JP2015200784A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an imaging apparatus in which the fluctuations of a cut-off frequency occurring because a light beam is made obliquely incident on an optical element having birefringence are reduced and the fluctuations of the aberration of an optical system are reduced.SOLUTION: The imaging apparatus includes the optical system, an imaging element and a low-pass filter arranged between the optical system and the imaging element. The low-pass filter includes a first optical element having positive refractive power and a second optical element having negative refractive power. The first optical element is made of a birefringent material. When the curvature radius of an optical surface whose curvature is larger in the optical surface of the first optical element is defined as R and the diagonal half image height of the imaging element is defined as Y, a conditional expression of 15.0<|R/Y|<40.0 is satisfied.

Description

  The present invention relates to an imaging apparatus having a low-pass filter with little image deterioration.

  In recent years, an image pickup device mounted on a digital camera or a video camera has been increased in size, and downsizing of an image pickup apparatus has been required.

  It is known that a negative lens group is arranged on the image sensor side of the optical system and the amount of movement of the variable power group accompanying zooming can be reduced by reducing the amount of movement of the variable power group, thereby reducing the size of the optical system. It has been. At this time, in the off-axis region (peripheral portion of the screen), the light beam is obliquely incident on the low-pass filter (LPF), so that it is difficult to obtain the desired image separation characteristics.

JP 2001-117139 A JP 2008-076691 A

  Patent Documents 1 and 2 disclose low-pass filters having different thicknesses for each region, but the aberration of the optical system fluctuates due to the difference in thickness of the low-pass filters.

  It is an object of the present invention to provide an imaging apparatus in which variation in cutoff frequency caused by oblique incidence of light on an optical element having birefringence is small and variation in aberration of an optical system is small. .

In order to achieve the above object, an imaging apparatus according to the present invention is an imaging apparatus having an optical system, an imaging element, and a low-pass filter disposed between the optical system and the imaging element. The filter includes a first optical element having a positive refractive power and a second optical element having a negative refractive power, and the first optical element is made of a birefringent material, and the first optical element When the radius of curvature of the optical surface having the larger curvature among the optical surfaces of the element is R, and the diagonal half-image height of the imaging element is Y,
15.0 <| R / Y | <40.0
The following conditional expression is satisfied.

  According to the present invention, it is possible to provide an imaging apparatus in which the variation in the cutoff frequency caused by the oblique incidence of the light beam on the optical element having birefringence is small and the variation in the aberration of the optical system is small. .

(A) Explanatory drawing of the incident angle with respect to a low-pass filter, (B) is sectional drawing of the optical system of the 1st Embodiment of this invention. It is a figure explaining the MTF frequency characteristic of an optical system. It is a figure explaining an image sensor. It is a figure explaining a color filter. It is sectional drawing of the optical system of the 2nd Embodiment of this invention. It is sectional drawing of the optical system of the 3rd Embodiment of this invention. It is sectional drawing of the optical system of the 4th Embodiment of this invention. (A) is an aberration diagram at the wide angle end of the first embodiment, and (B) is an aberration diagram at the telephoto end. (A) is an aberration diagram at the wide angle end of the second embodiment, and (B) is an aberration diagram at the telephoto end. It is an aberration diagram in the third embodiment. (A) is an aberration diagram at the wide-angle end of the fourth embodiment, and (B) is an aberration diagram at the telephoto end.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

<< First Embodiment >>
(Imaging device)
An imaging apparatus 100 according to the first embodiment of the present invention will be described with reference to FIGS. A configuration in which a light beam that has passed through an optical system (a zoom lens in this example) is incident on an image sensor 10 that can capture a wide angle (half angle of view is 40 °) through a low-pass filter 102 (hereinafter also referred to as LPF). It has become. Note that IP is an image plane, and an image sensor 10 such as a CCD is disposed. Hereinafter, the imaging element, the optical system, and the low-pass filter will be described in detail in this order.

(Image sensor)
As shown in FIG. 3, the imaging device 10 includes a semiconductor circuit substrate 11, a plurality of lower electrodes (pixel electrodes) 12 formed in a two-dimensional array on the semiconductor circuit substrate 11, and a plurality of pixel electrodes 12. And a photoelectric conversion layer 13 made of an organic material formed continuously. Furthermore, it is an opposing electrode which is formed on the photoelectric conversion layer and faces a plurality of pixel electrodes, and includes an upper electrode (common electrode) 14 provided as a single layer.

  A transparent insulating layer 15 is laminated on the upper electrode 14, and color filters 16 r, 16 g, and 16 b of two or more colors (three colors in the present embodiment) are provided on the insulating layer 15. . Further, a color filter layer CF including a transparent separation wall 17 that separates the color filters 16r, 16g, and 16b for each color is provided, and a low reflection layer 18 is provided on the color filter layer CF. Details of each component will be described below.

1) Semiconductor Circuit Substrate The semiconductor circuit substrate 11 includes a p-type well region 2 on the surface of an n-type silicon substrate 1 (hereinafter simply referred to as substrate 1), and the well region 2 includes an n-type impurity diffusion region 3. A plurality of are formed. The impurity diffusion region 3 is formed in a two-dimensional array corresponding to the lower electrode 12 formed on the semiconductor circuit substrate 11.

  Further, on the surface of the well region 2, in the vicinity of the impurity diffusion region 3, a signal reading unit 4 that outputs a signal corresponding to the charge accumulated in the impurity diffusion region 3 is provided. The signal reading unit 4 is a circuit that converts the charge accumulated in the impurity diffusion region 3 into a voltage signal and outputs the voltage signal, and can be configured by, for example, a known CCD or CMOS circuit.

  Further, an insulating layer 5 is laminated on the surface of the substrate 1 where the well region 2 is formed. A plurality of pixel electrodes 12 that are substantially rectangular when viewed from above are arranged on the insulating layer 5 at a predetermined interval. Each pixel electrode 12 is electrically connected to the impurity diffusion region 3 of the substrate through a connection portion 6 made of a conductive material so as to penetrate the insulating layer 5.

