sensitivity at the periphery, but noise increases with a low contrast, so Compared with the image quality is considerably deteriorated.
The peripheral light loss causes such a serious problem. This tendency becomes more prominent with an imaging optical system with a wide angle of view, and is one of the elements necessary to realize an imaging optical system with a wide angle of view.
The breakdown of the cos ω4 power law of the peripheral light ratio is
(A) The apparent focal length is increased according to the angle of view, so that the square of cos ω
(B) The apparent aperture diameter is reduced according to the angle of view, so that the first power of cos ω
(C) The angle of incidence on the imaging surface is increased according to the angle of view, which is the first power of cos ω.

In the imaging optical system in the imaging apparatus of the present embodiment, the radius of curvature of the imaging surface is set to be approximately equal to the focal length of the imaging optical system, and the apparent focal length can be made substantially the same at all angles of view.
As a result, the peripheral light amount ratio can be improved by the square of cos ω.
By satisfying the expression (2), a corresponding effect can be obtained.
That is, the peripheral light amount ratio can be improved from the fourth power of cosω to the second power of cosω.
To provide an imaging device capable of significantly improving the peripheral light amount ratio of an imaging optical system having a wide angle of view, and capable of shooting a high-quality image with high contrast and low noise over a wide angle of view. Can do.

Next, the significance of the above formulas (1) and (2) will be described in more detail.
Equation (1) is a condition for setting the focal length f_sys of the imaging optical system and the distance d_pup from the exit pupil of the imaging optical system to the imaging surface to be substantially equal, and is an optical system on the image side with respect to the aperture stop of the imaging optical system. Can have a configuration close to point symmetry.
If the expression (1) is satisfied, off-axis aberrations such as coma, astigmatism, distortion, and lateral chromatic aberration can be corrected well.
If the upper limit of the expression (1) is exceeded, the point symmetry of the optical system on the image side with respect to the aperture stop of the imaging optical system cannot be secured, and off-axis aberrations such as coma, astigmatism, distortion, and lateral chromatic aberration will occur. It occurs and becomes a problem.
If the lower limit of the expression (1) is exceeded, the point symmetry of the optical system on the image side with respect to the aperture stop of the imaging optical system cannot be secured, and off-axis aberrations such as coma, astigmatism, distortion, and lateral chromatic aberration occur. It occurs and becomes a problem.

Expression (2) is a condition for setting the radius of curvature R_img of the imaging surface to be approximately equal to the focal length f_sys of the imaging optical system, and is a condition for favorably correcting curvature of field and astigmatism.
If the expression (2) is satisfied, the image plane shape of the imaging apparatus can be brought close to the Petzval image plane, so that field curvature can be corrected without generating astigmatism over a wide angle of view.
When the upper limit of the expression (2) is exceeded, the deviation from the Petzval image plane becomes large at the periphery of the imaging surface, the field curvature occurs, and the imaging performance deteriorates.
If the lower limit of the expression (2) is exceeded, the deviation from the Petzval image plane increases at the periphery of the imaging surface, and field curvature occurs, resulting in degradation of imaging performance.
In the case of an imaging optical system with a bright F value, the allowable depth of field curvature is narrow because the depth of focus is narrow, and it is necessary to correct field curvature with high accuracy.

When the imaging surface is not spherical but is aspherical or stepped, the radius of curvature of the imaging surface is defined as follows.
First, when the shape of the imaging surface is an aspherical surface, the radius of curvature of the reference spherical surface is defined as “the radius of curvature of the imaging surface”.
An aspherical surface can be expressed by an α equation, and the reciprocal of the curvature c on the optical axis of the α equation is the radius of curvature.

Here, z is the sag amount (mm) in the optical axis direction of the aspherical shape, c is the curvature (1 / mm) on the optical axis, r is the distance (mm) from the optical axis in the radial direction, A, B, C, and D are fourth-order, sixth-order, eighth-order, and tenth-order coefficients, respectively.
Even if the imaging surface is aspherical, the radius of curvature of the reference curved surface can be obtained by measuring the radius of curvature on the optical axis.
Next, the case where the shape of the imaging surface is stepped will be described.
Strictly speaking, the imaging surface has a staircase shape when a small electronic imaging device is configured as an array or when an imaging surface curved by bundling optical fibers is configured.
In that case, a curved surface connecting one pixel of the electronic image sensor or one central point of the optical fiber can be regarded as an imaging surface.
If the curvature radius of the reference curved surface is calculated from the result of fitting the curved surface by the above-described α equation by the least square method, the curvature radius of the imaging surface can be obtained.

The imaging apparatus of the present invention adjusts the focus position by changing the distance between the imaging optical system and the imaging surface.
FIG. 17A and FIG. 17B show the imaging relationship when the object plane is arranged at a finite distance.
In FIG. 17A, OBJ is an object plane, SYS is an imaging optical system, IMG is an image plane, and the imaging optical system SYS forms an object point on the object plane OBJ on an image point on the image plane IMG. ing.
As shown in FIG. 17A, since the imaging optical system SYS forms an object point equidistant from the imaging optical system SYS on the Petzval image plane, the object plane OBJ at this time has a curved shape. Become.
However, in the imaging optical system, the object plane OBJ is preferably a flat surface.
As shown in FIG. 17B, an object point other than on the optical axis is not a curved object surface indicated by a broken line but a plane object surface indicated by a solid line. Then, as indicated by an arrow A in FIG. 17B, the object point moves away from the imaging optical system SYS, and the image point is also a solid line from the Petzval image plane indicated by a broken line as indicated by an arrow B. It moves to the image plane IMG shown in the direction closer to the imaging optical system SYS.
An example of the amount of movement on the image plane side as a defocus amount in the light beam traveling direction is shown in the graph of FIG. 18 in the model of the imaging optical system in FIG.

