JP2004029577A - Light quantity controller, imaging optical system, and imaging apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a light quantity controller which is provided with a multidensity optical filter using a multilayered film and prevents the occurrence of a ghost by the rereflection of the reflected light from an imaging device at the optical filter. <P>SOLUTION: The light quantity controller 13 is disposed within an imaging optical system for forming a subject image on the imaging device 12 and is provided with the optical filter 14 for attenuating the light quantity arriving at the imaging device by covering at least part of a diaphragm aperture, in which the optical filter comprises a base member 21 having light transparency, image side optical filters 23 and 24 covering the entire region of the surface of the base member on the subject side and a subject side optical film 22 covering a partial region of a ray effective section on the surface of the base member on the subject side and a plurality of density regions varying in light transmittance from each other are formed on at least either of the image side optical films and the subject side optical film. <P>COPYRIGHT: (C)2004,JPO

Description

【００９１】
なお、上記各実施形態にて説明したＮＤフィルタに設けられる多層膜の数および各多層膜の透過率はいずれも例に過ぎず、これら以外の多層膜数および透過率を選択してもよい。
【００９２】
また、上記各実施形態では、ＮＤフィルタに多層膜を形成した場合について説明したが、必ずしも多層膜を形成する必要はなく、単層膜でもよい。
【００９３】
【発明の効果】
以上説明したように本発明によれば、多濃度の光学フィルタでの反射に起因するゴーストの発生を抑制してクリアな画像を撮影できる光量調節装置、撮像光学系および撮像装置を実現することができる。
【図面の簡単な説明】
【図１】本発明の第１実施形態である撮像光学系の構成を示す断面図。
【図２】上記第１実施形態の撮像光学系内に設けられた光量調節装置のＮＤフィルタの構成を示す断面図。
【図３】上記ＮＤフィルタが絞り開口に対して移動する様子を示す図。
【図４】本発明の第３実施形態である光量調節装置のＮＤフィルタの構成を示す断面図。
【図５】本発明の第３実施形態である光量調節装置のＮＤフィルタの構成を示す断面図。
【図６】本発明の第４実施形態である光量調節装置のＮＤフィルタの構成を示す断面図。
【図７】ゴースト発生例を示す撮像光学系の断面図。
【図８】透過波面位相差が光学性能へ与える影響を説明するための図。
【図９】透過波面位相差と白色ＭＴＦ値の関係を示す図。
【符号の説明】
１１　対物レンズ
１２　固体撮像素子
１３　光量調節装置
１４，１４’，１４”，１４”’　ＮＤフィルタ
１５　水晶ローパスフィルタ
１６　カバーガラス
２１　ベース部材
２２，２３，２４，４１〜４３，５１〜５３，６１〜６３　多層膜
３１　絞り開口[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical filter mounted on an imaging device or an imaging optical system such as a video camera and a digital still camera.
[0002]
[Prior art]
2. Description of the Related Art In an imaging optical system of an imaging device such as a video camera, a light amount adjusting device that adjusts a light amount by changing an opening diameter formed by a plurality of aperture blades is used. In such a light amount adjusting device, if the aperture diameter is too small, optical performance degradation due to light diffraction becomes a problem.
[0003]
Therefore, in order to prevent the aperture diameter from becoming too small even under a bright subject condition, a light amount adjusting device using both an aperture blade and an ND (Neutral Density) filter has been proposed.
[0004]
For example, in Japanese Patent No. 2,592,949, an ND filter that covers an opening formed by an aperture blade is attached to the aperture blade, and the ND filter includes a plurality of density regions set to different uniform transmittances. The provided light amount adjusting device is disclosed. The plurality of regions of the ND filter are set so that the transmittance increases in order from the outside to the inside of the opening.
[0005]
Japanese Patent Application Laid-Open No. Sho 52-117127 discloses that a mechanical aperture blade is moved from an open state to a predetermined aperture area, and that a small aperture control below a certain aperture value continuously changes light transmittance depending on shading. There has been proposed a light amount adjusting device that causes the ND filter to enter the opening in order from the side with the highest transmittance.
[0006]
Further, Japanese Patent Application Laid-Open No. 2000-106649 proposes an imaging apparatus including an exposure control mechanism for reducing the influence of the light transmittance of an ND filter having a plurality of density regions on optical performance.
