JP2008076691A - Optical low-pass filter, imaging apparatus unit, digital camera, and personal digital assistant - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical low-pass filter which hardly spoils resolution while completely restraining false color or moire. <P>SOLUTION: The optical low-pass filter P1 having positive power is arranged between an image-formation optical system and an electronic imaging device. The optical low-pass filter P1 satisfies the following condition: 0.9<(M1(u)×M2(u))/(M10(u)×M20(u))<1.3 provided u=1/4p, when it is assumed that the pixel pitch of the electronic imaging device is p, MTF on the optical axis of the image-formation optical system on an e-line at spatial frequency u is M10(u), the MTF at the height of 80% of the maximum image height of the image-formation optical system on the e-line at the spatial frequency u is M1(u), the MTF of the optical low-pass filter on the optical axis at the spatial frequency u is M20(u), and the MTF of the optical low-pass filter at the height that a principal ray is made incident on the optical low-pass filter and is 80% of the maximum image height at the spatial frequency u is M2(u). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an optical low-pass filter, and more specifically, an optical low-pass filter that can reduce or eliminate noise such as color moire (false color) due to interference between an element pitch of an electronic image sensor and a specific spatial frequency of a subject. The present invention also relates to an imaging device unit having the optical low-pass filter, a digital camera having the imaging device unit, and a portable information terminal device.

In recent years, it is called a digital camera or an electronic camera, and a subject image is picked up by a solid-state image pickup device such as a CCD (charge coupled device) image pickup device, and a still image (still image) or a moving image (movie image) of the subject. The type of camera that obtains the image data and digitally records it in a non-volatile semiconductor memory such as a flash memory has become common. The market for such digital cameras has become very large, and user demands for digital cameras have become widespread and sophisticated. In particular, coupled with wide angle of view, downsizing, and low price, there is a strong demand for higher image quality.
By the way, in an optical system that uses an electronic image sensor such as a CCD as a light receiving element like a photographing optical system in such a digital camera or the like, it is necessary to make light rays incident on the image sensor almost perpendicularly, and the optical system. It is necessary to arrange filters such as an optical low-pass filter and an infrared cut filter between the sensor and the image sensor.
In equipment using an electronic image sensor such as a CCD as the image sensor, image formation is performed to prevent or reduce noise such as color moire due to interference between the element pitch of the electronic image sensor and a specific spatial frequency of the subject. Some optical low-pass filters are arranged between an optical system and an electronic image sensor.

However, if the effect of the optical low-pass filter is increased too much, the effect of reducing noise such as color moiré is increased, but there is also a problem that the resolving power is reduced and the reproducibility of a fine (high spatial frequency) subject is deteriorated. In general, the imaging optical system has a slightly lower performance around the screen than the center of the image plane, that is, the MTF around the screen is lower than the MTF around the screen. Therefore, in the conventional optical low-pass filter using a parallel plate of birefringent material, the frequency characteristic of the filter is uniquely determined with a constant thickness, so when the optimization is performed at the center of the screen, the resolution at the periphery of the screen decreases, Conversely, if optimization is performed at the periphery of the screen, there is a problem that noise tends to occur at the center of the screen.
By the way, as an invention made by paying attention to such a problem, there is Patent Document 1 (Japanese Patent Laid-Open No. 2001-117139).
In this Patent Document 1, a subject image is formed on an image sensor by an optical low-pass filter having a plurality of plate thicknesses that gradually change from the filter center corresponding to the optical axis of the image pickup lens to the periphery, and the image pickup lens. In an imaging apparatus that captures a subject image formed on the imaging element and records the image data, the low-pass filter described above is disposed with the center of the filter positioned on the optical axis of the imaging lens. An imaging device is described.

  The optical low-pass filter described in Patent Document 1 has a plate thickness that gradually decreases from the filter center to the periphery, so that false resolution and false color in the vicinity of the optical axis of the imaging lens with a low image height. And false resolution and false color near the periphery of the imaging lens having a high image height can be reduced to some extent. However, because the plate thickness is changed in multiple stages, there is a possibility that shadows of parts with different thicknesses may be reflected, and there is no description of specific solution means, and the resolution performance of the peripheral part is deteriorated. It is difficult to obtain a good image over the entire prevention image plane.