  When light enters the photoelectric conversion layer 13, the imaging element 10 moves, for example, holes to the upper electrode 14 among the charges (holes and electrons) generated in the photoelectric conversion layer 13, and moves the electrons to the lower electrode 12. Move to. Therefore, a bias voltage is applied between the lower electrode 12 and the upper electrode 14 by a voltage supply unit (not shown). In this case, the upper electrode 14 is a hole collecting electrode and the lower electrode 12 is an electron collecting electrode.

2) Electrode The upper electrode 14 and the lower electrode 12 are selected in consideration of adhesion to the photoelectric conversion layer 13, electron affinity, ionization potential, stability, and the like. Various methods are used to manufacture the upper electrode 14 and the lower electrode 12 depending on the material. For example, in the case of ITO, an electron beam method, a sputtering method, a resistance heating vapor deposition method, a chemical reaction method (such as a sol-gel method), an oxidation method, and the like. A film is formed by a method such as application of a dispersion of indium tin.

  The upper electrode 14 is made of a transparent conductive material because it is necessary to make light incident on the photoelectric conversion layer 13. Here, the transparent electrode material preferably has a transmittance of about 80% or more in the visible light range where the wavelength is in the range of about 420 nm to about 660 nm, for example. Specific examples of the material for the upper electrode 14 include conductive metal oxides such as tin oxide, zinc oxide, indium oxide, and indium tin oxide (ITO), and metals such as gold, silver, chromium, and nickel.

  Furthermore, mixtures or laminates of these metals and conductive metal oxides, inorganic conductive materials such as copper iodide and copper sulfide, organic conductive materials such as polyaniline, polythiophene, and polypyrrole, silicon compounds, and these and ITO And the like. The conductive metal oxide is preferable, and ITO, ZnO, and InO are particularly preferable in terms of productivity, high conductivity, transparency, and the like.

  The lower electrode 12 may be any conductive material and need not be transparent. However, when it is necessary to transmit light to the substrate side below the lower electrode 12, the lower electrode 12 also needs to be made of a transparent electrode material. At this time, as the transparent electrode material of the lower electrode 12, it is preferable to use ITO similarly to the upper electrode 14.

3) Photoelectric conversion layer The photoelectric conversion layer 13 made of an organic material is formed to have a thickness in the range of 0.1 μm to 1.0 μm. The thinner the photoelectric conversion layer 13 is, the more effective the color mixing of the image sensor is. However, there is a trade-off with light absorption, and it is desirable that the thickness is substantially 0.5 μm. As a material constituting the photoelectric conversion layer 13, various organic semiconductor materials such as those used in electrophotographic photosensitive materials can be used.

  Among them, the quinacridone skeleton is selected from the viewpoints of having high photoelectric conversion performance, excellent color separation at the time of spectroscopy, high durability against long-time light irradiation, and easy vacuum deposition. Particularly preferred are materials containing and organic materials containing a phthalocyanine skeleton. The organic material constituting such a photoelectric conversion layer preferably contains at least one of a p-type organic semiconductor and an n-type organic semiconductor.

  If the photoelectric conversion layer 13 is made of an organic material, the light absorption coefficient with respect to visible light is larger than a structure using a photodiode formed on a silicon substrate or the like as a photoelectric conversion portion. For this reason, the light incident on the photoelectric conversion layer 13 is easily absorbed. According to this property, light obliquely incident on the photoelectric conversion layer 13 is not easily leaked to the adjacent pixel portion, and is photoelectrically converted in the pixel portion, thereby improving transmission efficiency and suppressing crosstalk. it can.

4) Insulating layer The insulating layer 5 can be made of Al ₂ O ₃ , SiO ₂ , SiN, or a mixed film thereof.

5) Color filter layer As shown in FIG. 3, the color filter layer CF includes a plurality of color filters that respectively transmit light of different wavelengths. Here, the color filter CF includes color filters 16r, 16g, and 16b made of an organic material containing red / blue / green pigments or dyes for each pixel. A separation wall 17 made of a transparent material having a refractive index smaller than that of the color filter material is provided between the color filters.

  Each color filter transmits light having a different wavelength, and the color filter 16r functions as an R light color filter having a configuration that transmits light having a red wavelength among incident light. Similarly, the color filter 16g is a G light color filter having a configuration that transmits green light of the incident light, and the color filter 16b is configured to transmit blue light of the incident light. It functions as a B light color filter. Any one of the plurality of color filters 16r, 16g, and 16b is included in each pixel portion, and is arranged in a color pattern such as a Bayer arrangement according to the arrangement of the pixel portions.

  FIG. 4 shows, as an example, a Bayer array of color filters 16r, 16g, and 16b in four pixel portions. The arrangement of the plurality of color filters is not limited to that described in this configuration example, and can be arbitrarily changed.

  The refractive index of the color filter differs for each color of red, blue, and green, and also varies depending on the wavelength of incident light. Each of the color filters has a refractive index within a range of 1.5 to 1.8 with respect to an incident light wavelength (at least a wavelength of 400 nm to 700 nm in the visible light region). The thickness of each color filter is in the range of 0.3 μm to 1.0 μm.

  The separation walls 17 for separating the color filters from each other are formed in a substantially lattice shape as shown in FIG. 4 and are formed so as to individually surround each color filter. The width t of the separation wall 17 corresponding to the interval between the color filters is in the range of 0.05 μm to 0.2 μm, and the refractive index thereof is in the range of 1.22 to 1.34. As the refractive index of the separation wall 17 is lower, the characteristics as an image sensor are improved. However, if a very low material is used, the fragility as a film becomes a problem, so that it is effectively about 1.28 to 1.30. It is desirable to use materials.

6) Low reflection layer The low reflection layer 18 is provided to reduce reflection loss when light directly enters the color filter CF from the air. When the refractive index of the material used for the color filter CF (for example, the average value of the refractive indexes of the three color filters) is nc, a material having a refractive index of √nc is selected as the low reflective layer 18 and the layer thickness is selected. May be set to a ¼ nm film thickness of 550 nm, which is substantially the center wavelength of visible light.