[0007]
By the way, in an imaging apparatus that photoelectrically converts and captures a subject image using an imaging device, part of the light that passes through the ND filter and enters the imaging device is partially exposed to the surface of the imaging device (cover glass, quartz low-pass filter). , The surface of the infrared cut filter, etc.), returns to the subject side, and is reflected again on the imaging element side surface or the object side surface of the ND filter, and the reflected light is incident on the imaging element, thereby causing a ghost. May occur. As a countermeasure against this, Japanese Patent Application Laid-Open No. 2000-36917 proposes a light amount adjusting device in which an anti-reflection film is provided on a side surface of an image sensor of an ND filter.
[0008]
[Problems to be solved by the invention]
However, Japanese Patent Application Laid-Open No. 2000-36917 does not disclose a specific method for preventing reflection in the case of a multi-density ND filter, that is, for preventing ghost from occurring.
[0009]
In order to realize a multi-density ND filter, it is conceivable to form a multilayer film having a light amount attenuating effect with different areas on both surfaces of a transparent member serving as a base. However, such a multilayer film is used. There has not yet been proposed a configuration for preventing reflection in a multi-density ND filter.
[0010]
In addition, the cause of the deterioration of the optical performance in the intermediate stop state is the influence of diffraction caused by the transmittance difference in the ND filter, but the transmitted wavefront phase difference caused by the thickness component of the ND filter is also greatly affected.
[0011]
It is empirically known that the optical performance deteriorates when a thick filter covers a part of the aperture opening.However, it is analyzed how the thickness component of the filter affects the optical performance, There are no known examples of such measures.
[0012]
As a workaround for the influence of the thickness of the ND filter on the optical performance, Japanese Patent Application Laid-Open No. 6-259971 discloses a method in which an ND filter having a transparent portion and a portion where the transmittance changes continuously or stepwise is fixed to a circular diaphragm. There has been proposed a configuration in which the amount of transmitted light is adjusted by moving the opening so as to cover the entire opening.
[0013]
However, the light amount adjusting device proposed in Japanese Patent Application Laid-Open No. 6-265971 focuses only on a large phase difference between a portion that passes through the filter and a portion that does not pass through the filter, and realizes an ND filter that actually has a transmittance change. At this time, there is no description about a small transmitted wavefront phase difference equal to or less than the wavelength λ of light due to a small change in thickness or a small change in refractive index that would occur due to a change in transmittance.
[0014]
Accordingly, the present invention provides a light amount adjusting device and an imaging optical device that use a multi-density optical filter and reduce ghost generated when light reflected by the image sensor is further reflected by the optical filter and incident on the image sensor. It is an object to provide a system and an imaging device.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, according to the present invention, in order to attenuate the amount of light that is provided in an imaging optical system that forms a subject image on an imaging element and that covers at least a part of an aperture opening and reaches the imaging element. In the light amount adjusting device including an optical filter to be used, the optical filter includes a base member having a light-transmitting property, an image-side optical film that covers an entire exposed area of the base member on the image sensor side, and a base member. A plurality of density regions having different light transmittances in at least one of the image-side optical film and the subject-side optical film. Is formed.
[0016]
Thus, by providing the optical film (image-side optical film) on the surface of the base member on the image sensor side (the entire area exposed to the image sensor), the light reflected by the image sensor on the image sensor side of the base member It is possible to suppress the reflection on the surface and reduce the occurrence of ghost.
[0017]
In addition, by making each of the plurality of density regions have a light transmittance lower than the light transmittance of the base member, the light amount of the light that reaches the subject-side surface of the base member out of the light reflected by the image sensor. In addition, the amount of light reaching the subject-side surface of the base member can be suppressed, and the occurrence of ghost can be more effectively reduced.
[0018]
Here, when S is the area of the above-mentioned effective ray portion, and S11 is the area of the region of the effective ray portion on the subject side that is not covered with the subject-side optical film,
0.2 <S11 / S <0.6
It is better to satisfy the following conditions.
[0019]
Further, the film configuration of each of the plurality of concentration regions is different from each other, and the phase difference of light transmitted through these concentration regions due to the difference in the film configuration of the concentration regions adjacent to each other is suppressed to 1 / 5λ or less. The phase difference between the light transmitted through the subject-side optical film and transmitted through the base member and the light transmitted through the base member through a region of the base-side surface on the subject side where the subject-side optical film is not formed is calculated. By controlling to に λ or less, it is possible to suppress a decrease in MTF (Modulation Transfer Function).