FIG. 18 shows the relationship between the focus position and the imaging surface shape during focus adjustment in an example.
In this example, the parameters are such that the focal length of the imaging optical system is f_sys = 12.0 (mm) and the radius of curvature of the imaging surface is R_img = 12.0 (mm). The distance from the exit pupil of the imaging optical system to the imaging surface is d_pup = 12.0 (mm), the object distance is S = −300 (mm), and the field angle is ω = 60 (deg).
As described above, when the object plane is a plane, the focus position of each light beam at each angle of view is defocused from the Petzval image plane to the imaging optical system side.
The defocus amount in the traveling direction of the light beam is displayed as a circle number 1 on the graph of the focus position.
Further, the radius of curvature of the imaging surface is set equal to the focal length of the imaging optical system, which corresponds to the Petzval image surface shape when the object distance is infinity.
It is natural that the object position is infinite and the focus position of the circled numeral 1 and the imaging surface shape of the circled numeral 2 match, but FIG. 18 shows the focus position and the imaging even when the object distance is reduced to S = −300 (mm). It shows that the surface shapes match exactly.
This means that the plane object surface can be focused without causing field curvature at any distance from infinity to S = −300 (mm).
Further, the above can be realized in a wide range of angle of view from −60 to +60 (deg).
If the focal length of the imaging optical system is set approximately equal to the distance from the exit pupil of the imaging optical system to the imaging surface, and the radius of curvature of the imaging surface is set approximately equal to the focal length of the imaging optical system, the shape of the imaging surface changes. Without adjusting, it is possible to adjust the focus only by changing the distance between the imaging optical system and the imaging surface.
Therefore, it is important to satisfy the expressions (1) and (2).
Like the imaging device of the present embodiment described above, an imaging optical system with an extremely bright F value has a very narrow depth of focus, so that it is extremely important for the imaging device to be able to easily achieve high-precision focus adjustment. .

Examples of the present invention will be described below.
[ Reference Example 1]
As Reference Example 1, a configuration example of an imaging apparatus to which the present invention is applied will be described.
As shown in FIG. 1, the image pickup optical system used in the image pickup apparatus of this reference example includes four lenses G1, G2, G3, and G4 and an aperture stop STO installed in the fluid medium FM.
From the object side,
A first lens G1, which is a meniscus lens having a convex surface facing the object side;
A second lens G2, which is a meniscus lens having a convex surface facing the object side,
An aperture stop STO, a third lens G3 which is a biconvex lens,
A fourth lens G4, which is a meniscus lens having a convex surface facing the image side, is disposed.
IMG in FIG. 1 is an imaging surface.
In the imaging apparatus according to this reference example, the curved imaging surface IMG is formed by forming the electronic imaging element ICD in a spherical shape, and good imaging performance is realized over the entire imaging surface IMG.

Table 1 shows the configuration of the imaging apparatus of this reference example .
Surface number 1 is the entrance surface of the first lens G1, surface number 2 is the bonding surface of the exit surface of the first lens G1 and the second lens G2, surface number 3 is the exit surface of the second lens G2, and surface number 4 is An aperture stop STO.
Surface number 5 is the entrance surface of the third lens G3, surface number 6 is the bonding surface of the exit surface of the third lens G3 and the entrance surface of the fourth lens G4, surface number 7 is the exit surface of the fourth lens G4, and Surface number 8 is the imaging surface IMG.
In the table, R is the radius of curvature (mm), d is the surface separation (mm), Nd is the refractive index of the d-line, and νd is the Abbe number.

In the imaging device of this reference example , the space between the second lens G2 and the third lens G3 is filled with air (Air) that is a fluid medium FM. An aperture stop STO is disposed in the air, and the aperture stop STO has a variable aperture diameter.
Further, the refractive index of air is Nd = 1.000, and the refractive index is considerably lower than that of the optical glass, and the field angle light beam is refracted at a large angle at the interface between the optical glass and the air, resulting in a large aberration. That was a problem.
Also in the imaging device of this reference example , the refractive index Nd of the second lens G2 in contact with air = 1.54814 and the refractive index Nd3 = 1.7000 of the third lens G3 = 1.7000, whereas the refractive index Nd = 1.000. Is quite low.
Therefore, by making the exit surface (surface number 3) of the second lens that is closer to the object side than the aperture stop STO and in contact with the fluid medium FM to be a concave surface, the refraction angle of the light beam of view angle is relaxed, and the exit of the second lens. The coma generated on the surface is kept small.
By making the entrance surface (surface number 5) of the third lens that is on the image side of the aperture stop STO and in contact with the fluid medium FM a convex surface, the negative power given by the exit surface of the second lens is canceled. The curvature of field and lateral chromatic aberration are corrected.
Also, coma generated on the exit surface of the second lens is corrected.
As a result, even if a low-refractive-index fluid medium is placed inside an optical system close to point symmetry, the aberration of the beam of view angle can be corrected well, so that the aperture diameter can be varied while maintaining high optical performance. An imaging optical system can be realized.

Further, in the imaging apparatus of this reference example , the field angle light beam is largely refracted on the exit surface of the second lens in contact with air, and coma aberration, astigmatism, lateral chromatic aberration, color field curvature, and the like occur. A good aberration correction is realized by taking the following solution.
Specifically, the radius of curvature R3 = 15.6214 (mm) of the exit surface (surface number 3) of the second lens is the distance d3 = 0.2500 (mm) from the exit surface of the second lens to the aperture stop STO. Is bigger than.
Thereby, the refraction angle of each light beam on the exit surface of the second lens is weakened while avoiding the vignetting of the field angle light beam on the exit surface of the second lens.
Further, the radius of curvature R5 = 7.4401 (mm) of the entrance surface (surface number 5) of the third lens is larger than the radius of curvature R3 = 15.6214 (mm) of the exit surface (surface number 3) of the second lens. It is set small.
That is, the curvature radius Rf of the lens surface that is closer to the object side than the aperture stop STO and is in contact with the fluid medium FM, and the curvature radius Rr of the lens surface that is closer to the object side than the aperture stop STO and is in contact with the fluid medium FM. The relationship is configured to satisfy the following expression (3).

Thus, by giving a tighter radius of curvature to the entrance surface of the third lens than the exit surface of the second lens, it is possible to satisfactorily correct lateral chromatic aberration and color field curvature.
In particular, there is a relationship between the power φf of the lens surface on the object side from the aperture stop STO and in contact with the fluid medium FM, and the power φr of the lens surface on the object side from the aperture stop STO and in contact with the fluid medium FM. The following equation (4) is satisfied. As a result, the lateral chromatic aberration and the color field curvature can be corrected with a good balance.

Where Ni + 1 is the refractive index of the medium on the image side of the lens surface, Ni is the refractive index of the medium on the object side of the lens surface, and Ri is the radius of curvature of the lens surface, the power φi of each surface is (5) Expressed as an expression.