JP 2001-117139 A

The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an optical low-pass filter that sufficiently suppresses false colors and moire and does not impair the resolution as much as possible. is there.
An object of claim 2 of the present invention is to provide an optical low-pass filter capable of realizing a sealed structure with a simple configuration.
An object of the third aspect of the present invention is to provide an optical low-pass filter that can change the degree of change of the low-pass filter effect from the center toward the periphery more appropriately.
An object of claim 4 of the present invention is to provide an image pickup apparatus unit that sufficiently suppresses false colors and moire and does not impair the resolution as much as possible over the entire screen.
An object of claim 5 of the present invention is to provide a digital camera that sufficiently suppresses false colors and moire and does not impair the resolution as much as possible over the entire imaging screen.
An object of claim 6 of the present invention is to provide a portable information terminal device that sufficiently suppresses false colors and moire and does not impair the resolution as much as possible over the entire imaging screen.

In order to achieve the above-described object, the filter according to claim 1 has a positive power in an optical low-pass filter disposed between the imaging optical system and the electronic image sensor, and the filter of the electronic image sensor. The pixel pitch is p, the MTF on the optical axis of the imaging optical system at the e-line at the spatial frequency u is M10 (u), and the maximum image height of the imaging optical system at the e-line at the spatial frequency u is The MTF at 80% height is M1 (u), the MTF of the optical low-pass filter on the optical axis at the spatial frequency u is M20 (u), and the principal ray with the maximum image height at the spatial frequency u is 80%. When the MTF of the optical low-pass filter at the height incident on the optical low-pass filter is M2 (u), the following condition:
0.9 <(M1 (u) × M2 (u)) / (M10 (u) × M20 (u)) <1.3
However, u = 1 / 4p
It is characterized by satisfying.
The optical low-pass filter according to the invention described in claim 2 is characterized in that either one of the incident side and the exit side is a flat surface.
The optical low-pass filter according to the invention described in claim 3 is characterized in that either one of the incident side and the exit side is formed of an aspherical surface.

An imaging device unit according to a fourth aspect of the invention includes the optical low-pass filter according to any one of the first to third aspects.
According to a fifth aspect of the present invention, a digital camera includes the imaging device unit according to the fourth aspect.
According to a sixth aspect of the present invention, a portable information terminal device includes the imaging unit according to the fourth aspect.

According to the optical low-pass filter of the first aspect of the present invention, the optical low-pass filter disposed between the imaging optical system and the electronic image pickup device has a positive power and has a pixel pitch of the electronic image pickup device. P, MTF on the optical axis of the imaging optical system at the e-line at the spatial frequency u is M10 (u), and 80% of the maximum image height of the imaging optical system at the e-line at the spatial frequency u The MTF of the optical low-pass filter on the optical axis at the spatial frequency u is M20 (u), and the principal ray having a maximum image height of 80% at the spatial frequency u When the MTF of the optical low-pass filter at the height incident on the low-pass filter is M2 (u), the following conditions are satisfied:
0.9 <(M1 (u) × M2 (u)) / (M10 (u) × M20 (u)) <1.3
However, u = 1 / 4p
Therefore, there is provided an optical low-pass filter that can appropriately reduce or eliminate noise such as moire and false color without causing a reduction in resolution over the entire screen.
According to the optical low-pass filter of the second aspect, since either the incident side or the emission side is formed as a flat surface, a sealed structure that prevents foreign matter from adhering to the electronic imaging device is formed. At the same time, a sealed structure can be realized with a simple structure.

Further, according to the optical low-pass filter according to claim 3, since either one of the incident side and the emission side is formed of an aspherical surface, the center is compared with a simple spherical surface having positive power. It is possible to change the degree of change of the low-pass effect from to the periphery more appropriately, that is, more finely and more delicately.
According to the imaging device unit of the fourth aspect, since the optical low-pass filter according to any one of the first to third aspects is provided, the moiré or the fake is not caused without causing a reduction in resolution over the entire screen. Noise such as color can be reduced or removed accurately.
Further, according to the digital camera of the fifth aspect, since the image pickup apparatus unit according to the fourth aspect is provided, noise such as moire or false color is generated without causing a reduction in resolution over the entire screen. In an image pickup apparatus unit using an image pickup device such as a CCD, the exit pupil position of the image forming optical system can be set in order to make the light beam enter the image pickup device substantially perpendicularly. Although it is necessary to move away from the image sensor, it is effective to give positive power to the optical low-pass filter disposed immediately before the image sensor even when the exit pupil is moved away.
Further, according to the portable information terminal device of the sixth aspect, since the configuration has the imaging unit according to the fourth aspect, an effect equivalent to the effect produced by the digital camera according to the fifth aspect is obtained. Can play.