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Before describing the light amount adjusting device and the imaging optical system according to the embodiment of the present invention, how the transmitted wavefront phase difference affects an image will be described.
[0021]
First, what phenomenon occurs optically when a thick filter covers a part of the aperture will be described with reference to FIG.
[0022]
FIGS. 8A to 8E show a case in which a filter P and an aperture stop AS are arranged in front (on the object side) of an ideal lens L having no aberration, and parallel light rays, which are plane waves of monochromatic light having a wavelength λ, are incident. The point image intensity distribution Q near the geometrical optical imaging point I is shown.
[0023]
FIG. 8A shows a state where the filter P has a thickness of zero and does not affect the transmitted wavefront. In this case, the intensity distribution Q becomes a diffraction image based on the relationship between the F number F and the wavelength λ of the light beam.
[0024]
When the aperture stop AS has a circular aperture, one high-intensity point image Qa having a radius of 1.22 Fλ is formed, and a ring-like weak light diffracted light is formed around the point image Qa. Here, the thickness of the lower region of the filter P corresponding to half of the aperture of the aperture stop AS is increased by a small amount, and as shown in FIG. 8B, the transmitted wavefront phase difference becomes (４) λ. When set to, a point image Qb with a relatively high intensity appears beside the point image with a high intensity.
[0025]
When the thickness of the lower region of the filter P is further increased and the phase difference of the transmitted wavefront is set to (2/4) λ as shown in FIG. 8C, the intensity distribution Q does not become one point image. Then, two point images Qc having large intensities separated vertically in the paper surface appear. This is because the phase of the wavefront of the light passing through the upper half of the pupil of the paper and the wavefront of the lower half of the paper among the light to be condensed at the geometrical optical imaging point I are shifted by (1/2) λ. Therefore, a wave canceling phenomenon occurs in the wave optics, and the intensity on the image forming point I becomes zero. On the other hand, since the light energy to be condensed on the imaging point I does not disappear according to the law of conservation of energy, the light energy is dispersed in the vertical direction of the imaging point I and converges on two points.
[0026]
When the thickness of the lower region of the filter P is further increased and the phase difference of the transmitted wavefront is set to (3/4) λ as shown in FIG. 8D, the upper point image Qc of the two point images is obtained. Is weaker than the intensity of the lower point image Qc.
[0027]
Further, if the thickness of the lower region of the filter P is increased and the transmitted wavefront phase difference is set to (4/4) λ as shown in FIG. 8E, the intensity distribution Q becomes one point image Qe again. It becomes a diffraction image and returns to the state similar to FIG.
[0028]
When the thickness of the lower region of the filter P is further increased, the intensity distribution Q periodically changes from the state shown in FIGS. 8A to 8D according to the transmitted wavefront phase difference. .
[0029]
The interval Δy between the two-point images Qc separated when the transmitted wavefront phase difference is (1/2) λ shown in FIG.
Δy ≒ 2Fλ
And is proportional to the F number and the wavelength λ. For example, when the F number F = 4 and the wavelength λ = 550 nm, the interval between the two point images is
Δy ≒ 4.4 μm
become. This is based on the same principle as that of the low-pass filter of the two-point separation. The same effect as when a low-pass filter with a cutoff frequency of 1 / (2Δy) = 114 lines / mm is applied, and the MTF of the optical system deteriorates. Means that.
[0030]
Incidentally, the light actually used in the imaging optical system is not monochromatic light, but white light in which lights of various wavelengths are mixed. In this case, in FIG. 8C, the thickness of the filter P at which the intensity becomes zero at the image forming point differs for each wavelength.
[0031]
Conversely, for a specific filter thickness, the phase difference varies depending on the wavelength, so that when the specific wavelength is in the state of FIG. 8C, the wavelength that deviates from this wavelength is closer to FIG. 8B or 8D. The state is shifted. Since white light is affected by the intensity distribution of each wavelength, even if the center wavelength corresponds to FIG. 8C, the intensity does not become zero as white light.
[0032]
A color weight having a sensitivity peak near 550 nm in the range of visible light of 400 nm to 700 nm in accordance with the standard relative luminosity is set as white light, and the relationship between the transmitted wavefront phase difference and the MTF in white light is shown in FIG. Note that FIG. 9 does not consider the influence of diffraction caused by the transmittance of the ND filter, and shows only the change in the MTF value due to the transmitted wavefront phase difference caused by the thickness component of the filter.