In the imaging apparatus of this reference example , the power φ3 = −0.0351 of the exit surface of the second lens, the power φ5 = 0.0973 of the entrance surface of the third lens, and the power of the exit surface of the second lens The power ratio of the entrance surfaces of the three lenses is φ5 / φ3 = −2.772.
Since the exit surface of the second lens and the entrance surface of the third lens are configured so as to satisfy the expression (4) , the lateral chromatic aberration and the curvature of field can be corrected with a good balance.
Accordingly, it is possible to further improve the imaging performance of the field-angle light beam in the imaging device having a bright F-number with a variable aperture diameter and a wide field angle.
Table 2 shows the specifications of the imaging apparatus of this reference example .

The imaging device of this reference example has a small F value of F / 1.2, a field angle of 120.0 (deg), and a super wide field angle, and a total length of 6.229 (mm). This is an example of an imaging apparatus that simultaneously achieves brightness, high resolution, an ultra-wide field angle, and compactness.
Table 3 shows the values of the expressions (1) and (2) in the imaging apparatus of this reference example .

The value of the formula (1) is 1.03, which satisfies the range of the formula (1).
As a result, the optical system on the image side from the aperture stop of the imaging optical system can be configured to be nearly point-symmetric, and coma, astigmatism, and lateral chromatic aberration can be favorably corrected.
The value of formula (2) is 1.02, which satisfies the range of formula (2). Thereby, it is possible to satisfactorily correct field curvature and astigmatism over a wide field angle of 120.0 (deg).
By satisfying equations (1) and (2), it is possible to adjust the focus from infinity to the closest distance by simply changing the distance between the imaging optical system and the imaging surface without changing the shape of the imaging surface. Become.
In the imaging apparatus of the present invention, the radius of curvature of the imaging surface is set to be approximately equal to the distance from the exit pupil of the imaging optical system to the imaging surface.
Equation (6) shown below is a condition for setting the radius of curvature R_img of the imaging surface to be approximately equal to the distance d_upp from the exit pupil of the imaging optical system to the imaging surface.
0.8 ≦ | R_img | /d_pup≦1.5 (6)
If the expression (6) is satisfied, the imaging optical system can be configured to be closer to point symmetry.

In the imaging apparatus of this reference example , focus adjustment is performed by changing the distance between the imaging optical system and the imaging surface. At that time, by satisfying the expression (6), the field curvature due to the focus adjustment can be suppressed to be very small at a wide range of object distances from infinity to the closest distance, and high-resolution imaging is possible.
In the imaging device of this reference example , the value of equation (6) is 1.05, which satisfies the range of equation (6).
As a result, the imaging optical system has a configuration close to point symmetry, and coma, astigmatism, distortion, and lateral chromatic aberration are corrected well. Furthermore, focus adjustment is possible while maintaining high resolution in a wide range of focus adjustment from infinity to close range.

FIG. 2 shows a longitudinal aberration diagram in the imaging optical system of this reference example , and FIG. 3 shows a lateral aberration diagram.
As shown in FIG. 2, spherical aberration, axial chromatic aberration, astigmatism, field curvature, and chromatic spherical aberration are corrected well. Here, the spherical aberration of color is defined as the difference between the spherical aberration amount of each wavelength (for example, C line, F line, g line, etc.) with respect to the spherical aberration amount of the reference wavelength (for example, d line).
As shown in FIG. 3, good performance is obtained for each luminous flux at each angle of view, and coma aberration, curvature of field, and lateral chromatic aberration are corrected well. In particular, the effect of the present invention is sufficiently exerted with respect to a light beam with a large angle of view (for example, a field angle light beam with an angle of view ω = 35 (deg)), and good imaging with a bright F number over a wide angle of view. Has gained performance.

Next, the effect | action which improves peripheral light quantity fall is demonstrated.
In the imaging optical system in the imaging apparatus of this reference example, the radius of curvature of the imaging surface is set to be approximately equal to the focal length of the imaging optical system, and the focal length can be made substantially the same at all angles of view.
As a result, the peripheral light amount ratio can be improved by the square of cos ω.
In this reference example, the maximum half angle of view ω = 60.0 (deg), and when cos 4ω = 0.0625, cos 2ω = 0.25 can be obtained, and the peripheral light amount can be improved four times.
By satisfying the expression (2), a corresponding effect can be obtained.
Further, in the imaging optical system in the imaging apparatus of the present reference example, the radius of curvature of the imaging surface is set to be substantially equal to the distance from the exit pupil of the imaging optical system to the imaging surface, and the incident angle on the imaging surface is made substantially vertical. Can be set.
As a result, the peripheral light amount ratio can be improved by the first power of cos ω.
In this reference example, the maximum half angle of view ω = 60.0 (deg). When cos 4ω = 0.0625, cos 3ω = 0.125 can be obtained, and the peripheral light amount can be improved by a factor of two.
By satisfying the expression (6), a corresponding effect can be obtained.
By simultaneously satisfying the expressions (2) and (6), the peripheral light amount ratio can be improved by the third power of cos ω, and the peripheral light amount can be increased to eight times the conventional amount.
As a result, the peripheral light amount ratio of the imaging optical system with a wide angle of view can be greatly improved. Therefore, an imaging apparatus that can capture high-quality images with high contrast and low noise over a wide angle of view. Can be provided.

As described above, by using the effect of the present invention, an imaging apparatus having a good imaging performance over a wide angle of view even with a bright F value can be realized with a compact configuration.
In addition, the drop in the amount of light from the periphery can be greatly improved, and a very bright imaging optical system can be realized over a wide field angle.
As a result, the exposure time can be greatly shortened, so that it is possible to provide an imaging apparatus capable of capturing a high-quality image with favorable reduction of camera shake, subject blur, and noise.
In addition, it is possible to provide an imaging optical system capable of giving a large blur to a defocused subject with a compact configuration.
In addition, the above-described high-performance imaging optical system can be adjusted in focus in a wide range from infinity to a close range with a simple configuration with almost no deterioration in imaging performance.