  Although the image sensor according to the present embodiment has been described above, the present embodiment has the following effects. That is, compared with a surface irradiation type imaging device in which a charge transfer path is formed on a conventional substrate on which a photodiode is formed and a color filter layer is provided on the surface with a planarization film or the like interposed therebetween, the color fill CF And the distance d between the photoelectric conversion layer 13 (FIG. 3) can be shortened. Specifically, the distance d (FIG. 3) from the lower surface of the color filter CF to the upper surface of the photoelectric conversion layer 13 can be 3 μm or less. In addition, although the image pick-up element showed the laminated type image pick-up element by which the photoelectric converting layer was laminated | stacked above the board | substrate as an example, this invention is not this limitation.

(Optical system)
Next, the zoom lens according to the present embodiment will be described as the optical system 101 (FIG. 1A). In the optical system according to the present embodiment, by setting the back focus shorter than the effective diagonal length of the image sensor, the power arrangement can be made symmetrical so that aberrations can be corrected more easily, and a more compact design is possible. It becomes. Here, the back focus is a distance between the optical surface closest to the image sensor of the optical system and the image sensor. For simple evaluation, the wavelength is d-line evaluation.

  In addition, with regard to the zoom lens according to each embodiment including the present embodiment, the wide-angle end and the telephoto end are zoom positions when the zooming lens groups are positioned at both ends of a range that can move on the optical axis due to the mechanism. .

  The optical system of the present embodiment is a negative, positive, and negative three-group zoom lens in order from the object side. In zooming from the wide-angle end to the telephoto end, the first to third lens groups are all moved during zooming. Regarding the configuration of the first lens group, a total of three concave lens elements and a convex lens element are arranged in order from the object side. The concave meniscus lens disposed closest to the object side is provided with an aspheric surface, and mainly a field curvature or the like. Can be suppressed.

  The configuration of the second lens group is composed of a total of four convex lenses, two concave lenses, and convex lenses, and in this embodiment, aspherical surfaces are adopted for the most object side and image side convex lenses. Mainly spherical and coma can be reduced. The configuration of the third lens group is a total of two lenses: a convex lens and a concave lens with a strong concave surface facing the object side. In this embodiment, an aspherical surface is adopted for the concave lens arranged closest to the image side, and this makes it possible to favorably correct curvature of field at the peripheral image height.

  As in this embodiment, it is possible to increase the refractive power of the second lens unit disposed on the object side by increasing the negative refractive power of the third lens unit disposed on the most image side. The amount of movement during zooming can be designed to be small.

(Low-pass filter)
When the above optical system is used, the angle of light incident on the low-pass filter and the image sensor 10 is increased particularly at the outermost angle of view at the wide-angle end due to the refractive power arrangement as described above. Here, the low-pass filter separates the light beam in the Y direction (FIG. 1A), but there are the following concerns. That is, there is a concern that the beam separation width off-axis becomes larger (the amount of blur increases) than the desired separation width set on the axis (on the optical axis) due to the obliquely incident light beam. is there.

  Here, in general, a low-pass filter uses a crystal plate having a crystal axis orientation inclined by 45 ° as a birefringent plate. The quartz plate separates ordinary rays and extraordinary rays in the Y direction (vertical direction in FIG. 1A). FIG. 2 shows the MTF frequency characteristics (Y-section light flux) of the optical system when a general crystal is used for the birefringent plate.

  It can be seen that even if the optical system 101 is an ideal lens, the cutoff frequency decreases from 100 lines / mm to about 80 lines / mm as it goes from the optical axis toward the peripheral angle of view. This is because the light beams are incident on the birefringent plate at different angles on the optical axis and the peripheral image height. In particular, at the peripheral image height that is obliquely incident on the birefringent plate, the cutoff frequency as described above is shifted to the low frequency side.

  Therefore, in this embodiment, the shift to the low frequency side of the cutoff frequency is suppressed by the following configuration. That is, a low-pass filter 102 in which the first optical element 103 and the second optical element 104 are joined is used. Here, the first optical element 103 is provided between the optical system 101 and the image sensor 10 in the optical path, and the second optical element 104 is provided between the first optical element 103 and the image sensor 10 in the optical path. Between.

  The first optical element 103 is an optical element having a positive refractive power and birefringence, and has a convex surface facing the image sensor in the optical path, and the second optical element. Reference numeral 104 denotes an optical element having negative refractive power and not having birefringence. Here, the material (birefringent material) of the first optical element 103 having birefringence is a material satisfying ne-no ≧ 0.0005, and the second optical element 104 not having birefringence. This material does not satisfy ne-no ≧ 0.0005.

  In the present embodiment, the optical surface having the strongest curvature in the first optical element 103 is the optical surface (bonding surface) on the second optical element side. The optical surface having the strongest curvature in the first optical element 103 is a convex surface toward the image sensor 10 (an optical surface having a convex shape on the image sensor side).

  When the radius of curvature of the optical surface having the larger curvature among the optical surfaces of the first optical element 103 having positive refractive power is R and the diagonal half-image height of the image sensor is Y, the following conditional expression: To be satisfied.

15.0 <| R / Y | <40.0 (1)
Conditional expression (1) defines the ratio between the radius of curvature of the optical surface having the larger curvature of the convex lens as the first optical element having birefringence and the diagonal half-image height Y of the imaging element. Is. Exceeding the lower limit is not preferable because the radius of curvature becomes too small, the plate thickness for securing the necessary edge portion of the birefringent plate becomes large, and the cut-off frequency becomes too small (lower). On the contrary, if the upper limit is exceeded, the radius of curvature is too large, and the effect of suppressing the shift of the cutoff frequency due to the light beam obliquely incident on the low-pass filter becomes too small, which is not preferable.

  More preferably, by setting the range as shown in the following conditional expression (1a), it is possible to provide an image pickup apparatus that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

17.0 <| R / Y | <35.0 (1a)
In this embodiment, by setting R = −400 mm and | R / Y | = 29.3, the cut-off frequency from the optical axis to the periphery can be about 120 / mm or more in the entire zoom range.

  The imaging device according to the present embodiment has a light receiving rate of 0.8 Yv at a first angle where the incident angle with respect to the imaging device is greater than 30 °, where Yv is a light receiving rate with respect to a light beam perpendicularly incident on the imaging device. It has an incident angle characteristic obtained.

