JP5084115B2 - Light amount adjusting device, optical system having light amount adjusting device, and photographing apparatus - Google Patents

Description

  The present invention relates to a light amount adjusting device used in an optical apparatus such as a digital video or a digital still camera. In particular, the present invention relates to a light amount adjusting device that can suppress deterioration of optical performance and is suitable for a camera taking lens using an image pickup device having a small pixel pitch.

In an imaging optical system such as a digital still camera or digital video, a light amount adjusting device (light amount diaphragm device) that adjusts the light amount by changing the aperture diameter formed by the diaphragm blades is used.
In such a light amount adjusting device, when an aperture diameter becomes small when photographing a high-luminance subject, optical performance deteriorates due to light diffraction. For this reason, conventionally, many light amount adjusting devices using both diaphragm blades and ND (Neutral Density) filters have been proposed. For example, Patent Document 1 proposes a light amount adjustment device that performs aperture control using mechanical aperture blades up to a certain aperture value and performs small aperture control using an ND filter below a certain aperture value. . In this light quantity adjusting device, a mechanical aperture blade is moved from the open position to a predetermined aperture area, and aperture control is performed by this mechanical aperture blade. In addition to such control, in small aperture control below a certain aperture value, an ND filter whose translucency changes continuously depending on the shading is used, and the aperture control is made to enter the aperture in order from the filter portion with the highest transmittance. Configured to do.

However, in the configuration as in Patent Document 1, when the edge of the ND filter is positioned on the aperture opening, a part of the light passes through the filter part and the remaining light passes through the part without the filter. Therefore, a phase difference due to the difference in thickness occurs between the two, causing a problem that optical performance deteriorates.
For this reason, for example, Patent Document 2 proposes a transmitted light amount adjustment mechanism that avoids a deterioration in optical performance by preventing a phase difference due to a difference in thickness.
Here, an ND filter composed of a transparent portion and a filter portion in which the transmittance changes continuously, or a transparent portion and a filter portion having a low transmittance is used. The ND filter configured as described above is configured to move in a state of covering all of the fixed circular aperture and adjust the amount of transmitted light.

Further, in Patent Document 3, the optical performance is deteriorated not only by a large phase difference between the part with and without the filter as in Patent Document 2 described above, but also by a transmitted wavefront phase difference below the wavelength order of light. Proposed proposals have been made.
In other words, here, when realizing an ND filter having a change in transmittance, attention is paid to a change in a minute thickness caused to give a change in transmittance, or a transmitted wavefront phase difference less than a wavelength λ of light due to a change in minute refractive index. Proposals have been made.
The ND filter here has a gradation ND region (gradation density distribution region) in which the transmittance continuously changes and a transparent partial region having a transmittance of 80% or more and a uniform transmittance. Such an ND filter is configured such that the phase difference of light having a predetermined wavelength λ that passes through the boundary between the gradation ND region and the transparent partial region is λ / 5 or less. This avoids deterioration of optical performance due to a transmitted wavefront phase difference equal to or less than the wavelength order of light.
Japanese Patent Laid-Open No. 52-117127 JP-A-6-265971 JP 2004-205951 A

By the way, in the above-described conventional example, each countermeasure is taken as described above for the deterioration of the optical performance when the ND filter is used.
For example, in Patent Document 2 described above, measures are taken to avoid deterioration of optical performance due to a large phase difference between a portion with and without a filter.
Further, in the above-mentioned Patent Document 3, when realizing the ND filter, the optical performance of the transmission wavefront phase difference below the wavelength λ of the light due to the change in the minute thickness caused by the change in transmittance or the change in the minute refractive index is realized. Measures are taken to avoid degradation. However, there is no indication about the problem of optical performance degradation caused by factors other than these and countermeasures. In fact, in an ND filter having a gradation ND region in which the transmittance changes continuously, the transmitted wavefront phase difference caused by the length of the gradation ND region with respect to the aperture opening also affects the deterioration of the optical performance.