Hereinafter, based on an embodiment of the present invention, an optical low-pass filter, an imaging device unit, a digital camera, and a portable information terminal device of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a sectional view showing a configuration of an imaging optical system and an optical low-pass filter which are numerical examples as well as the first embodiment of the present invention.
The imaging optical system according to the present invention is a single focus lens, which is a first lens E1 composed of a negative meniscus lens having a convex surface toward the object side in order from the object side to the image surface side, and a convex surface toward the object side. A second lens E2 composed of a negative meniscus lens facing the lens, a third lens E3 composed of a biconvex lens having a strong convex surface facing the object side, a fourth lens E4 composed of a biconvex lens facing a strong convex surface toward the image surface side, and the object side A fifth lens E5 composed of a biconcave lens with a strong concave surface facing the lens, and a sixth lens E6 composed of a biconvex lens with a strong convex surface directed to the image surface side. The fourth lens E4 and the fifth lens E5 are cemented lenses that are in close contact with each other and are integrally joined.
The front lens group GF is configured by integrally supporting the first lens E1, the second lens E2, and the third lens E3, and the fourth lens E4, the fifth lens E5, and the sixth lens E6 are integrally formed. By supporting, the rear group GR is configured, and by moving the rear group GR, the focus position is adjusted by changing the object distance, that is, the subject distance, that is, the focus is adjusted. By adopting such a configuration, it is possible to achieve a small size, a wide angle of view and a large aperture, and to effectively compensate for deterioration of the flatness of the image plane during focusing when the angle of view is widened. Regardless of, high performance can be obtained.

Here, when the subject approaches from infinity to a short distance, the best surface of the peripheral image changes to the plus side of the optical axis direction with respect to the center of the screen (the direction from the subject to the image plane is plus). On the other hand, when the distance between the front group and the rear group is changed to be narrow, the best surface of the peripheral image changes to the minus side in the optical axis direction. When the subject position changes from a state in focus on an infinitely distant subject, if the rear group is moved to focus (focus), the rear group moves to the subject side. Become. That is, the interval between the front group and the rear group is narrowed. Therefore, the change in the best surface of the peripheral image due to the change in the subject distance is offset by the movement of the best surface in the peripheral image due to the change in the distance between the front group and the rear group due to the focus adjustment operation. As a result, the position of the best surface of the peripheral image is kept substantially constant even when the subject distance changes, and performance deterioration due to the subject distance change can be prevented.
In FIG. 1, one positive refractive power is provided behind the imaging optical system, that is, between the image side surface of the sixth lens E6 having the positive refractive power closest to the image plane and the image plane FS. An optical filter OF composed of the optical low-pass filter P1 and the plane parallel glass P2 is inserted, and functions to appropriately remove false resolution and false color.
The optical low-pass filter P1 will be described in detail in the description of the embodiment of the present invention.

Next, specific numerical examples based on the above-described embodiment of the present invention will be described in detail. Examples described below are examples of specific configurations based on specific numerical examples of the imaging optical system according to the present invention. Thereafter, an embodiment of an imaging device unit, a digital camera, or a portable information terminal device using an imaging optical system as shown in the examples will be described.
In the embodiment showing the imaging optical system according to the present invention, the configuration of the imaging optical system and specific numerical examples thereof are shown.
It will be apparent from the Examples that by configuring the imaging optical system as in the present invention, very good imaging performance can be ensured while achieving a sufficiently small size.
In the description related to the following embodiments, the following various symbols are used.
When the height from the optical axis is H, the amount of displacement in the optical axis direction from the surface apex is S, the radius of curvature is R, and the aspheric coefficient is A2i. .

また、ｆは全系の焦点距離、面間隔はレンズ厚またはレンズ間隔に相当し、Ｎｄはｄ線の屈折率、そしてνｄはｄ線のアッベ数を示している。さらに、Ｋは、非球面の円錐定数、Ａ４は、４次の非球面係数、Ａ６は、６次の非球面係数、Ａ８は、８次の非球面係数、Ａ１０は、１０次の非球面係数とする。

Further, f represents the focal length of the entire system, the surface interval corresponds to the lens thickness or the lens interval, Nd represents the d-line refractive index, and νd represents the d-line Abbe number. K is an aspherical conic constant, A4 is a fourth-order aspheric coefficient, A6 is a sixth-order aspheric coefficient, A8 is an eighth-order aspheric coefficient, and A10 is a tenth-order aspheric coefficient. And