  This prescribes the sensitivity angle characteristic of the light beam incident on the image sensor. If the incident angle at which a light receiving rate of 0.8 Yv is obtained is smaller than 30 °, the image sensor is reduced in size for the purpose of miniaturization. Therefore, it becomes impossible to effectively capture a desired light beam incident obliquely.

  In the present embodiment, when the distance on the optical axis between the optical surface closest to the image sensor of the low-pass filter 102 and the image sensor (image plane) is L, the following conditional expression is satisfied.

0.00 <L / Y <0.10 (2)
Conditional expression (2) defines the arrangement and position of the low-pass filter 102. If the upper limit is exceeded, the distance between the low-pass filter and the image sensor is too large, and the off-axis light beam passes through the low-pass filter. The height is lowered and the desired effect cannot be obtained.

  More preferably, by setting the range as shown in the following conditional expression (2a), it is possible to provide an image pickup apparatus that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

0.00 <L / Y <0.06 (2a)
In the present embodiment, when the radius of curvature of the optical surface of the first optical element 103 of the low-pass filter 102 opposite to the second optical element 104 is Ra, the following conditional expression is satisfied. To do.

0.00 ≦ | Y / Ra | <0.10 (3)
Conditional expression (3) defines the radius of curvature Ra of the optical surface of the first optical element 103 opposite to the second optical element 104. When the upper limit is exceeded, aberration variations when a system like an interchangeable lens, that is, a plurality of types of lenses is attached, cannot be ignored.

  More preferably, by setting the range as shown in the following conditional expression (3a), it is possible to provide an imaging device that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

0.00 ≦ | Y / Ra | <0.03 (3a)
Further, when the optical system is a zoom lens as in the present embodiment, when the distance on the optical axis between the exit pupil position of the optical system at the wide angle end and the image sensor is tk, the following conditional expression (4) is satisfied. I am satisfied. When the optical system is a single focus lens, the distance on the optical axis between the exit pupil position of the optical system and the image sensor is tk.

0.50 <| tk | / Y <2.50 (4)
If the lower limit is exceeded, the pupil position of the optical system is close to the image plane, and the light beam is obliquely incident on the low-pass filter 102, which degrades performance. On the other hand, if the upper limit is exceeded, the imaging apparatus cannot be downsized, which is not preferable.

  More preferably, by setting the range as shown in the following conditional expression (4a), it is possible to provide an image pickup apparatus that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

0.70 <| tk | / Y <2.00 (4a)
In addition, when the focal length of the lens unit having the negative refractive power arranged closest to the image side is fn, and the optical system is a zoom lens, the focal length of the optical system at the telephoto end is f, and the optical system is a single focus lens. When f is the focal length of the optical system, the following conditional expression (5) is satisfied.

0.10 <| fn / f | <12.00 (5)
Conditional expression (5) defines the ratio between the focal length fn of the lens group having the negative refractive power arranged on the most image side of the optical system and the focal length f of the optical system. If the lower limit is exceeded, the negative refracting power of the lens unit arranged closest to the image side becomes too strong, and the performance deterioration at the periphery of the screen cannot be ignored. On the other hand, if the upper limit is exceeded, the imaging device becomes larger, which is not preferable.

  More preferably, by setting the range as shown in the following conditional expression (5a), it is possible to provide an imaging apparatus that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

0.20 <| fn / f | <0.40 (5a)
Regarding the low-pass filter 102, for the d-line, when the ordinary ray refractive index of the first optical element 103 is Np and the refractive index of the second optical element 104 is Nn, the following conditional expression (6) is satisfied. ing.

0.000 ≦ (Np−Nn) <0.100 (6)
Conditional expression (6) defines the difference between the ordinary ray refractive index of the first optical element 103 having birefringence and the refractive index of the second optical element 104 having no birefringence. If the upper limit is exceeded, the influence of aberration generated by the low-pass filter 102 becomes large, which is not preferable.

  More preferably, by setting the range as shown in the following conditional expression (6a), it is possible to provide an imaging apparatus that is smaller and suppresses moire and resolution reduction to the periphery of the screen.

0.000 ≦ (Np−Nn) <0.060 (6a)
(Function)
As described above, in this embodiment, the first optical element 103 is provided with birefringence to suppress image quality deterioration due to moire or the like. Then, by providing a radius of curvature with the convex surface facing the image surface side, the birefringence plate thickness is set to be thinner in the peripheral region through which the off-axis light beam passes than the axial (on the optical axis) light beam passage region. And suppresses the cut-off frequency shift. The first optical element of this embodiment is made of quartz having a thickness of 0.64 mm.

  In the present embodiment, the first optical element has a positive refractive power, and the second optical element has a negative refractive power in order to reduce the influence of aberration fluctuations caused by the positive refractive power. . By reducing the combined refractive power of the first and second optical elements, it is possible to obtain a preferable imaging device for various optical systems (for example, interchangeable lenses). The second optical element of the present embodiment is made of 1.04 mm S-BSL7 (OHARA trademark) optical glass.

  Here, the low-pass filter 102 to which the first and second optical elements are joined is preferably close to a flat plate. That is, the optical surface of the first optical element opposite to the second optical element is a flat optical surface, and the optical surface of the second optical element opposite to the first optical element is an optical surface. The surface is preferably a flat surface. Further, the position where the low-pass filter 102 is disposed is preferably a position as close to the image sensor 10 as possible. This is because if they are separated from each other, the difference between the light beam passage position and the light beam area on the optical axis is reduced, and the effect of suppressing the shift of the cutoff frequency is reduced.

(Effect of this embodiment)
According to the present embodiment, it is possible to suppress a decrease in the cutoff frequency for an off-axis light beam incident obliquely on the low-pass filter.

<< Second Embodiment >>
Hereinafter, an imaging apparatus according to the second embodiment of the present invention will be described with reference to FIG. The same members as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. Also in this embodiment, the light beam that has passed through the optical system is configured to enter the image sensor through a low-pass filter, and details of the optical system will be described below.