[0033]
FIG. 9 shows the white color in the ideal aberration-free lens system when the thickness of the filter in the lower half region of the aperture opening is gradually increased from zero and the transmitted wavefront phase difference (corresponding to the film thickness) is changed to 6λ. This is a calculated value of the MTF of light wave optics.
[0034]
FIG. 9A is a graph showing white MTF values at a spatial frequency of 50 lines / mm, and FIG. 9B is a graph showing white MTF values at a spatial frequency of 100 lines / mm. The vertical axis of the graph is the MTF value, and the horizontal axis is the transmitted wavefront phase difference at a wavelength λ = 550 nm. The MTF values in the state where the F number of the circular aperture is F1, F1.4, F2, F2.8, F4, F5.6, and F8 are shown in the graph.
[0035]
Note that the limit spatial frequency at which the image sensor can resolve is up to 1 / (2 × pixel pitch), but the spatial frequency important for image quality evaluation is about half of the limit frequency. Thus, if the evaluation frequency is defined as 1 / (4 × pixel pitch), the evaluation frequency is 50 lines / mm and 100 lines / mm at a pixel pitch of 5 μm and 2.5 μm, respectively.
[0036]
In FIG. 9, a so-called valley portion where the MTF falls at a position where the phase difference is an integral multiple of λ / 2 occurs. In the case of a single wavelength of 550 nm, the MTF is zero at any valley portion, but is not zero due to the influence of other wavelengths.
[0037]
Further, when comparing the valley portions, the larger the phase difference, the more the wavelength is different from that of FIG. 8C at other wavelengths, so that the MTF value at the position of the valley portion tends to increase.
[0038]
Further, comparing the so-called peak portions of the MTF, the larger the phase difference is, the more the wavelength is different from that of FIG. 8E at other wavelengths, so that the MTF at the position of the peak portion tends to decrease. As a result, as a result, the difference between the peak and the valley of the MTF becomes small when the phase difference is about 5λ, and a convergence tendency is observed.
[0039]
Specifically, a region where the phase difference is 5λ or more corresponds to a state where the filter end face is half in the optical path (a state where half of the aperture is covered with the filter). In addition, the region where the phase difference is within 2λ is, specifically, a region having a vapor deposition film in a state where the filter has completely entered the optical path (a state where the entire aperture opening is covered with the filter) and a region having no vapor deposition film. Are applied, or a case where a plurality of deposited film regions having different film thicknesses are applied.
[0040]
In these cases, it is effective to reduce the phase difference as much as possible in order to prevent the MTF from decreasing. In order to improve the performance when the phase difference is 5λ or more, light passing through the region having a deposition film and deposition It is preferable that the phase difference of the light of wavelength λ passing through the region without the film be λ / 4 or less.
[0041]
In particular, when a filter is formed by providing a plurality of adjacent vapor-deposited films, it is preferable to devise a film configuration of each vapor-deposited film region so as to reduce a phase difference as much as possible. In this case, it is preferable to prevent the MTF from being lowered by setting the phase difference of the light having the wavelength λ transmitted through these regions caused by the difference in the film configuration of the adjacent deposited film regions to be λ / 5 or less.
[0042]
As the ND filter, there are known a type in which an organic dye or a pigment which absorbs light is mixed and kneaded in a filter material, and a type in which an optical thin film is deposited on the surface of a base material (base member). ing.
[0043]
The feature of the kneading type ND filter is that a large number of filters having a uniform density can be processed inexpensively. As the ND filter used for the adjusting device, there are many vapor deposition types.
[0044]
The deposition type ND filter can reduce the wavelength dependence of the spectral transmittance by stacking a plurality of metal films and dielectric films, and can also have the deposited film also function as an antireflection film.
[0045]
(1st Embodiment)
FIG. 1 shows a configuration of an imaging optical system including a light amount adjusting device having an ND filter according to a first embodiment of the present invention.
[0046]
In FIG. 1, reference numeral 11 denotes an objective lens for forming a subject image, and reference numeral 12 denotes a solid-state image sensor such as a CCD or a CMOS, which photoelectrically converts the subject image and outputs an electric signal. Reference numeral 12a denotes a light receiving surface of the image sensor 12.