  FIG. 1 shows the configuration of an imaging optical system according to an embodiment of the present invention, and schematically shows a longitudinal section along the optical axis. The imaging optical system shown in FIG. 1 includes a first lens E1 composed of a negative meniscus negative lens having a convex surface directed toward the object side in order from the object side to the image surface, and a negative meniscus having a convex surface directed toward the object side. A second lens E2 composed of a negative lens of the type, a third lens E3 which is a biconvex positive lens having a strong convex surface facing the object side, an aperture FA, and a biconvex positive lens having a strong convex surface facing the image surface side A cemented lens obtained by closely bonding a fourth lens E4 that is a negative concave lens having a strong concave surface facing the object side and a fifth lens E5 that is a negative lens having a strong concave surface facing the object side; a biconvex lens having a strong convex surface facing the image surface side The sixth lens E6, which is a positive lens of the type, is arranged. The first lens E1 to the third lens E3 constitute the front group GF, and the fourth lens E4 to the sixth lens E6 constitute the rear group GR. Thereafter, the group GR is extended and moved together with the aperture FA to perform focusing.

For example, in a photographing optical system of a camera using a solid-state image sensor such as a CCD image sensor such as a digital still camera, the optical low-pass filter according to the present invention is provided between the final surface of the sixth lens E6 and the image surface FS. For example, an optical low-pass filter P1 having a positive power and, for example, an infrared cut filter P2, which constitute at least one of an infrared cut filter and a cover glass for protecting the light receiving surface of the CCD image sensor, are inserted. Therefore, the MTF characteristic diagrams shown in FIGS. 4 and 6 show the characteristics in a state where two filters P1 and P2 are inserted between the final surface 13 of the imaging lens and the image plane FS. In FIG. 1, surface numbers of the respective optical surfaces are also given.
In this embodiment, the focal length f = 5.9 mm, the F number = 2.4, and the half angle of view ω = 38.6 °.
The characteristics of each optical system are as shown in the following table (Table 1).

  In Table 1, the optical surfaces of the fourth surface, the twelfth surface, the thirteenth surface, and the fourteenth surface with an asterisk “*” attached to the surface number are aspherical surfaces, and the parameters in the above equation (1) for each aspherical surface Is as shown in the following table (Table 2).

Next, examples of the optical low-pass filter according to the present invention will be described.
The embodiment of the optical low-pass filter P1 according to the present invention uses an imaging lens having a focal length f = 5.9 mm, an F number 2.4, and an electronic image sensor having a pixel pitch p = 2.5 × 10 ⁻³ m. An optical low-pass filter P1 made of quartz having a paraxial radius of curvature on the subject side of 47.200 mm and an infinite (plane) paraxial radius of curvature on the image plane is disposed between them. (See FIG. 1).
In this embodiment, the number of the optical low-pass filter P1 is one, and the image separation direction is 46 ° with respect to the horizontal / vertical direction of the image sensor.
Here, the conditions of the optical low-pass filter which is the gist of the present invention will be described.
The optical low-pass filter has positive power, the object-side surface is aspherical, the pixel pitch of the electronic image sensor is p, and the above-mentioned connection at the e-line (546.07 mu) at the spatial frequency u. The MTF on the optical axis of the image optical system is M10 (u), and the MTF at 80% of the maximum image height of the imaging optical system at the e-line at the spatial frequency u is M1 (u). The MTF of the optical low-pass filter on the optical axis at the frequency u is M20 (u), and the optical low-pass filter has a height at which the principal ray having a maximum image height of 80% at the spatial frequency u is incident on the optical low-pass filter. When MTF is M2 (u), the following conditions:
0.9 <(M1 (u) × M2 (u)) / (M10 (u) × M20 (u)) <1.3 (2)
However, u = 1 / 4p
By satisfying the above, it is possible to reduce or eliminate noise such as false color and color moire without reducing the resolution of the entire screen.