In view of the above problems, the present invention provides a light amount adjusting device, an optical system, and a photographing device that can suppress degradation of optical performance caused by a gradation ND filter whose transmittance changes continuously. It is for the purpose.
The present invention also provides a light amount adjusting device, an optical system, and a photographing device capable of minimizing a phenomenon in which the amount of transmitted light at a diaphragm opening is reduced in a light amount adjusting device having diaphragm blades using a gradation ND filter. It is intended to provide.
It is another object of the present invention to provide a light amount adjusting device, an optical system, and a photographing device that are effective against performance deterioration due to light diffraction at the time of a small aperture.

In order to solve the above-described problems, the present invention provides a light amount adjusting device configured as follows, an optical system having the light amount adjusting device, and an imaging device.
That is, the light amount adjusting device of the present invention is a light amount adjusting device having a diaphragm blade for forming an opening and a filter member attached to the diaphragm blade for adjusting the light amount of light passing through the opening.
The filter member has a gradation ND region in which the transmittance continuously changes, and has a transparent portion region continuous with a region having the maximum transmittance in the gradation ND region, and the transmittance of the transparent portion region is 80. % Uniform transmittance ,
The filter member has an ND region having a uniform transmittance continuous with a region having the minimum transmittance in the gradation ND region, and the transmittance of the ND region having the uniform transmittance is lower than the minimum region. Yes,
The filter member is set so as to cover the entire opening,
A light quantity adjusting device satisfying the following conditional expression, where L _GND is a length of the gradation ND region and D is an entrance pupil diameter at the aperture.
0.1 <L _GND /D<0.5
At that time, the numerical range of the conditional expression is
0.15 <L _GND /D<0.45
It is desirable to set so as to satisfy
Furthermore, it is more desirable to set L _GND /D=0.39.
The optical system of the present invention is characterized by having the above-described light amount adjusting device, and can adopt a configuration for forming an image on a photoelectric conversion element.
An optical system according to the present invention includes an optical system having the above-described light amount adjusting device and a photoelectric conversion element that receives an image formed by the optical system.

According to the present invention, it is possible to minimize degradation of optical performance caused by a gradation ND filter whose transmittance changes continuously.
In addition, according to the present invention, in the light amount adjusting device having the diaphragm blades by the gradation ND filter, it is possible to minimize the phenomenon that the transmitted light amount at the aperture opening is reduced.
In addition, according to the present invention, it is possible to provide a light amount adjusting device that is effective against performance degradation due to light diffraction at the time of small aperture.

With the above-described configuration, it is possible to minimize degradation of optical performance caused by the gradation ND filter. However, in the embodiment of the present invention, the following configuration can be further adopted.
The filter member has an ND region having a uniform transmittance continuous with a region having the minimum transmittance in the gradation ND region, and the transmittance of the ND region having the uniform transmittance is further lower than the minimum region. It can be set as the structure which has.
In addition, the filter member has a transparent region that is continuous with a region having the maximum transmittance in the gradation ND region, and the transmittance of the transparent region is 80% or more. be able to.
The filter member may be configured to cover part or all of the opening.
In addition, the diaphragm blades to which the filter member is attached may be inclined with respect to the image plane.

Examples of the present invention will be described below. In the following description of the embodiments, the wavelength used is the visible wavelength range, and the wavelength at that time is λ = 550 nm.
[Example 1]
As a first embodiment of the present invention, a configuration example of a gradation ND filter in a light amount adjusting device to which the present invention is applied will be described.
FIG. 1 shows a schematic configuration of an opening portion of a diaphragm blade and a gradation ND filter in the present embodiment.
In FIG. 1, 1 and 2 are diaphragm blades, and 3 is a gradation ND filter. The aperture blades 1 and 2 adjust the aperture state of the aperture by being moved in opposite directions.
The gradation ND filter 3 includes a gradation ND region 5 in which an ND deposition film is deposited in an inclined manner so that the transmittance continuously changes, and an ND region 6 in which the ND deposition film is uniformly deposited. The gradation ND regions 5 and 6 can attenuate the amount of light transmitted through the surface of the film-like resin filter 4 that is the base.
Further, the gradation ND filter 3 has a transparent part region 7 that is continuous with the region having the maximum transmittance in the gradation ND region 5, and the transmittance of the transparent part region is 80% or more and is uniform. ing.