[0047]
Reference numeral 13 denotes a diaphragm device for changing the F-number of the imaging optical system, and 14 denotes an ND filter for attenuating the amount of light passing through the diaphragm opening of the diaphragm device 13. The ND filter 14 is obtained by depositing an optical film as a multilayer film on a base member formed of a transparent resin into a film shape. The stop device 13 and the ND filter 14 constitute a light amount adjusting device for adjusting the light amount of the imaging optical system.
[0048]
The ND filter 14 may be integrally attached to the diaphragm blade of the diaphragm device 13, or may be provided separately from the diaphragm blade. When provided separately from the aperture blades, a filter drive mechanism that drives the ND filter 14 in a direction to advance and retreat with respect to the imaging optical path is provided in the aperture device 13. In any case, the actuator is driven by an actuator (not shown) so as to advance and retreat in the imaging optical path, and attenuates the amount of light reaching the imaging element 12 by covering at least a part of the aperture opening.
[0049]
Reference numeral 15 denotes a quartz low-pass filter, and reference numeral 16 denotes a cover glass provided in front of the light receiving surface 12a of the image sensor 12. The imaging device 12 is configured as a unit including a light receiving surface 12a and a cover glass 16.
[0050]
Of the components described above, the imaging optical system is configured by a portion excluding the imaging element 12 and the cover glass 16. This imaging optical system may be provided integrally with an imaging device body (not shown) (such as a compact digital camera or a lens-integrated video camera) or an imaging device body (a single-lens reflex digital camera or a lens-interchangeable video camera). Etc.).
[0051]
FIG. 2 shows an example of deposition of the ND filter 14. FIG. 2 shows a cross section of the ND filter 14, in which the left side of the paper is the subject side and the right side of the paper is the image sensor side. When the ND filter 14 is retracted from the optical path (when the ND filter 14 does not cover the aperture opening at all), the end face near the optical axis is the end face on the upper side of the paper.
[0052]
Reference numeral 21 denotes a film-like base member formed of a transparent resin, and reference numerals 22, 23, and 24 denote multilayer films as optical films for attenuating the amount of light. The multilayer films 22 and 23 have a transmittance of about 32%, and the multilayer film 24 has a transmittance of about 10%. With these films, the ND filter 14 is divided into three regions 2A, 2B, and 2C having different light transmittances, and the transmittance of each region is 32%, 10%, and 0.32%, respectively.
[0053]
The ratio of the area s of each region is
s (2A): s (2B): s (2C) = 1: 1: 2
And In FIG. 2, the thicknesses of the base member 21 and the multilayer films 22, 23, 24 are exaggerated for the sake of explanation.
[0054]
Further, two density regions having different transmittances (densities) are formed on the image pickup element side of the base member 21 by the multilayer film 23 and the multilayer film 24.
[0055]
As a material of the base member 21, a synthetic resin film such as cellulose acetate, polyethylene terephthalate, vinyl chloride, and acrylic resin is used, and has a thickness of about 50 to 100 μm.
[0056]
The multilayer films 22, 23, and 24 are formed from the ND layer and the AR coat layer from the base member 21 side. Further, by providing a dielectric film close to the refractive index of the base member 21 between the base member 21 and the ND layer in the multilayer film 23, the transmitted wavefront phase difference with the adjacent multilayer film 24 can be reduced. Is possible.
[0057]
FIG. 3 shows how the ND filter 14 moves to attenuate the amount of light. Reference numeral 31 denotes an aperture opening (small aperture state) formed by the aperture blades of the aperture device 13. FIG. 3A shows a state in which the ND filter 14 has retracted outside the aperture opening 31, and FIG. 2C shows a state in which the area 2C covers the aperture 31 and (b), (c), and (d) show states in the course of (a) to (e). In FIG. 3, the diaphragm blade and the ND filter 14 are driven separately, and the light amount is attenuated only by the ND filter 14 while keeping the area of the diaphragm opening 31 constant. This prevents a decrease in MTF due to small-aperture diffraction.
[0058]
In the embodiment shown in FIG. 2, multilayer films 23 and 24 are formed on the entire surface of the base member 21 of the ND filter 14 on the image sensor side (here, the entire surface is exposed to the image sensor). ing.
[0059]
This is because light reflected by the cover glass 16 or the like of the image sensor 12 and returned to the subject side is reflected by the surface of the ND filter 14 (base member 21) on the image sensor side and enters the image sensor 12 again. This is effective for suppressing ghosts caused by the above.