[ Reference Example 2]
As a reference example 2, a configuration example of an imaging apparatus having a different form from the reference example 1 will be described.
As shown in FIG. 4, the image pickup optical system used in the image pickup apparatus of this reference example includes five lenses G1, G2, G3, G4, and G5 and an aperture stop STO installed in a fluid medium.
In order from the object side, a first lens G1, which is a meniscus lens having a convex surface facing the object side,
A second lens G2, which is a meniscus lens having a convex surface facing the object side,
A third lens G3 which is a meniscus lens having a convex surface facing the object side;
An aperture stop STO, a fourth lens G4 which is a biconvex lens with a convex surface facing the image side,
A fifth lens G5, which is a meniscus lens having a convex surface facing the image side, is disposed.

The aperture stop STO in the imaging apparatus of the present reference example is disposed in the air layer between the third lens G3 and the fourth lens G4, and a variable stop can be installed.
In the imaging apparatus according to this reference example, the curved imaging surface IMG is formed by forming the electronic imaging element ICD in a spherical shape, and good imaging performance is realized over the entire imaging surface IMG.
Further, IMG in FIG. 4 is an imaging surface, which is an incident surface of the optical transmission means OTM. The curved imaging surface IMG is formed by forming the imaging surface in a spherical shape, and good imaging performance is realized over the entire area of the imaging surface IMG.
The optical transmission means OTM of this reference example is an image fiber configured by bundling optical fibers with a pitch of several microns, and plays a role of transmitting an image formed on the image plane of the imaging optical system to the electronic imaging device ICD.
The imaging unit ICU is configured by making the incident surface of the optical transmission means OTM curved in a spherical shape and connecting it in close contact with the electronic imaging element ICD with the emission surface as a plane.
By making the incident surface shape of the optical transmission means OTM follow the curvature of field of the imaging optical system, good imaging is realized over the entire area of the imaging surface IMG.

Although the optical transmission means OTM is used in this reference example , one surface of the optical transmission means OTM is formed into a spherical shape and the other surface is used as the electronic image pickup device ICD as compared with the case where the imaging device itself is formed into a spherical shape. The connected electronic imaging unit ICU has an advantage that it is easier to manufacture.

Table 4 shows the configuration of the imaging apparatus of this reference example .
Surface number 1 is the entrance surface of the first lens G1, surface number 2 is the bonding surface of the exit surface of the first lens G1 and the entrance surface of the second lens G2, and surface number 3 is the exit surface of the second lens G2. It is a bonding surface with the incident surface of the three lens G3. Surface number 4 is the exit surface of the third lens G3 and is connected to the air layer.
Surface number 5 is an aperture stop STO, which is disposed in the air layer.
Surface number 6 is the entrance surface of the fourth lens G4, surface number 7 is the bonding surface of the exit surface of the fourth lens G4 and the entrance surface of the fifth lens G5, and surface number 8 is the exit surface of the fifth lens G5. .
Surface number 9 is the imaging surface IMG, which is the incident surface of the optical transmission means OTM of the electronic imaging unit ICU. The exit surface of the optical transmission means OTM is connected to the electronic image sensor ICD.
In the table, R is the radius of curvature (mm), d is the surface separation (mm), Nd is the refractive index of the d-line, and νd is the Abbe number.

In the imaging device of the present reference example , the space between the third lens G3 and the fourth lens G4 is filled with air as the fluid medium FM. An aperture stop STO is disposed in the air, and the aperture diameter is variable.
The lens surface on the object side of the aperture stop STO and in contact with the air layer is the exit surface (surface number 4) of the third lens, and is on the image side of the aperture stop STO and in contact with the air layer. The lens surface is an incident surface (surface number 6) of the fourth lens G4.
The shape of the exit surface of the third lens is a concave surface, and the shape of the entrance surface of the fourth lens G4 is a convex surface.
Accordingly, it is possible to provide an imaging apparatus that realizes high imaging performance over a wide angle of view and has a variable aperture diameter.

Further, in the imaging apparatus of this reference example , the field angle light beam is greatly refracted on the exit surface of the third lens in contact with air, and coma aberration, astigmatism, lateral chromatic aberration, color field curvature, and the like occur. A good aberration correction is realized by taking the following solution.
Specifically, the radius of curvature R4 = 43.5438 (mm) of the exit surface of the third lens is made larger than the distance d4 from the exit surface of the third lens to the aperture stop STO = 0.6507 (mm). Yes.
Thus, the angle of refraction is weakened while avoiding the vignetting of the field angle light beam on the exit surface of the third lens.
Further, the radius of curvature R6 = 25.3784 (mm) of the entrance surface of the fourth lens is set to be smaller than the radius of curvature R4 = 43.5438 (mm) of the exit surface of the third lens. Satisfied with the relationship.
As described above, the chromatic aberration of magnification and the curvature of field of the color can be favorably corrected by giving the entrance surface of the fourth lens a radius of curvature that is tighter than the exit surface of the third lens.

Further, the power φ4 = −0.0154 of the exit surface of the third lens, the power φ6 = 0.0348 of the entrance surface of the fourth lens, and the power of the exit surface of the third lens and the power of the entrance surface of the fourth lens. The ratio φ6 / φ4 = −2.258.
Since the exit surface of the third lens and the entrance surface of the fourth lens are configured so as to satisfy the expression (4) , the lateral chromatic aberration and the curvature of field can be corrected with a good balance.
Accordingly, it is possible to further improve the imaging performance of the field-angle light beam in the imaging device having a bright F-number with a variable aperture diameter and a wide field angle.
Table 5 shows the specifications of the imaging apparatus of this reference example .

The image pickup apparatus of this reference example is an example of an image pickup optical system in which an aperture stop is disposed in an air layer and a variable stop is adopted.
Although it has a simple configuration with five lenses, it achieves a bright optical system with F / 1.6.
Further, the total length is 17.959 (mm) with respect to the focal length of 11.997 (mm), and this is a compact optical system having L_sys / f_sys = 1.50.
Table 6 shows the values of the expressions (1) and (2) in the imaging apparatus of this reference example .

The value of the formula (1) is 0.85, which satisfies the range of the formula (1). As a result, the optical system on the image side from the aperture stop of the imaging optical system can be configured to be nearly point-symmetric, and coma, astigmatism, and lateral chromatic aberration can be favorably corrected.
The value of formula (2) is 1.04, which satisfies the range of formula (2). This makes it possible to satisfactorily correct field curvature and astigmatism over an angle of view of 65.5 (deg).
By satisfying equations (1) and (2), it is possible to adjust the focus from infinity to the closest distance by simply changing the distance between the imaging optical system and the imaging surface without changing the shape of the imaging surface. Become.
The value of formula (6) is 1.05, which satisfies the range of formula (6). Thereby, the field curvature due to focus adjustment can be suppressed to a very small range at a wide range of object distances from infinity to the closest distance, and high-resolution imaging is possible.
As described above, also in this reference example , an imaging apparatus having a good imaging performance over a wide angle of view even with a bright F value can be realized with a compact configuration.