  The optical system of the present embodiment is a positive, negative, positive, negative four-group zoom lens in order from the object side. In zooming from the wide-angle end to the telephoto end, the first to fourth lens groups are all moved during zooming. The configuration of the first lens group is a two-lens configuration in which a concave lens and a convex lens are cemented in order from the object side. As for the configuration of the second lens group, a total of three concave lens elements and two convex lens elements are arranged in order from the object side, and the concave meniscus lens disposed closest to the object side is provided with a double-sided aspheric surface. Surface curvature can be suppressed.

  The configuration of the third lens group is composed of a total of four convex lenses, two concave lenses, and convex lenses. In the examples of the present embodiment, all convex lenses employ aspherical surfaces. Comatic aberration can be reduced. The fourth lens group has a total of two lenses: a convex lens and a concave lens with a strong concave surface facing the object side. In the example of the present embodiment, a double-sided aspherical surface is used for the concave lens disposed closest to the image side, and this makes it possible to satisfactorily correct curvature of field at the peripheral image height.

  As in the example of the present embodiment, the refractive power of the second lens unit disposed on the object side can be increased by increasing the negative refractive power of the fourth lens unit disposed on the most image side. This is possible, and the amount of movement during zooming can be designed to be small.

  The low-pass filter in the present embodiment is configured by joining a first optical element and a second optical element, the first optical element is configured by a crystal having a thickness of 0.65 mm, and the second optical element is It is composed of S-FPM3 (OHARA trademark) having a thickness of 1.05 mm. In the present embodiment, the relation of the ratio between the LPF junction interface R = −400 mm and the imaging element diagonal half-image height Y is 29.3 (conditional expression (1)). Thereby, the cutoff frequency by the low-pass filter from the on-axis to the periphery can be set to about 120 lines / mm or more in the entire zoom range.

<< Third Embodiment >>
Hereinafter, an imaging apparatus according to the third embodiment of the present invention will be described with reference to FIG.
The light beam that has passed through the optical system is configured to enter the image sensor through a low-pass filter, and details of the optical system will be described below. The optical system of the present embodiment is a single-focus lens having a symmetrical power arrangement. As described above, the light beam on the optical axis is incident obliquely on the low-pass filter and the image sensor as described above. It is the composition which does.

  The specific configuration includes, in order from the object side, a concave lens, a front group of convex lenses, a convex-concave joint, a diaphragm, an intermediate group composed of concave-convex cemented lenses, and a rear group composed of convex lenses and concave lenses. Aspherical surfaces are used for the front group concave and convex lenses to correct field curvature, etc., and all middle lens convex lenses are used to correct spherical aberration, coma, etc. An aspheric surface is used for the concave lens to correct curvature of field.

  In the example of this embodiment, as a low-pass filter, S-FPM3 (OHARA trademark) having a thickness of 1.05 mm which is the second optical element and a crystal having a thickness of 0.65 mm which is the first optical element are bonded. It is composed. A low-pass effect is provided by a convex lens birefringent plate to suppress image quality deterioration due to moire or the like.

  In the present embodiment, the second optical element and the first optical element are arranged in order from the object side to the image side. At this time, the thickness of the birefringent plate can be set to be thinner than the optical axis in the peripheral region through which the off-axis light beam passes by giving a radius of curvature with the convex surface facing the object side of the joint surface. A shift in the cut-off frequency can be suppressed.

  Also in the present embodiment, as in the first embodiment, the optical surface having the strongest curvature in the first optical element is the optical surface (bonding surface) on the second optical element side. The optical surface having the strongest curvature in the first optical element is not a convex surface facing the image sensor as in the first embodiment, but is a concave surface facing the image sensor.

  In the example of the present embodiment, the radius of curvature of the air interface, which is the optical surface on the air side, also has a radius of curvature of about Ra = 1000 mm. Assuming a system in which the optical system is exchanged, it is desirable that it be as close to a plane as possible in consideration of aberration fluctuations in each lens.

  In the example of the present embodiment, the relationship of the ratio between the LPF junction interface R = 425 mm and the diagonal half-image height Y of the image sensor is 19.6 (conditional expression (1)). Thereby, the cutoff frequency by the low-pass filter from the on-axis to the periphery can be about 100 lines / mm or more in the entire zoom range.

<< Fourth Embodiment >>
Hereinafter, an imaging apparatus according to the fourth embodiment of the present invention will be described with reference to FIG.
The light beam that has passed through the optical system is configured to enter the image sensor through a low-pass filter, and details of the optical system will be described below. The optical system of the present embodiment is a negative, positive, and negative three-group zoom lens in order from the object side. In zooming from the wide-angle end to the telephoto end, the first to third lens groups are all moved during zooming.

  Regarding the configuration of the first lens group, a total of three concave lens elements and a convex lens element are arranged in order from the object side. The concave meniscus lens disposed closest to the object side is provided with an aspheric surface, and mainly a field curvature or the like. Can be suppressed. The second lens group is composed of a total of four lenses including a convex-concave cemented lens and two convex lenses. In this example, the aspherical surface is adopted for the most convex lenses on the object side and the image side. In addition, spherical aberration and coma can be reduced. About the structure of a 3rd lens group, it is set as the total 2 sheet structure of a convex lens and a concave lens.

  In the example of the present embodiment, an aspherical surface is used for the concave lens disposed closest to the image side, and thereby, it is possible to satisfactorily correct curvature of field at the peripheral image height. Then, by increasing the negative refractive power of the third lens group arranged closest to the image side, the refractive power of the second lens group arranged on the object side can be increased, and the amount of movement during zooming can be reduced. Can be designed small.

  In the example of the present embodiment, the low-pass filter is configured by bonding a 0.65 mm thick quartz crystal as the first optical element and 1.0 mm S-FPM2 (OHARA trademark) as the second optical element. . A convex lens birefringent plate that is a first optical element can provide a low-pass effect and suppress image quality deterioration due to moire or the like.

  In the present embodiment, the relationship between the LPF junction interface R = −350 mm and the imaging element diagonal half-image height Y is 25.6 (conditional expression (1)). Thereby, the cut-off frequency by the low-pass filter from the optical axis to the periphery can be set to about 120 lines / mm or more in the entire zoom range.