In the gradation ND region 5 of the present embodiment, the film thickness is continuously changed so that the ND density continuously changes in the moving direction of the gradation ND filter 3 from ND = 0.1 to ND = 1.0. An ND deposition film is deposited on the substrate. In the ND region 6, an ND vapor deposition film is deposited so that the film thickness is constant and the ND concentration is ND = 1.0. The gradation ND filter 3 is set so that the aperture opening has an F number (Fno) = 4.0, and the ND regions 5 and 6 cover the opening.
Here, the transmittance of the ND region is calculated with a 10- ^ND concentration. For example, ND0.1 has a transmittance of 79.4%, and ND1.0 has a transmittance of 10%.

In the present embodiment, the ratio of the gradation ND region 5 to the aperture opening diameter is set so as to satisfy the following conditional expression (1).
0.1 <L _GND /D<0.5 (1)
Here, L _GND is the length of the gradation ND filter 3 in the moving direction of the gradation ND filter 3, and D is the entrance pupil diameter at the aperture.
In the above conditional expression, if the upper limit value is exceeded, the gradation ND region 5 increases with respect to the aperture diameter of the aperture, and the average density decreases, so the effect on light diffraction at the time of small aperture is weakened. On the other hand, if the lower limit value is exceeded, the gradation ND region 5 is reduced, so that a transmitted wavefront phase difference due to a minute thickness is generated, causing image quality degradation.
In addition, since the amount of applied uniform density increases at the aperture of the aperture, a phenomenon occurs in which the transmitted light amount decreases (hereinafter, the phenomenon in which the transmitted light amount decreases is referred to as “T number (Tno) decrease. Tno”). Fno / √ (transmittance%) × 10).
Therefore, by setting so as to satisfy the above conditional expression, the optical due to the transmitted wavefront phase difference between the light beam that passes through the gradation ND region and the light beam that passes through the filter member having a high transmittance, while taking into account the Tno drop. It becomes possible to suppress performance degradation. At that time, the numerical range of the conditional expression (1) is set to 0.15 <L _GND /D<0.45 (1) ′.
It is desirable to set so as to satisfy the above in order to reduce performance deterioration and further improve the image quality.

Here, it is desirable that the gradation ND filter is provided with an antireflection film in order to prevent surface reflection. By setting in this way, it is possible to suppress the ghost intensity caused by the reflection of the filter surface.
Further, it is desirable that the gradation ND filter is inclined to the light source so as to face down (the direction opposite to the direction facing the light source). Usually, since a high-intensity light source such as the sun is on the upper side, ghost rays caused by reflection on the ND filter surface can be released outside the imaging apparatus by tilting face down (that is, downward) with respect to the light source. For example, it is inclined 4 ° downward with respect to a plane perpendicular to the optical axis.

FIG. 2 is a schematic diagram of a zoom lens using the gradation ND filter of this embodiment.
Table 1 shows numerical examples of the zoom lens using the gradation ND filter of this embodiment.

In Table 1, ri is the radius of curvature of the i-th surface from the object side, di is the surface interval between the i-th surface and the i + 1-th surface from the object side, and ni is the refraction of the i-th lens at the d-line. Rate. Here, ν i indicates the Abbe number of the i-th lens at the d-line.
Also, k is a conic constant, A ′, B, B ′, C, C ′, D, D ′, E, E ′, and F are 3rd, 4th, 5th, 6th, 7th, 8th, Nineth, tenth, eleventh, and twelfth aspheric coefficients are used.
Further, the displacement in the optical axis direction at the position of the height h from the optical axis is assumed to be x with reference to the surface vertex.
Thus, the aspheric shape is expressed by the following expression.
x = (h ² / R) / [1+ [1- (1 + K) (h / R) ² ] ^1/2 ] + A′h ³ + Bh ⁴ + B′h ⁵ + Ch ⁶ + C′h ⁷ + Dh ⁸ + D′ h ⁹ + Eh ¹⁰ + E'h ¹¹ + Fh ¹²
Here, R is a radius of curvature, and “e−X” means “× 10 ^−X ”. In addition, the aspherical surface is marked with * on the left side of the surface number in each table.