FIG. 5 shows a longitudinal aberration diagram in the imaging optical system of this reference example , and FIG. 6 shows a lateral aberration diagram.
As shown in FIG. 5, spherical aberration, axial chromatic aberration, astigmatism, field curvature, and color spherical aberration are corrected well.
Since spherical aberration and chromatic spherical aberration are corrected well, it can be realized with good imaging performance even at a bright F value.
In addition, since astigmatism and field curvature are well corrected, good imaging performance can be realized even at a wide angle of view.
In particular, astigmatism is corrected, and the effects of the present invention are sufficiently exhibited.
As shown in FIG. 6, coma aberration, curvature of field, and lateral chromatic aberration are corrected well, and good performance is obtained for each field angle light flux.
In particular, coma is corrected, and the effects of the present invention are fully exhibited.

[Example 3]
As Example 3, a configuration example of an imaging apparatus having a different form from each of the above reference examples will be described.
As shown in FIG. 7, the image pickup optical system of the image pickup apparatus according to the present embodiment includes four lenses G1, G2, G3, and G4 and an aperture stop STO installed in a fluid medium.
From the object side,
A first lens G1, which is a meniscus lens having a convex surface facing the object side;
A second lens G2, which is a meniscus lens having a convex surface facing the object side,
An aperture stop STO, a third lens G3 which is a biconvex lens,
A fourth lens G4, which is a meniscus lens having a convex surface facing the image side, is disposed.
Similarly, in the imaging apparatus of the present embodiment, the aperture stop STO is disposed in the air layer between the second lens G2 and the third lens G3, and a variable stop is provided.
The imaging unit ICU in the present embodiment is the same as that of the reference example 2.

Table 7 shows the configuration of the imaging apparatus of the present embodiment.
Surface number 1 is the incident surface of the first lens G1, and has a rotationally symmetric aspherical shape expressed by a polynomial expressed by the α equation .
Surface number 2 is a bonding surface of the exit surface of the first lens G1 and the entrance surface of the second lens G2.
Surface number 3 is the exit surface of the second lens G2, and has a rotationally symmetric aspherical shape expressed by a polynomial expressed by the α equation . Surface number 4 is an aperture stop STO.
Surface number 5 is the entrance surface of the third lens G3, and has a rotationally symmetric aspherical shape expressed by a polynomial expressed by the α equation.
Surface number 6 is a bonding surface between the exit surface and entrance surface of the fourth lens G 4 of the third lens G3. The surface number 7 is an exit surface of the fourth lens G 4, and has a rotationally symmetric aspherical surface shape is expressed by a polynomial indicated by α expression. Surface number 8 is the imaging surface IMG, which is the incident surface of the optical transmission means OTM. An emission surface of the optical transmission means OTM not shown is connected to the electronic image sensor ICD.
In the table, R is the radius of curvature (mm), d is the surface separation (mm), Nd is the refractive index of the d-line, and νd is the Abbe number. The surface with “*” is an aspherical surface.

The aspherical surface in the image pickup apparatus of the present embodiment is a rotationally symmetric aspherical surface centered on the optical axis, and is expressed by the following equation (7).

Here, z is the sag amount (mm) in the optical axis direction of the aspherical shape, c is the curvature (1 / mm) on the optical axis, r is the distance (mm) from the optical axis in the radial direction, A, B, C, and D are fourth-order, sixth-order, eighth-order, and tenth-order coefficients, respectively.
Table 8 (A) shows the aspheric coefficient of surface number 1 in the imaging apparatus of the present embodiment, Table 8 (B) shows the aspheric coefficient of surface number 3, and Table 8 (C) shows the aspheric coefficient of surface number 5. Table 8 (D) shows the aspherical coefficient of surface number 7.

In the image pickup apparatus of the present embodiment, the space between the second lens G2 and the third lens G3 is filled with air as the fluid medium FM. An aperture stop STO is disposed in the air, and the aperture diameter is variable.
The lens surface on the object side of the aperture stop STO and in contact with the air layer is the exit surface (surface number 3) of the second lens, and is on the image side of the aperture stop STO and in contact with the air layer. The lens surface is the incident surface (surface number 5) of the third lens G3.

Also in the imaging apparatus of the present embodiment, the exit surface (surface number 3) of the second lens is a concave surface, and the entrance surface (surface number 5) of the third lens G3 is a convex surface.
Accordingly, it is possible to provide an imaging apparatus that realizes high imaging performance over a wide angle of view and has a variable aperture diameter.
In addition, the imaging apparatus according to the present embodiment further improves the fact that the light flux at the angle of view is greatly refracted on the exit surface of the second lens that is in contact with air, thereby generating coma, astigmatism, lateral chromatic aberration, color field curvature, and the like. A good aberration correction is realized by taking the following solution.
Specifically, the radius of curvature R3 = 52.9741 (mm) of the exit surface (surface number 3) of the second lens is set to a distance d3 = 1 from the exit surface (surface number 3) of the second lens to the aperture stop STO. It is larger than 7051 (mm).
Thereby, the angle of refraction is weakened while avoiding the vignetting of the field angle light beam on the exit surface of the second lens.

Further, the radius of curvature R5 = 23.2147 (mm) of the entrance surface (surface number 5) of the third lens G3 is greater than the radius of curvature R3 = 52.9741 (mm) of the exit surface (surface number 3) of the second lens. Is set to be small, which satisfies the relationship of the expression (3).
Thus, by giving a tighter radius of curvature to the entrance surface of the third lens than the exit surface of the second lens, it is possible to satisfactorily correct lateral chromatic aberration and color field curvature.
Further, the power φ3 = −0.0117 of the exit surface of the second lens, the power φ5 = 0.0299 of the entrance surface of the third lens, and the power of the exit surface of the second lens and the power of the entrance surface of the third lens. The ratio φ5 / φ3 = −2.549.
Since the exit surface of the third lens and the entrance surface of the fourth lens are configured so as to satisfy the expression (4) , the lateral chromatic aberration and the curvature of field can be corrected with a good balance.
Accordingly, it is possible to further improve the imaging performance of the field-angle light beam in the imaging device having a bright F-number with a variable aperture diameter and a wide field angle.
In the imaging apparatus of the present embodiment, the distance d5 = 0.3000 (mm) from the aperture stop STO to the entrance surface of the third lens G3, and the distance d3 = 1 from the exit surface of the second lens to the aperture stop STO. .7051 (mm).
Therefore, the lateral chromatic aberration can be corrected more satisfactorily.
Accordingly, it is possible to further improve the imaging performance of a light beam with a field angle in an imaging apparatus with a bright F value and a wide field angle.