(Numerical example)
Next, data in each embodiment is shown below. i indicates the order of the surfaces from the object surface, ri is the radius of curvature of the lens surface, di is the lens thickness and air spacing between the i-th surface and the (i + 1) -th surface, and ni and vi are the refractive indices for the d-line, respectively. And Abbe number. Further, k, A, B, C, D, E, etc. described in the aspheric surface are aspheric coefficients. The aspherical shape is defined by the following expression when the displacement in the optical axis direction at the position of the height h from the optical axis is x with respect to the surface vertex.

x = (h ² / R) / [1+ {1− (1 + k) (h / R) ² } ^1/2 ]
+ Ah ⁴ + Bh ⁶ + Ch ⁸ + Dh ¹⁰ + Eh ¹²
Here, R is a radius of curvature.

(Numerical example 1)
Unit mm

Surface data surface number rd nd vd Effective diameter
1 * 26.413 0.99 1.85400 40.4 18.52
2 15.427 4.13 16.57
3 -25.646 0.88 1.88300 40.8 16.24
4 58.928 0.10 16.05
5 29.195 2.03 1.92286 18.9 16.12
6 289.412 (variable) 15.90
7 * 26.582 2.16 1.69350 53.2 11.90
8 -47.810 0.10 12.00
9 19.278 2.94 1.48749 70.2 11.95
10 -23.118 0.70 1.91082 35.3 11.65
11 20.441 1.07 11.50
12 16.325 4.38 1.48749 70.2 12.11
13 * -11.465 1.17 12.13
14 (Aperture) ∞ (Variable) 10.38
15 -36.738 2.54 1.74077 27.8 11.37
16 -16.272 1.63 11.79
17 * -7.684 0.82 1.69350 53.2 11.74
18 * -65.160 (variable) 13.51
19 ∞ 0.64 1.54431 70.0 28.00
20 -400.000 1.04 1.51633 64.1 28.00
21 ∞ 0.66 28.00
Image plane ∞

Aspheric data 1st surface
K = -1.18620e + 000 A 4 = -6.36796e-006 A 6 = -4.84459e-008 A 8 = -4.46951e-010 A10 = 2.03047e-012

7th page
K = -1.30831e + 001 A 4 = -3.51337e-006 A 6 = -1.02725e-006 A 8 = -5.32564e-009

Side 13
K = -3.59349e-001 A 4 = 1.34844e-005 A 6 = -2.29851e-007 A 8 = 5.45046e-009

17th page
K = 1.13414e-001 A 4 = 1.07843e-004 A 6 = 3.63209e-006 A 8 = -4.67839e-008 A10 = 2.56149e-009

18th page
K = 0.00000e + 000 A 4 = -4.10344e-005 A 6 = -5.67666e-008 A 8 = 2.03975e-008 A10 = -1.65396e-010

Various data Zoom ratio 3.36
Wide angle Medium Telephoto focal length 18.30 47.13 61.50
F number 2.68 5.45 6.83
Angle of view 33.89 16.16 12.52
Image height 12.29 13.66 13.66
Total lens length 56.1 63.5 70.9
BF 0.66 0.66 0.66

d 6 12.58 2.02 0.44
d14 7.54 7.46 7.45
d18 7.96 26.05 35.03

Entrance pupil position 15.70 13.26 12.78
Exit pupil position -16.30 -34.42 -43.45
Front principal point position 14.25 -2.93 -11.47
Rear principal point position -17.64 -46.47 -60.84

Zoom lens group data group Start surface Focal length Lens configuration length Front principal point position Rear principal point position
L1 1 -22.64 8.13 2.78 -3.43
L2 7 15.08 12.52 4.72 -5.76
L3 15 -19.43 4.99 3.20 -0.31
LPF 19 14296.39 1.68 0.41 -0.69

Single lens Data lens Start surface Focal length
1 1 -45.31
2 3 -20.14
3 5 35.05
4 7 24.93
5 9 22.07
6 10 -11.82
7 12 14.57
8 15 37.45
9 17 -12.64
10 19 734.88
11 20 -774.70

(Numerical example 2)
Unit mm

Surface data surface number rd nd vd Effective diameter
1 83.844 1.52 1.95906 17.5 41.47
2 68.987 5.01 1.59282 68.6 40.66
3 -454.641 (variable) 40.12
4 * -415.585 1.10 1.85400 40.4 27.84
5 * 26.775 5.32 24.20
6 -39.193 0.76 1.77250 49.6 23.97
7 77.745 0.10 24.01
8 41.717 3.16 1.95906 17.5 24.28
9 513.701 (variable) 24.03
10 ∞ (variable) 18.06
11 ∞ (variable) 19.46
12 * 62.629 1.54 1.85400 40.4 18.03
13 539.324 0.17 17.96
14 * 17.531 5.71 1.55332 71.7 17.99
15 -25.309 0.17 17.74
16 35.451 0.68 2.00069 25.5 15.76
17 15.626 8.22 14.78
18 219.796 2.54 1.49710 81.6 13.76
19 * -14.905 1.69 13.65
20 (Aperture) ∞ (Variable) 11.33
21 -717.855 3.61 1.72825 28.5 13.19
22 -14.319 0.52 13.50
23 * -11.224 0.68 1.85400 40.4 13.39
24 * 39.272 (variable) 14.34
25 ∞ 0.65 1.54431 70.0 28.00
26 -400.000 1.05 1.53775 74.7 28.00
27 ∞ 0.90 28.00
Image plane ∞

Aspheric data 4th surface
K = 0.00000e + 000 A 4 = -1.33581e-005 A 6 = 5.94910e-008 A 8 = -8.66080e-011 A10 = 2.57718e-014

5th page
K = 9.88643e-001 A 4 = -1.25595e-005 A 6 = 3.62300e-008 A 8 = 1.56077e-011 A10 = 7.16901e-013

12th page
K = -2.23343e + 001 A 4 = -6.16318e-005 A 6 = -8.35200e-008 A 8 = -4.60666e-010

14th page
K = -1.54872e-001 A 4 = 2.69538e-005 A 6 = -1.61055e-007 A 8 = -7.46453e-010

19th page
K = -9.21547e-001 A 4 = 2.00554e-005 A 6 = -2.14223e-007 A 8 = 4.96902e-009 A10 = -4.85606e-011