When the upper limit of the above equation (2) is exceeded, noise such as false color and color moire tends to appear on a subject with a high spatial frequency, and when the lower limit of the above equation (2) is not reached, the resolving power Decreases, and the reproducibility of a subject with a high spatial frequency deteriorates.
M10 (u) and M1 (u) that are MTFs of the imaging optical system in the above-described embodiment, and M20 (u) and M2 (u) that are MTFs of the optical low-pass filter alone are calculated and calculated.
Conditional expression (2) = 0.948 (T)
Conditional expression (2) = 1.018 (R)
was gotten.
At this time, spatial frequency u = 1 / 4p = 100 / mm
Calculated by
The total MTF passing through the imaging optical system and the optical low-pass filter is 58% on the axis (center of the image plane), and the MTF at the 80% of the maximum image height on the image plane is tangential ( 55% in the T) direction and 59% in the radial (R) direction.
Therefore, as described above, the value of conditional expression (2) is 0.948 in the T direction and 1.017 in the R direction.
In this embodiment of the imaging optical system, when a conventional flat plate is used as the optical low-pass filter, if the thickness is 0.6 mm, the axial MTF is the same as above, but the image height is 80%. The MTF is 46% in the T direction and 50% in the R direction.

Therefore, the value of conditional expression (1) is only 0.793 in the T direction and 0.862 in the R direction.
That is, when viewed in the T direction, it can be seen that the imaging performance is reduced by 20% or more with respect to the center.
FIG. 2 is an MTF characteristic curve diagram of the optical low-pass filter according to the present invention. The horizontal axis represents the spatial frequency (lines / mm), and the vertical axis represents MTF (%). The MTF (response) is almost “0” around the spatial frequency “200 (lines / mm)”.
FIG. 3 is an MTF characteristic curve on the axis of the imaging optical system described in the embodiment. FIG. 4 shows a state in which the optical low-pass filter according to the present invention and the imaging optical system are combined, and shows an MTF characteristic curve diagram when the MTFs of the two are multiplied. At the time of book / mm, it is almost “0”.
FIG. 5 is an MTF characteristic curve diagram at 80% of the maximum image height of the imaging optical system according to the embodiment of the present invention. Here, the solid line is the radial (R) direction and the broken line is the tangential (T) direction MTF (the same applies hereinafter).

FIG. 6 is an MTF characteristic curve diagram at the height of 80% of the maximum image height of the optical low-pass filter at the e line and the imaging optical system according to the embodiment of the present invention.
FIG. 7 is obtained by multiplying the MTF characteristic of the optical low-pass filter when a conventional flat crystal is used as the optical low-pass filter and the MTF of 80% of the image height of the imaging optical system according to the embodiment of the present invention described above. It is a MTF characteristic curve figure.
Of the above-described MTF characteristic curves, for example, referring to FIG. 4, in this case, the MTF of the optical low-pass filter according to the embodiment of the present invention is multiplied by the MTF on the axis of the imaging optical system according to the embodiment. At a spatial frequency of 150 lines / mm in the MTF characteristic curve diagram, the MTF is about 28%.
On the other hand, when comparing the MTF of the conventional low-pass filter as an optical low-pass filter with the MTF of the imaging optical system according to the above embodiment, FIG. It can be seen that the MTF is about 20%, causing a decrease in resolution.
However, in the MTF obtained by multiplying the MTF of 80% of the image height of the MTF imaging optical system of the optical low-pass filter according to the present invention shown in FIG. 6, the MTF at a spatial frequency of 150 lines / mm is about 30%. It was confirmed that the resolving power was comparable to the optical axis center.

Further, when viewed at a spatial frequency of 100 lines / mm, the total MTF that has passed through the imaging optical system and the optical low-pass filter of the above embodiment is 58 on the axis (center of the image plane) as shown in FIG. The MTF at the position where the maximum image height is 80% on the image plane is 55% in the tangential (T) direction and 59% in the radial (R) direction as shown in FIG. is there. The value of conditional expression (2) at this time is 0.948 in the T direction and 1.017 in the R direction, and is within the range of conditional expression (2).
If the optical low-pass filter is a conventional flat plate, the thickness is 0.6 mm (the axial thickness of the optical low-pass filter with positive power in this embodiment is also 0.6 mm), the axis The upper MTF is the same as that in the above embodiment, but the MTF at an image height of 80% is as low as 46% in the T direction and 50% in the R direction, as can be seen from FIG. When the value of conditional expression (2) in this case is calculated, the MTF value is only 0.793 in the T direction and 0.862 in the R direction, and clearly from the range of the conditional expression (2). It is off.