FIG. 8A shows an aspherical quantity with surface number 1, FIG. 8B shows an aspherical quantity with surface number 3, FIG. 9A shows an aspherical quantity with surface number 5, and FIG. ) Indicates the aspherical surface number 7.
Here, the aspherical amount is a sag amount ΔZASP in which the aspherical surface is displaced from the reference spherical surface in the optical axis direction, and is obtained by subtracting the sag amount of the spherical surface from the sag amount of the aspherical polynomial expressed by equation (7). It is.
Specifically, it is expressed by the following equation (8).

The aspherical amount of surface number 1 is negative at the periphery of the lens surface and is displaced from the reference spherical surface to the object side.
The aspherical amount of surface number 3 is positive at the periphery of the lens surface and is displaced from the reference spherical surface to the image side. The aspherical amount of surface number 5 is negative at the periphery of the lens surface, and is displaced from the reference spherical surface to the object side.
The aspherical amount of surface number 7 is negative at the periphery of the lens surface, and is displaced from the reference spherical surface to the object side.
With these aspherical shapes, better imaging performance is obtained.
In particular, the exit surface of the second lens (surface number 3) and a concave surface, displaced to the image side from the reference spherical surface at the periphery of the lens surface (aspherical amount becomes positive) aspherical Grant, near When a negative power component is added to the part, the aberration generated by the air layer can be corrected satisfactorily.

Further, the entrance surface (surface number 5) of the third lens G3 is a convex surface, and an aspherical shape displaced from the reference spherical surface to the object side (the aspherical amount becomes negative) is given at the periphery of the lens surface, When a negative power is applied to the part, the aberration generated by the air layer can be corrected more satisfactorily.
Furthermore, if both are realized simultaneously, aberrations can be corrected more satisfactorily and high imaging performance can be obtained.
Table 9 shows the specifications of the image pickup apparatus of the present embodiment.

The image pickup apparatus of this embodiment is extremely bright with an F value of F / 1.2 and a wide angle of view of 70.0 (deg), and the overall length is 16.263 (mm) and is kept compact. This is an example of an imaging device that simultaneously achieves brightness, high resolution, and compactness.
Table 10 shows the values of the expressions (1) and (2) in the image pickup apparatus of the present embodiment.

The value of the formula (1) is 1.07, which satisfies the range of the formula (1). Thereby, the optical system on the image side from the aperture stop of the imaging optical system can be configured to be nearly point-symmetric, and coma, astigmatism, distortion, and lateral chromatic aberration can be corrected well.
The value of formula (2) is 1.04, which satisfies the range of formula (2). This makes it possible to satisfactorily correct field curvature and astigmatism over a wide field angle of 70.0 (deg).
The value of equation (6) is 1.11, which satisfies the range of equation (6).
As a result, the imaging optical system has a configuration close to point symmetry, and coma, astigmatism, distortion, and lateral chromatic aberration are corrected well.
Furthermore, in the imaging apparatus of the present embodiment, focus adjustment is performed by changing the distance between the imaging optical system and the imaging surface. Since the expression (6) is satisfied, focus adjustment can be performed while maintaining high resolution in a wide range of focus adjustment from infinity to close range.

FIG. 10 is a longitudinal aberration diagram in the imaging optical system of the present embodiment, and FIG. 11 is a transverse aberration diagram.
As shown in FIG. 10, spherical aberration, axial chromatic aberration, astigmatism, field curvature, and chromatic spherical aberration are corrected well. In particular, the light beam can be condensed on the image plane over the entire range from a light beam having a low incident height to a high light beam, and the spherical aberration can be corrected very well.
In addition, axial chromatic aberration and chromatic spherical aberration can be corrected very well, and high imaging performance is obtained.
As shown in FIG. 11, coma aberration, curvature of field, and lateral chromatic aberration are corrected satisfactorily, and good performance can be obtained for each field angle light flux.
As in this embodiment, both the most object-side lens surface and the most image-side lens surface are aspherical surfaces to which an aspherical amount obtained by displacing the reference spherical surface from the reference spherical surface to the outside of the imaging optical system at the periphery of the lens surface; This makes it possible to correct spherical aberration with high accuracy over a wide angle of view.

[Example 4]
As a fourth embodiment, a configuration example of an imaging apparatus having a different form from the above embodiments will be described.
As shown in FIG. 12, the image pickup optical system of the image pickup apparatus according to the present embodiment includes four lenses G1, G2, G3, and G4 and an aperture stop STO installed in a fluid medium.
From the object side,
A first lens G1, which is a meniscus lens having a convex surface facing the object side;
A second lens G2, which is a menis lens having a convex surface facing the object side;
A third lens G3 which is a biconvex lens having a convex surface facing the image side;
A fourth lens G4, which is a meniscus lens having a convex surface facing the image side, is disposed.
Further, between the second lens G2 and the third lens G3, silicon oil as the fluid medium FM is filled, and an aperture stop STO is disposed therein.
The imaging unit ICU in the present embodiment is the same as that of the reference example 2.

Table 11 shows the configuration of the imaging apparatus of the present example.
Surface number 1 is the incident surface of the first lens G1, and surface number 2 is the bonding surface of the exit surface of G1 and the incident surface of the second lens G2.
Surface number 3 is the exit surface of the second lens G2, and has a rotationally symmetric aspherical shape expressed by a polynomial expressed by the α equation . Surface number 3 is a lens surface that is on the object side of the aperture stop and is in contact with the fluid medium on which the aperture stop is disposed. Surface number 4 is an aperture stop STO.
Surface number 5 is the entrance surface of the third lens G3, and has a rotationally symmetric aspherical shape expressed by a polynomial expressed by the α equation.
Surface number 6 is a bonding surface of the exit surface of the third lens G3 and the entrance surface of the fourth lens G4.
The surface number 7 is an exit surface of the fourth lens G 4, and has a rotationally symmetric aspherical surface shape is expressed by a polynomial indicated by α expression.
Surface number 8 is the imaging surface IMG, which is the incident surface of the curved electronic imaging device.
In the table, R is the radius of curvature (mm), d is the surface separation (mm), Nd is the refractive index of the d-line, and νd is the Abbe number. The surface with “*” is an aspherical surface.