23rd page
K = -8.92198e-001 A 4 = 4.01767e-005 A 6 = -2.29731e-006 A 8 = 5.41593e-008 A10 = -4.81021e-010

24th page
K = 3.46548e + 000 A 4 = 3.46372e-006 A 6 = -1.70971e-006 A 8 = 4.07900e-008 A10 = -3.10015e-010

Various data Zoom ratio 4.99
Wide angle Medium telephoto focal length 17.53 49.55 87.44
F number 2.06 4.20 6.00
Angle of view 33.52 15.41 8.88
Image height 11.61 13.66 13.66
Total lens length 81.0 100.3 122.0
BF 0.90 0.90 0.90

d 3 0.69 21.37 36.09
d 9 21.88 5.70 0.45
d10 -5.07 -2.54 0.00
d11 5.07 2.54 0.00
d20 6.63 5.37 5.06
d24 6.71 22.77 35.34

Entrance pupil position 28.41 63.23 99.67
Exit pupil position -14.33 -29.78 -42.21
Front principal point position 25.76 32.74 9.75
Rear principal point position -16.63 -48.65 -86.55

Zoom lens group data group Start surface Focal length Lens configuration length Front principal point position Rear principal point position
L1 1 134.34 6.53 0.13 -3.80
L2 4 -22.85 10.44 1.49 -6.40
SSP1 10 ∞ 0.00 0.00 -0.00
SSP2 11 ∞ 0.00 0.00 -0.00
L3 12 19.98 20.72 9.76 -12.66
L4 21 -21.86 4.81 3.33 0.33
LPF 25 60984.91 1.70 0.42 -0.68

Single lens Data lens Start surface Focal length
1 1 -427.36
2 2 101.40
3 4 -29.42
4 6 -33.64
5 8 47.19
6 12 82.85
7 14 19.65
8 16 -28.41
9 18 28.18
10 21 20.02
11 23 -10.16
12 25 734.88
13 26 -743.84

(Numerical Example 3)
Unit mm

Surface data surface number rd nd vd Effective diameter
1 * 41.185 1.25 1.62230 53.2 31.12
2 12.034 6.18 22.39
3 * 36.234 2.78 1.83400 37.2 22.26
4 65.324 10.72 21.08
5 * 14.621 4.57 1.56883 56.4 12.57
6 -14.052 0.95 1.72047 34.7 11.01
7 -25.508 1.95 10.36
8 (Aperture) ∞ 1.95 7.42
9 -17.664 0.95 1.73800 32.3 8.46
10 16.292 4.57 1.83481 42.7 8.54
11 * -24.463 10.34 10.38
12 -39.931 2.75 1.69680 55.5 21.80
13 -25.676 3.94 23.21
14 -13.749 1.25 1.75520 27.5 23.39
15 * -41.195 3.00 29.13
16 1000.000 1.05 1.53775 74.7 44.00
17 425.000 0.65 1.54431 70.0 44.00
18 1000.000 0.30 44.00
Image plane ∞

Aspheric data 1st surface
K = 2.93750e + 000 A 4 = 5.02474e-006 A 6 = -2.72892e-008 A 8 = 1.15072e-010 A10 = -2.08454e-013

Third side
K = -3.27015e-001 A 4 = -6.63083e-006 A 6 = 2.04029e-008 A 8 = -2.09015e-010 A10 = 2.49091e-013

5th page
K = -1.56147e + 000 A 4 = 7.73944e-005 A 6 = 1.82575e-007 A 8 = 1.52651e-010

11th page
K = -1.04975e + 001 A 4 = -8.80698e-006 A 6 = 1.11904e-006 A 8 = -4.72936e-009

15th page
K = -3.07779e + 001 A 4 = -6.95493e-005 A 6 = 6.52660e-008 A 8 = -1.98710e-010 A10 = 5.62295e-013

Various data Zoom ratio 1.00

Focal length 20.91
F number 3.60
Angle of view 45.98
Statue height 21.64
Total lens length 59.2
BF 0.30


Entrance pupil position 15.76
Exit pupil position -18.84
Front principal point position 13.83
Rear principal point position -20.61

Zoom lens group data group Start surface Focal length Lens configuration length Front principal point position Rear principal point position
1 1 20.91 58.87 13.83 -20.61

Single lens Data lens Start surface Focal length
1 1 -27.78
2 3 93.50
3 5 13.37
4 6 -44.99
5 9 -11.35
6 10 12.34
7 12 95.63
8 14 -27.87
9 16 -1375.37
10 17 1357.38

(Numerical example 4)
Unit mm

Surface data surface number rd nd vd Effective diameter
1 53.848 1.83 1.58313 59.4 32.68
2 * 11.439 7.45 22.59
3 37.484 1.83 1.69680 55.5 21.21
4 13.943 4.34 18.50
5 16.132 2.52 2.00069 25.5 18.23
6 22.403 (variable) 17.25
7 * 13.752 2.08 1.85135 40.1 8.94
8 53.997 0.98 1.80809 22.8 8.05
9 13.853 1.20 7.31
10 29.812 1.53 1.55332 71.7 6.84
11 -189.853 1.28 6.61
12 (Aperture) ∞ 1.56 6.36
13 32.670 1.81 1.55332 71.7 7.07
14 * -35.183 (variable) 7.63
15 64.323 2.48 1.49700 81.5 11.64
16 -17.952 0.80 11.96
17 * -54.165 1.28 1.85135 40.1 12.04
18 33.089 (variable) 12.55
19 ∞ 0.65 1.54431 70.0 28.00
20 -350.000 1.00 1.59522 67.7 28.00
21 ∞ 0.05 28.00
Image plane ∞

Aspheric data 2nd surface
K = -2.71305e-001 A 4 = -8.01168e-007 A 6 = -2.27782e-008 A 8 = 5.10509e-010 A10 = -2.24487e-012

7th page
K = 0.00000e + 000 A 4 = -3.13026e-005 A 6 = 1.90170e-007 A 8 = -1.14077e-008 A10 = 8.57994e-011