Next, a case where the diaphragm blades and the gradation ND filter of this embodiment are used together will be described.
FIG. 3 shows the relationship between the Tno and the MTF value when the gradation ND filter of this embodiment is attached to a diaphragm blade.
As is clear from FIG. 3, by using the gradation ND filter of this embodiment, 53.6% when Tno = 8, 37.0% when Tno = 16, and 27.5 when Tno = 22. Degradation of image quality remains at about%. The phase change rate of the transmitted wavefront generated at this time is 0.93λ / A. Here, A represents the ratio of the length of the gradation ND region to the aperture diameter of the stop. Further, Tno at the time of opening the aperture remains at a drop of 2.1.

[Comparative Example 1]
As Comparative Example 1, a configuration example including only aperture blades that do not use a gradation ND filter will be described.
FIG. 4 shows the relationship between the Fno and the MTF value when the gradation ND filter in Comparative Example 1 is not used together and the aperture is narrowed down from the aperture opening state to the small aperture state only with the aperture blades of the zoom lens shown in FIG.
In the graph, the MTF value (axial optical performance value) is shown on the left vertical axis, Fno (opening F number) is shown on the horizontal axis, and Tno is shown on the right vertical axis. Moreover, the schematic diagram of each aperture opening state is shown on the upper side of the graph.
The calculation condition of MTF is that of wave optics with white color weight, and the evaluation spatial frequency was 111 lines / mm.
As is clear from FIG. 4, when the ND filter of Comparative Example 1 is used in combination, the optical performance is the best when the aperture is opened, but the optical performance is degraded due to the diffraction that occurs when the aperture is reduced. .
At the time of the small aperture (Fno = 11.2), the MTF value is 9.4%, so that the subject image falls to a level at which the subject image is not resolved. In this state, the lens cannot be used for a high brightness subject.

[Comparative Example 2]
As Comparative Example 2, a configuration example in which an ND filter having a uniform density capable of handling a high-luminance subject is used will be described.
FIG. 5 shows a schematic configuration of an opening portion formed by an ND filter having a uniform density and a diaphragm blade capable of handling a high-luminance subject in Comparative Example 2.
In FIG. 5, 11 and 12 are diaphragm blades, and 13 is an ND filter having a uniform density.
The aperture blades 11 and 12 adjust the aperture state of the aperture by being moved in opposite directions.
The uniform density ND filter 13 includes a uniform density ND region 16 with ND = 1.0.
In the configuration example of Comparative Example 2, an ND filter having an ND region 16 having a density of ND = 1.0 (transmittance 10.0%) is attached to the diaphragm blade, and the aperture opening is set to Fno = 4. It is set so as to cover the entire ND area.
FIG. 6 shows the relationship between the Tno and the MTF value when the ND filter of ND = 1.0 in Comparative Example 2 is attached to the diaphragm blade.
As is apparent from FIG. 6, the image quality deterioration remains at about 31.7% even with a small aperture (Tno = 22) by using the ND filter of Comparative Example 2 together. However, the optical performance is deteriorated by the influence of the transmitted wavefront phase difference due to the minute thickness around Tno = 6. The transmitted wavefront phase difference generated at this time is 0.37λ.