Table 12 (A) shows the aspheric coefficient of surface number 3 in the imaging apparatus of the present embodiment, Table 12 (B) shows the aspheric coefficient of surface number 5, and Table 12 (C) shows the aspheric coefficient of surface number 7. Shown in

In the imaging apparatus of the present embodiment, the space between the second lens G2 and the third lens G3 is filled with silicon oil that is the fluid medium FM. An aperture stop STO is disposed in the silicon oil, and the aperture diameter is variable.
The refractive index of silicon oil is Nd = 1.404, which is higher than the refractive index of air, and is close to the refractive index of the optical glass of the second lens and the third lens in contact with the silicon oil.
For this reason, the angle of refraction of the field-angle light beam on the exit surface of the second lens and the entrance surface of the third lens can be reduced, and the effect of suppressing the generated aberration is exhibited. In this embodiment, silicon oil is used for the fluid medium FM, but water may be used. The refractive index of water is Nd = 1.333, which is higher than the refractive index of air Nd = 1.000 and close to the refractive index of optical glass. Therefore, an effect almost equivalent to that of silicon oil can be exhibited.
As described above, the fluid medium FM in which the aperture stop is disposed preferably has a refractive index of Nd> 1.000. This can suppress the aberration to be small, and is effective in realizing high resolving power.

In the imaging apparatus of the present embodiment, the lens surface that is closer to the object side than the aperture stop and is in contact with the fluid medium is the exit surface (surface number 3) of the second lens G2, and is closer to the image side than the aperture stop. The lens surface in contact with the fluid medium is the incident surface (surface number 5) of the third lens G3.
Also in the imaging apparatus of the present embodiment, the exit surface (surface number 3) of the second lens is a concave surface, and the entrance surface (surface number 5) of the third lens G3 is a convex surface.
Accordingly, it is possible to provide an imaging apparatus that realizes high imaging performance over a wide angle of view and has a variable aperture diameter.
In the imaging apparatus of the present embodiment, the field angle light beam is largely refracted on the exit surface of the second lens in contact with the silicone oil, and coma, astigmatism, lateral chromatic aberration, color field curvature, and the like are generated. A further solution is taken to achieve good aberration correction.
Specifically, the radius of curvature R3 = 25.6855 (mm) of the exit surface (surface number 3) of the second lens is the distance d3 = 1 from the exit surface (surface number 3) of the second lens to the aperture stop STO. It is larger than .0000 (mm).
Thereby, the angle of refraction is weakened while avoiding the vignetting of the field angle light beam on the exit surface of the second lens.

Further, the radius of curvature R5 = 20.4801 (mm) of the entrance surface (surface number 5) of the third lens G3 is based on the radius of curvature R3 = 25.6855 (mm) of the exit surface (surface number 3) of the second lens. Is set to be small, which satisfies the relationship of the expression (3).
Thus, by giving a tighter radius of curvature to the entrance surface of the third lens than the exit surface of the second lens, it is possible to satisfactorily correct lateral chromatic aberration and color field curvature.
Further, the power φ3 = −0.0186 of the exit surface of the second lens, the power φ5 = 0.0231 of the entrance surface of the third lens, and the power of the exit surface of the second lens and the power of the entrance surface of the third lens. The ratio φ5 / φ3 = −1.241.
Since the exit surface of the third lens and the entrance surface of the fourth lens are configured so as to satisfy the expression (4) , the lateral chromatic aberration and the curvature of field can be corrected with a good balance.
Accordingly, it is possible to further improve the imaging performance of the field-angle light beam in the imaging device having a bright F-number with a variable aperture diameter and a wide field angle.

In the imaging apparatus of the present embodiment, the distance d5 = 0.500 (mm) from the aperture stop STO to the entrance surface of the third lens G3, and the distance d3 = 1 from the exit surface of the second lens to the aperture stop STO. It is set shorter than .000 (mm).
Therefore, the lateral chromatic aberration can be corrected more satisfactorily.
Accordingly, it is possible to further improve the imaging performance of a light beam with a field angle in an imaging apparatus with a bright F value and a wide field angle.
Thus, by giving a stronger power to the entrance surface of the third lens G3 than the exit surface of the second lens, it is possible to satisfactorily correct lateral chromatic aberration and color field curvature.
In the imaging apparatus of the present embodiment, the distance d5 = 0.5000 (mm) from the aperture stop STO to the entrance surface of the third lens G3, and the distance d3 = 1 from the exit surface of the second lens to the aperture stop STO. It is set shorter than .0000 (mm).
Therefore, coma can be corrected well while correcting chromatic aberration of magnification.
Accordingly, it is possible to further improve the imaging performance of a light beam with a field angle in an imaging apparatus with a bright F value and a wide field angle.

FIG. 13 (a) shows the aspherical quantity with surface number 3, FIG. 13 (b) shows the aspherical quantity with surface number 5, and FIG. 14 shows the aspherical quantity with surface number 7.
The aspherical amount of surface number 3 is positive at the periphery of the lens surface, and is an aspherical shape displaced from the reference spherical surface to the image side. The aspherical amount of surface number 5 is negative at the periphery of the lens surface, and is an aspherical shape displaced from the reference spherical surface toward the object side.
The aspherical amount of surface number 7 is negative at the periphery of the lens surface, and is an aspherical shape displaced from the reference spherical surface toward the object side.
With these aspherical shapes, better imaging performance is obtained.
In particular, the exit surface (surface number 3) of the second lens is a concave surface, and an aspherical shape displaced from the reference spherical surface to the image side (aspherical amount becomes positive) at the peripheral portion of the lens surface is provided, and the peripheral portion When a negative power component is added to, aberration generated by the air layer can be corrected well.