[0060]
FIG. 7 shows an example of a ghost. Reference numeral 71 denotes a light beam which enters the objective lens 11 and becomes a surface reflection ghost. After being reflected by the cover glass 16, the light beam 71 is further reflected by the surface of the ND filter 14 on the image sensor side, reaches the light receiving surface 12 of the image sensor 12, and forms a ghost image.
[0061]
Ghosts in such a path are particularly noticeable in image forming properties with an afocal objective lens in an ND filter. In order to reduce the intensity of the ghost, it is effective to form an anti-reflection film on the surface of the ND filter 14 on the image sensor side.
[0062]
In the present embodiment, the multilayer films 23 and 24 formed on the entire surface of the ND filter 14 on the image sensor side have a function as an anti-reflection film so as to achieve both light quantity suppression and anti-reflection functions. When a plurality of ND films are used, it is preferable to set the film thickness step so that the phase difference is λ / 5 or less as described above. This makes it possible to reduce MTF degradation due to the phase difference and to reduce ghost intensity due to ND reflection.
[0063]
From the viewpoint of ghost reduction, it is more preferable to apply a vapor deposition film (multilayer film) having an antireflection effect to the surface of the ND filter 14 on the subject side. This is to suppress the reflection of light incident from the image sensor side of the ND filter 14 on the back surface of the base member 21. In addition, the light incident from the image sensor side and the light reflected from the back surface are subjected to the light amount attenuating action in the multilayer films 23 and 24, so that the light amount itself decreases, and the light amount reaching the image sensor 12 becomes extremely small.
[0064]
However, in order to cover the entire surface on the object side (light ray effective portion) with the desired density pattern, the number of films equal to or greater than the number of density steps is required as a whole, and thus there is a problem in cost.
[0065]
Therefore, in the present embodiment, only a part of the surface of the base member 21 on the subject side is covered with the multilayer film 22 having the light amount attenuation function and the anti-reflection effect, and the number of evaporations is the minimum necessary to obtain a desired density step. The configuration is effective in reducing ghost strength.
[0066]
In FIG. 2, 50% of the object side surface is covered with the multilayer film 22. The multilayer film 22 is set so that the transmission phase difference between the light passing therethrough and the light passing through the region without the multilayer film is λ / 4 or less, so that the deterioration of the MTF due to the phase difference is minimized. Preferably, it is reduced.
[0067]
(2nd Embodiment)
FIG. 4 shows an ND filter 14 'used in the light amount adjusting device according to the second embodiment of the present invention. In FIG. 4, reference numerals 41, 42, and 43 denote multilayer films deposited on the surface of the base member 21 on the subject side (left side in the figure) and on the imaging element side (right side in the figure) to attenuate the light amount. The multilayer films 41 and 42 have a transmittance of about 32%, and the multilayer film 43 has a transmittance of about 10%.
[0068]
Further, the entire surface of the base member 21 on the image sensor side (region exposed to the image sensor) is covered with the multilayer film 42 and the multilayer film 43, and the surface of the base member 21 on the subject side (light-effective area) is covered with the multilayer film 41. Part) is covered. The multilayer film 42 and the multilayer film 43 form two density regions having different transmittances (densities) on the imaging element side of the base member 21.
[0069]
With these multilayer films, the ND filter 14 'is divided into three regions 4A, 4B, and 4C having different transmittances, and the transmittances of the respective regions are 32%, 10%, and 0.32%, respectively.
[0070]
The ratio of the area s of each region is
s (4A): s (4B): s (4C) = 1: 1: 2
It is.
[0071]
The transmittance (density) pattern of the ND filter 14 'of this embodiment is the same as that of the ND filter 14 shown in FIG. 2, but the deposition range of each multilayer film is different. In particular, the multilayer film 41 occupies 75% of the subject-side surface (light-effective portion) of the base member 21.
[0072]
Thereby, similarly to the ND filter 14 shown in FIG. 2, the effect of suppressing the light from the image sensor side from being reflected on the surface of the base member 21 on the image sensor side can be obtained, and compared with the ND filter 14 of FIG. Therefore, there is an advantage that the range in which the reflection of the ghost light on the back surface of the base member 21 can be suppressed is wide.