[Comparative Example 3]
As Comparative Example 3, a configuration example in which the gradation ND region is 0.07 in the ratio to the entrance pupil diameter will be described.
FIG. 7 shows a schematic configuration of an opening formed by an ND filter having a short gradient area of gradation ND and a diaphragm blade in Comparative Example 3. Here, the same reference numerals are given to the same components as those shown in FIG.
In FIG. 7, the gradation ND filter 13 includes a gradation ND region 15 in which an ND vapor deposition film is deposited in an inclined manner so that the transmittance continuously changes, and an ND region 16 in which the ND vapor deposition film is uniformly deposited. Has been.
In the configuration example of the comparative example 3 shown in FIG. 7, unlike the first embodiment, the gradation ND region 15 is set to 0.07 as a ratio to the entrance pupil diameter. Further, the ND density of the gradation ND at this time is ND = 1.0 to ND = 0.1, and the diaphragm opening is F4 and is attached to the diaphragm so that the ND area covers the entire opening.
FIG. 8 shows the relationship between the Tno and the MTF value when the diaphragm blades in Comparative Example 3 and the gradation ND filter with the above specifications are used in combination.
It can be seen from FIG. 8 that the MTF value suddenly drops after the aperture is fully opened.
This is based on the fact that the gradient width of the gradation ND is short with respect to the opening diameter. That is, in the light flux passing through the aperture opening, an abrupt film thickness difference is generated between the light beam passing through the ND region and the filter region having a high transmittance, so that the optical performance is deteriorated due to the transmitted wavefront phase difference.
The phase change rate of the transmitted wavefront generated at this time is 4.85λ / A, and the phase difference change rate due to the inclination is 5.2 times that of Numerical Example 1.
In addition, Tno at the time of opening the aperture drops to 2.24 despite the same density as in Numerical Example 1, and the effect as gradation ND is not sufficiently obtained.
As described above, when a gradation ND filter having a large phase change rate is used together, similarly to a uniform density ND filter, it is affected by optical performance deterioration due to a transmitted wavefront phase difference caused by a minute thickness.

[Example 2]
As a second embodiment of the present invention, a configuration example different from the first embodiment will be described.
FIG. 9 shows a schematic configuration of the opening portion of the diaphragm blade and the gradation ND filter in the present embodiment. Here, the same components as those shown in FIG. 1 in the first embodiment are denoted by the same reference numerals, and description of portions common to the first embodiment will be omitted.
In the gradation ND region 5 of this embodiment, the film thickness is continuously changed, and vapor deposition is performed so that the ND concentration continuously changes from ND = 0.1 to ND = 1.5 (transmittance of 3.2%). The ND region 6 is deposited so that the film thickness is constant and the ND concentration is ND1.5.
The gradation ND filter is set so that the diaphragm aperture is F4 and the ND covers all the aperture, and L _GND /D=0.39.

FIG. 10 shows the relationship between the Tno and the MTF value when the gradation ND filter in the present embodiment is attached to the diaphragm blade.
Only the diaphragm blade of Comparative Example 1 is different from the case of using the uniform density ND filter of Comparative Example 2 at the time of 50.3% when Tno = 8, 43.1% when Tno = 16, and Tno = 22. 38.
The image quality is only about 9%. The phase change rate of the transmitted wavefront generated at this time is 1.45λ / A.
Further, Tno at the time of opening the diaphragm remains at 2.14 even though the minimum transmittance of ND is 3.2%.
As described above, the influence of the concentration on the phase change rate of the transmitted wavefront is small compared to the influence of the gradient ND on the phase change rate of the transmitted wavefront.
However, when the ND concentration is increased, it is advantageous for diffraction occurring at the time of small aperture, but an ND filter region having a low transmittance and a transparent portion region having a high transmittance can be formed at the aperture of the aperture, and transmitted in the vertical direction. Illuminance imbalance occurs due to the difference in rate. Therefore, the ND concentration is desirably ND = 1.5 or less.

As described above, by using the gradation ND filter of each embodiment in combination as an aperture of an optical system such as a photographing optical system, performance deterioration due to a transmitted wavefront phase difference particularly in a normal Tno region (Tno = 4 to 16) from the aperture opening. Can be reduced. Thereby, the image quality can be improved.
It is also possible to suppress performance degradation due to diffraction at the time of small aperture while minimizing Tno drop due to the ND region.