Further, the entrance surface (surface number 5) of the third lens G3 is a convex surface, and an aspherical shape displaced from the reference spherical surface to the object side (the aspherical amount becomes negative) is given at the periphery of the lens surface, When a negative power is applied to the part, the aberration generated by the air layer can be corrected more satisfactorily.
Furthermore, if both are realized simultaneously, aberrations can be corrected more satisfactorily and high imaging performance can be obtained.
Table 15 shows the specifications of the image pickup apparatus of the present embodiment.

The imaging apparatus according to the present embodiment has a bright F value of F / 1.2 and an ultra wide angle of view of 120.0 (deg), and the overall length is 6.044 (mm) and is compact. This is an example of an imaging apparatus that simultaneously achieves brightness, high resolution, an ultra-wide field angle, and compactness.
Table 16 shows the values of the expressions (1) and (2) in the imaging apparatus of the present embodiment.

The value of the formula (1) is 0.96, which satisfies the range of the formula (1). Thereby, the optical system on the image side from the aperture stop of the imaging optical system can be configured to be nearly point-symmetric, and coma, astigmatism, distortion, and lateral chromatic aberration can be corrected well.
The value of formula (2) is 1.01, which satisfies the range of formula (2). Thereby, it is possible to satisfactorily correct field curvature and astigmatism over a wide field angle of 120.0 (deg).
The value of equation (6) is 0.97, which satisfies the range of equation (6).
As a result, the imaging optical system has a configuration close to point symmetry, and coma, astigmatism, distortion, and lateral chromatic aberration are corrected well.
Furthermore, in the imaging apparatus of the present embodiment, focus adjustment is performed by changing the distance between the imaging optical system and the imaging surface. Since the expression (6) is satisfied, focus adjustment can be performed while maintaining high resolution in a wide range of focus adjustment from infinity to close range.

FIG. 15 shows a longitudinal aberration diagram in the imaging optical system of the present embodiment, and FIG. 16 shows a lateral aberration diagram.
As shown in FIG. 15, spherical aberration, axial chromatic aberration, astigmatism, field curvature, and chromatic spherical aberration are corrected fairly well.
In particular, the light beam can be condensed on the image plane over the entire range from a light beam having a low incident height to a high light beam, and the spherical aberration can be corrected very well.
In addition, axial chromatic aberration and chromatic spherical aberration can be corrected very well, and high imaging performance is obtained.

As shown in FIG. 16, good performance is obtained for each field angle light beam, and coma aberration, field curvature, and lateral chromatic aberration are corrected fairly well.
As in this embodiment, both the most object-side lens surface and the most image-side lens surface are aspherical surfaces to which an aspherical amount obtained by displacing the reference spherical surface from the reference spherical surface to the outside of the imaging optical system at the periphery of the lens surface; This makes it possible to correct spherical aberration with high accuracy over a wide angle of view.
The configuration of the present invention including the embodiments described above can be used for products using an imaging device such as a digital camera, a digital video camera, a mobile phone camera, and a surveillance camera.

G1, G2, G3, G4 Lens STO Aperture stop Asph Aspherical surface IMG Imaging surface OTM Optical transmission means ICD Electronic imaging device ICU Imaging unit

Claims (9)

An imaging apparatus having an imaging surface that is concave toward the object side, and an imaging optical system that forms an object image on the imaging surface,
The imaging optical system includes at least two lenses and an aperture stop disposed in a fluid medium that fills between the two lenses.
The lens surface in contact with the fluid medium on the object side of the aperture stop is a concave surface,
The lens surface in contact with the fluid medium on the image side with respect to the aperture stop is an aspherical surface having a peripheral surface displaced toward the object side with respect to the reference surface when a convex spherical surface is used as a reference surface.
When the focal length of the imaging optical system is f_sys, the distance from the exit pupil of the imaging optical system to the imaging surface is d_upp, and the radius of curvature of the imaging surface is R_img, the following expression is satisfied: An imaging device.
0.8 ≦ f_sys / d_pup ≦ 1.5
0.8 ≦ | R_img | /f_sys≦1.5 An imaging apparatus having an imaging surface that is concave toward the object side, and an imaging optical system that forms an object image on the imaging surface,
The imaging optical system includes at least two lenses and an aperture stop disposed in a fluid medium that fills between the two lenses.
The lens surface in contact with the fluid medium on the object side from the aperture stop is a concave surface, and the lens surface in contact with the fluid medium on the image side from the aperture stop is a convex surface,
The fluid medium is a liquid;
When the focal length of the imaging optical system is f_sys, the distance from the exit pupil of the imaging optical system to the imaging surface is d_upp, and the radius of curvature of the imaging surface is R_img, the following expression is satisfied: An imaging device.
0.8 ≦ f_sys / d_pup ≦ 1.5
0.8 ≦ | R_img | /f_sys≦1.5 The lens surface that is in contact with the fluid medium on the image side of the aperture stop is an aspherical surface having a peripheral surface displaced toward the object side with respect to the reference surface when a convex spherical surface is used as the reference surface. The imaging apparatus according to claim 2, characterized in that: The imaging apparatus according to claim 1, wherein the following expression is satisfied when a radius of curvature of the imaging surface is R_img.
0.8 ≦ | R_img | /d_pup≦1.5   5. The curvature radius of a lens surface that is in contact with the fluid medium on the object side of the aperture stop is larger than a distance from the lens surface to the aperture stop. 6. Imaging device.   The curvature radius of the lens surface in contact with the fluid medium on the object side of the aperture stop is larger than the curvature radius of the lens surface in contact with the fluid medium on the image side of the aperture stop. The imaging device according to any one of 5. When the power of the lens surface in contact with the fluid medium on the object side of the aperture stop is φf and the power of the lens surface in contact with the fluid medium on the image side of the aperture stop is φr, the following expression is satisfied. The image pickup apparatus according to claim 1, wherein the image pickup apparatus is an image pickup apparatus.
−5 ≦ φr / φf ≦ −1   The distance from the aperture stop to the lens surface in contact with the fluid medium on the image side of the aperture stop is shorter than the distance from the lens surface in contact with the fluid medium on the object side to the aperture stop than the aperture stop. The imaging apparatus according to claim 1, wherein The lens surface that is in contact with the fluid medium on the object side of the aperture stop is an aspherical surface with a peripheral portion displaced toward the image side with respect to the reference surface when a concave spherical surface is used as the reference surface. The imaging apparatus according to claim 1, wherein the imaging apparatus is characterized.