(Third embodiment)
FIG. 5 shows an ND filter 14 ″ used in the light amount adjusting device according to the third embodiment of the present invention. In FIG. 5, reference numerals 51, 52, and 53 denote an object on the base member 21 for attenuating the light amount. The multilayer film 51 has a transmittance of about 32%, and the multilayer film 53 has a transmittance of about 10%. The multilayer film 52 is an anti-reflection film provided so as not to cause a phase difference with light transmitted through the multilayer film 53, and has a transmittance substantially equal to the transmittance of the base member 21 (a transmittance close to 100%). ).
[0073]
By these multilayer films 51 to 53, the ND filter 14 ″ is divided into three regions 5A, 5B, and 5C having different transmittances, and the transmittance of each region is 95%, 32%, and 0.32%, respectively. Become.
[0074]
The ratio of the area s of each region is
s (5A): s (5B): s (5C) = 1: 1: 2
It is.
[0075]
Note that the transmittance of the area 5A does not become 100% mainly because there is light reflected on the surface on the subject side.
[0076]
The entire surface of the base member 21 on the image sensor side (the area exposed to the image sensor) is covered by the multilayer films 52 and 53, and the surface of the base member 21 on the subject side (light-effective area) is covered by the multilayer film 51. Part) is covered. The multilayer film 52 and the multilayer film 53 form two density regions having different transmittances (densities) on the imaging element side of the base member 21.
[0077]
The difference from the ND filter 14 'shown in FIG. 4 is that the multilayer film 52 is an anti-reflection film having no ND function. In this case, even if the filter tip at the top of the paper is inserted into the imaging optical path, there is no effect of attenuating the light amount. Therefore, compared to the filter configuration of FIG. It is necessary to increase the amount of insertion of the filter 14 "into the imaging optical path. This reduces the frequency with which the tip of the ND filter 14" half-hangs on the imaging optical path. As described above, when the filter tip is half-hung on the imaging optical path, the MTF is reduced when the phase difference in FIG. 9 is 5λ or more, but this embodiment has the effect of reducing this frequency.
[0078]
In addition, this embodiment has the same effect as the ND filter 14 'of the second embodiment.
[0079]
(Fourth embodiment)
Fig. 6 shows an ND filter 14 "'used in a light amount adjusting device according to a fourth embodiment of the present invention. In Fig. 6, 61, 62 and 63 are multilayer films for attenuating the light amount. The multilayer films 61 and 63 have a transmittance of about 32%, and the multilayer film 62 has a transmittance of about 10%.
[0080]
With these multilayer films 61 to 63, the ND filter 14 "'is divided into three regions 6A, 6B, and 6C having different transmittances, and the transmittances are 32%, 10%, and 0.32%, respectively. .
[0081]
The ratio of the area s of each region is
s (6A): s (6B): s (6C) = 1: 1: 2
It is.
[0082]
The entire surface of the base member 21 on the image sensor side (the area exposed to the image sensor) is covered with the multilayer film 63, and the surface of the base member 21 on the subject side (light-effective area) is covered with the multilayer films 61 and 62. Part) is covered. The multilayer film 61 and the multilayer film 62 form two density regions having different transmittances (densities) on the subject side of the base member 21.
[0083]
The transmittance (density) pattern of the ND filter 14 "" of the present embodiment is the same as that of the ND filter 14 'shown in FIG. 4 except that the multilayer film on the object side has two types of densities, and the multilayer film on the image sensor side. Are different from each other.
[0084]
In order to mass-produce the ND filter at low cost, it is effective to deposit a multilayer film for a plurality of filters on a large transparent sheet and cut the ND filter into a required size. At this time, if the filter configuration as shown in FIG. 6 is used, since the multilayer film 63 has no film boundary within the cutting dimension, it is sufficient to uniformly vapor-deposit the entire transparent sheet and then cut, and the surface on the image sensor side is complicated. There is an advantage that a simple evaporation mask is not required.
Here, in each of the embodiments described above, it is preferable that the ND filter satisfies the following conditional expressions.
[0085]
0.2 <S11 / S <0.6 (1)
Here, S is the area of the effective ray portion of the base member 21, and S11 is the area of the region of the base member 21 on the object side that is not covered with the multilayer film.
[0086]
Conditional expression (1) defines the area of the region on the object side of the ND filter where there is no multilayer film. If the area of the region without the multilayer film is too small below the lower limit of the conditional expression (1), the boundary (film boundary) between the region with the multilayer film and the region without the multilayer film approaches the tip of the filter. This makes it easier for both the filter tip and the film boundary to hang over the aperture opening.