14th page
K = 0.00000e + 000 A 4 = 2.06383e-005 A 6 = 1.87654e-007

17th page
K = 3.15338e + 001 A 4 = -7.40197e-005 A 6 = 1.01986e-008 A 8 = -1.54104e-009 A10 = 2.73948e-011

Various data Zoom ratio 1.90
Wide angle Medium Telephoto focal length 11.30 16.42 21.49
F number 4.12 4.92 5.71
Angle of view 45.78 39.76 32.44
Image height 11.61 13.66 13.66
Total lens length 70.7 65.6 65.1
BF 0.05 0.05 0.05

d 6 16.28 6.48 1.33
d14 7.48 6.45 6.03
d18 12.27 18.00 23.10

Entrance pupil position 15.25 13.70 12.58
Exit pupil position -24.79 -29.65 -34.37
Front principal point position 21.41 21.04 20.66
Rear principal point position -11.27 -16.37 -21.42

Zoom lens group data group Start surface Focal length Lens configuration length Front principal point position Rear principal point position
L1 1 -19.71 17.97 2.65 -12.25
L2 7 19.01 10.43 4.03 -5.03
L3 15 -230.40 4.56 16.62 12.60
LPF 19 -6874.80 1.65 0.42 -0.63

Single lens Data lens Start surface Focal length
1 1 -25.31
2 3 -32.91
3 5 47.94
4 7 21.17
5 8 -23.31
6 10 46.68
7 13 30.91
8 15 28.53
9 17 -23.97
10 19 643.02
11 20 -588.02
Table 1 below shows the values of the conditional expressions of the numerical embodiments.

  Aberration diagrams of the respective embodiments are shown in FIGS. FIG. 8A is an aberration diagram at the wide-angle end of the first embodiment, and FIG. 8B is an aberration diagram at the telephoto end. FIG. 9A is an aberration diagram at the wide-angle end of the second embodiment, and FIG. 9B is an aberration diagram at the telephoto end. FIG. 10 is an aberration diagram in the third embodiment. FIG. 11A is an aberration diagram at the wide-angle end of the fourth embodiment, and FIG. 11B is an aberration diagram at the telephoto end.

(Modification)
As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment, A various deformation | transformation and change are possible within the range of the summary.

(Modification 1)
In the above-described embodiment, the LPF separation is the Y-direction two-point separation method, but this is not necessarily limited, and four-point separation may be used.

(Modification 2)
As an optical system, the above-described zoom lens may perform aperture stop control in order to reduce F value fluctuation at the time of zooming.

(Modification 3)
In addition, when an imaging device having an imaging element that converts an optical image formed on the light receiving surface into an electrical signal and a low-pass filter are integrated, electrical correction may be applied depending on the amount of distortion. good.

(Modification 4)
The first optical element having a positive refractive power and birefringence, and the second optical element having a negative refractive power and not birefringence are respectively a convex lens and a transmissive low-pass filter as a concave lens. A low-pass filter may be used. Further, the lens may use a diffraction grating.

(Modification 5)
An imaging device in which the imaging element and the above-described low-pass filter provided in front of the imaging element are integrated can also be provided.

DESCRIPTION OF SYMBOLS 10 ... Imaging device 101 ... Optical system 102 ... Low pass filter 103 ... First optical element 104 ... Second optical element

Claims (11)

An imaging apparatus having an optical system, an imaging element, and a low-pass filter disposed between the optical system and the imaging element,
The low-pass filter includes a first optical element having a positive refractive power and a second optical element having a negative refractive power, and the first optical element is made of a birefringent material,
When the radius of curvature of the optical surface having the larger curvature among the optical surfaces of the first optical element is R, and the diagonal half-image height of the imaging element is Y,
15.0 <| R / Y | <40.0
An imaging device characterized by satisfying the following conditional expression:   2. The image pickup apparatus according to claim 1, wherein an optical surface having a larger curvature among the optical surfaces of the first optical element is an optical surface convex toward the image pickup element.   The imaging apparatus according to claim 1, wherein the first optical element and the second optical element are joined.   Of the optical surfaces of the first optical element, the optical surface opposite to the second optical element is a flat surface, and among the optical surfaces of the second optical element, the optical surface is opposite to the first optical element. The imaging device according to claim 1, wherein the optical surface of the imaging device is a flat surface. When the ordinary ray refractive index for the d-line of the first optical element is Np and the refractive index of the second optical element is Nn,
0.000 ≦ (Np−Nn) <0.100
5. The imaging apparatus according to claim 1, wherein the following conditional expression is satisfied.   The image pickup device has an incident angle at which a light reception rate of 0.8 Yv is obtained at a first angle where the incident angle with respect to the image pickup device is greater than 30 °, where Yv is a light reception rate with respect to a light beam perpendicularly incident on the image pickup device. 6. The imaging apparatus according to claim 1, wherein the imaging apparatus has characteristics. When the distance on the optical axis between the optical surface closest to the image sensor and the image sensor among the optical surfaces of the first optical element and the second optical element is L,
0.00 <L / Y <0.10
The imaging apparatus according to claim 1, wherein the following conditional expression is satisfied. When the curvature radius of the optical surface opposite to the second optical element among the optical surfaces of the first optical element is Ra,
0.00 ≦ | Y / Ra | <0.10
The imaging apparatus according to claim 1, wherein the following conditional expression is satisfied. When the optical system is a single focus lens, the distance on the optical axis between the exit pupil position of the optical system and the image sensor is tk, and when the optical system is a zoom lens, the optical system at the wide angle end. When the distance on the optical axis between the exit pupil position and the image sensor is tk,
0.50 <| tk | / Y <2.50
The imaging apparatus according to claim 1, wherein the following conditional expression is satisfied. When the optical system is a single focal lens, the focal length of the optical system is f, and when the optical system is a zoom lens, the focal point of the lens group having the negative refractive power arranged closest to the image side. When the distance is fn and the focal length of the optical system at the telephoto end is f,
0.10 <| fn / f | <12.00
The imaging apparatus according to claim 1, wherein the following conditional expression is satisfied.   The imaging apparatus according to claim 1, wherein a back focus of the optical system is shorter than an effective diagonal length of the imaging element.