[Example 3]
As Embodiment 3 of the present invention, a photographing apparatus having a light amount adjusting device configured by attaching the gradation ND filter of the present invention to the aperture blade will be described.
In FIG. 11, reference numeral 20 denotes a photographing apparatus body, and 10 denotes a photographing optical system.
Reference numeral 11 denotes a light amount adjusting device composed of diaphragm blades to which the gradation ND filter of the present invention is attached.
12 is a solid-state image sensor (photoelectric conversion element) that receives a subject image formed by the photographing optical system 10, 13 is a recording medium that records the subject image received by the image sensor 12, and 14 is a viewfinder for observing the subject image. It is. As the finder 14, a finder of a type that observes a subject image displayed on a display element such as an optical finder or a liquid crystal panel can be considered.
In this way, by applying the light amount adjusting device of the present invention to a photographing device that forms a subject image on a solid-state image sensor such as a video camera or a digital still camera, a photographing device with good performance can be realized.

Schematic which shows the structure of the opening part by the gradation ND filter and aperture blade in Example 1 of this invention. 1 is a schematic diagram of a zoom lens that uses a gradation ND filter together in Example 1 of the present invention. FIG. The figure which shows the relationship between Tno and a MTF value at the time of attaching and using the gradation ND filter in Example 1 of this invention to an aperture blade. The figure which shows the relationship between Fno and MTF value at the time of restrict | squeezing from an aperture opening state to a small aperture state only by an aperture blade without using ND filter in the comparative example 1. FIG. Schematic which shows the structure of the opening part by the ND filter of uniform density which can respond to the high-intensity subject in the comparative example 2, and an aperture blade. The figure which shows the relationship between Tno and MTF value at the time of attaching and using the ND filter in the comparative example 2 to an aperture blade. Schematic which shows the structure of the opening part by the ND filter with a short inclination area | region of the gradation ND in comparative example 3, and an aperture blade. The figure which shows the relationship between Tno and a MTF value at the time of using together ND filter with a short inclination area | region of gradation ND in the comparative example 3. FIG. Schematic which shows the structure of the opening part by the gradation ND filter and aperture blade in Example 2 of this invention. The figure which shows the relationship between Tno and a MTF value at the time of attaching and using the gradation ND filter in Example 2 of this invention to an aperture blade. 1 is a schematic view of a photographing apparatus having a light amount adjusting device configured by attaching a gradation ND filter of the present invention to a diaphragm blade.

Explanation of symbols

1: Diaphragm blade 2: Diaphragm blade 3: Gradation ND filter (filter member)
4: Film-shaped resin filter as a base 5: Gradation ND region 6 in which transmittance changes continuously 6: ND region LND deposition film uniformly deposited L _GND : Length of gradation ND region D: Aperture aperture Entrance pupil diameter at

Claims (5)

In a light quantity adjusting device having a diaphragm blade for forming an opening and a filter member attached to the diaphragm blade for adjusting the light quantity of light passing through the opening,
The filter member has a gradation ND region in which the transmittance continuously changes, and has a transparent portion region continuous with a region having the maximum transmittance in the gradation ND region, and the transmittance of the transparent portion region is 80. % Uniform transmittance ,
The filter member has an ND region having a uniform transmittance continuous with a region having the minimum transmittance in the gradation ND region, and the transmittance of the ND region having the uniform transmittance is lower than the minimum region. Yes,
The filter member is set so as to cover the entire opening,
A light quantity adjusting device satisfying the following conditional expression, where L _GND is a length of the gradation ND region and D is an entrance pupil diameter at the aperture.
0.1 <L _GND /D<0.5 2. The light quantity adjusting device according to claim 1, wherein the diaphragm blades to which the filter member is attached are arranged to be inclined with respect to the image plane. An optical system comprising the light amount adjusting device according to claim 1 . The optical system according to claim 3 , wherein an image is formed on the photoelectric conversion element. An imaging apparatus comprising: an optical system having the light amount adjusting device according to claim 1 ; and a photoelectric conversion element that receives an image formed by the optical system.

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