[0087]
The filter tip causes an MTF reduction at a phase difference of 5λ or more shown in FIG. 9, and a membrane boundary causes an MTF reduction at a wavelength of λ / 4 or less shown in FIG. Therefore, if both the filter tip and the film boundary hang over the aperture opening, there are a plurality of MTF reduction factors in the aperture opening, and the MTF reduction becomes remarkable, which is not preferable.
[0088]
If the area of the region without the multilayer film is too large beyond the upper limit of the conditional expression (1), the range for suppressing the back surface reflection on the base member 21 described above becomes narrow, and the ghost reduction effect is weakened. Absent.
[0089]
Table 1 below shows values of the conditional expression (1) in each of the above embodiments.
[0090]
[Table 1]

[0091]
Note that the number of multilayer films and the transmittance of each multilayer film provided in the ND filter described in each of the above embodiments are merely examples, and other numbers of multilayer films and transmittances may be selected.
[0092]
Further, in each of the above embodiments, the case where a multilayer film is formed on the ND filter has been described. However, it is not always necessary to form a multilayer film, and a single layer film may be used.
[0093]
【The invention's effect】
As described above, according to the embodiments of the present invention, it is possible to realize a light amount adjustment device, an imaging optical system, and an imaging device that can capture a clear image while suppressing generation of ghost due to reflection by a multi-density optical filter. it can.
[Brief description of the drawings]
FIG. 1 is a sectional view showing a configuration of an imaging optical system according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view illustrating a configuration of an ND filter of the light amount adjusting device provided in the imaging optical system according to the first embodiment.
FIG. 3 is a diagram showing a state in which the ND filter moves with respect to a diaphragm aperture.
FIG. 4 is a cross-sectional view illustrating a configuration of an ND filter of a light amount adjusting device according to a third embodiment of the invention.
FIG. 5 is a cross-sectional view illustrating a configuration of an ND filter of a light quantity adjusting device according to a third embodiment of the present invention.
FIG. 6 is a cross-sectional view illustrating a configuration of an ND filter of a light amount adjusting device according to a fourth embodiment of the present invention.
FIG. 7 is a cross-sectional view of an imaging optical system showing an example of occurrence of a ghost.
FIG. 8 is a diagram for explaining the effect of a transmitted wavefront phase difference on optical performance.
FIG. 9 is a diagram illustrating a relationship between a transmitted wavefront phase difference and a white MTF value.
[Explanation of symbols]
11 Objective lens
12 solid-state imaging device
13 Light intensity adjustment device
14, 14 ', 14 ", 14"' ND filter
15 Crystal low-pass filter
16 Cover glass
21 Base member
22,23,24,41-43,51-53,61-63 Multilayer film
31 Aperture aperture

Claims (7)

A light amount adjustment device provided in an image pickup optical system that forms a subject image on an image pickup device, and having an optical filter that attenuates a light amount reaching the image pickup device by covering at least a part of an aperture opening,
The optical filter,
A base member having optical transparency,
An image-side optical film that covers the entire exposed area of the surface of the base member on the image sensor side;
A subject-side optical film that covers a partial area of the ray effective portion on the subject-side surface of the base member,
At least one of the image-side optical film and the subject-side optical film has a plurality of density regions having different light transmittances from each other. The light amount adjusting device according to claim 1, wherein each of the plurality of density regions in the optical filter has a light transmittance lower than a light transmittance of the base member. The light amount adjusting device according to claim 1, wherein the following conditions are satisfied.
0.2 <S11 / S <0.6
Here, S is the area of the light beam effective portion of the optical filter, and S11 is the area of the region of the light effective portion on the subject side of the optical filter that is not covered with the subject side optical film. The film configuration of the plurality of density regions in the optical filter is different from each other, and the phase difference of light of wavelength λ passing through these density regions due to the difference in the film configuration of the density regions adjacent to each other is 1 / 5λ. The light amount adjusting device according to claim 1, wherein: On the subject side of the base member, the phase difference between the light of wavelength λ passing through the subject-side optical film and the light of wavelength λ passing through the area where the subject-side optical film is not formed is １ ／ λ or less. The light amount adjusting device according to claim 1, wherein: An imaging optical system comprising: an objective optical system; and the light amount adjusting device according to claim 1. An imaging apparatus comprising: an imaging element; and the imaging optical system according to claim 6.

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