Next, according to an embodiment of the present invention in which an imaging apparatus unit configured using the imaging optical system according to the present invention as shown in the above-described embodiment is employed as a photographing optical system to configure a camera. A digital camera will be described with reference to FIG. FIG. 8 is a perspective view showing the external appearance of the digital camera as seen from the back side, which is the photographer side. Although a camera is described here, a camera in which a camera function is incorporated in a personal digital assistant such as a so-called PDA (personal data assistant) or a mobile phone has recently appeared. Such a portable information terminal device also includes substantially the same function and configuration as a camera, although the appearance is slightly different. The photographing optical system according to the present invention is used as a photographing optical system in such a portable information terminal device. A system may be adopted.
As shown in FIG. 8, the digital camera includes an imaging device unit 101, a release button 102, an optical viewfinder 104, a liquid crystal display unit 105, a liquid crystal monitor 106, a main switch 107, and the like.
The digital camera has an imaging device unit 101 and a light receiving element (not shown) as an area sensor such as a CCD (charge coupled device) imaging element, and an image of an object to be photographed, that is, a subject, An image is formed by the image pickup apparatus unit 101 that is a photographing optical system and is read by a light receiving element. As the imaging device unit 101, the imaging optical system according to the present invention as described in the embodiment is used.

The output of the light receiving element is processed by a signal processing device (not shown) controlled by a central processing unit (CPU) (not shown) and converted into digital image information. Image information digitized by the signal processing device is subjected to predetermined image processing in an image processing device (not shown) that is also controlled by a central processing unit, and then a semiconductor memory (not shown) such as a nonvolatile memory. Not recorded). In this case, the semiconductor memory may be a memory card loaded in a memory card slot or the like, or may be a semiconductor memory built in the camera body. On the liquid crystal monitor 106, an image being photographed can be displayed as an electronic viewfinder, or an image recorded in a semiconductor memory can be displayed. The image recorded in the semiconductor memory can be transmitted to the outside via a communication card or the like loaded in a communication card slot or the like.
The imaging device unit 101 is in a retracted state when the camera is carried and is buried in the body of the camera. When the user operates the main switch 107 to turn on the power, the lens barrel is extended as shown in FIG. Projecting from the body.

In many cases, focusing, that is, focusing is performed by pressing the release button 102 halfway. In this case, focusing in the imaging optical system according to the present invention can be performed by moving the rear group GR, that is, the fourth lens E4 to the sixth lens E6. When the release button 102 is further pushed down to the fully depressed state, photographing is performed, and then the processing as described above is performed.
As described above, the imaging optical system as shown in the embodiment can be used for the camera or the portable information terminal device as described above. Therefore, it is possible to achieve a high-quality and small camera or portable information terminal device. In this case, the portable information terminal device can take a high-quality image and transmit the image to the outside.

It is sectional drawing along the optical axis which shows the structure of the imaging optical system which concerns on the Example of this invention. It is a MTF characteristic curve figure of the optical low pass filter concerning the example of the present invention. It is a MTF characteristic curve figure on the axis | shaft of the imaging optical system which concerns on the Example of this invention. It is a MTF characteristic curve figure which multiplied MTF on the axis | shaft of the imaging optical system which concerns on MTF of the optical low-pass filter which concerns on this invention, and the Example. It is a MTF characteristic curve figure of 80% of image heights of the image formation optical system concerning the example of the present invention. It is a MTF characteristic curve diagram which multiplied MTF of the optical low-pass filter which concerns on the Example of this invention, and MTF of 80% of image height of the imaging optical system which concerns on the Example of this invention. It is a total MTF characteristic curve figure which multiplied MTF of the optical low-pass filter at the time of using the conventional quartz flat plate, and MTF of 80% of image heights of the image formation optical system concerning the example of the present invention. 1 is a perspective view schematically showing a configuration of a main part of a digital camera including an imaging optical system of the present invention.

Explanation of symbols

GF Front group optical system GR Rear group optical system E1 to E6 First lens to sixth lens FA Aperture P1 Optical low pass filter P2 Parallel flat glass (infrared cut filter, dustproof glass, etc.)
101 Imaging Device Unit 102 Release Button 104 Optical Viewfinder 105 Liquid Crystal Display Unit 106 Liquid Crystal Monitor 107 Main Switch

Claims (6)

In the optical low-pass filter disposed between the imaging optical system and the electronic image sensor, the imaging optical system has a positive power, the pixel pitch of the electronic image sensor is p, and the imaging optical at the e line at the spatial frequency u The MTF on the optical axis of the system is M10 (u), the MTF at 80% of the maximum image height of the imaging optical system at the e-line at the spatial frequency u is M1 (u), and the spatial frequency u The MTF of the optical low-pass filter on the optical axis at M is M20 (u), and the MTF of the optical low-pass filter at a height at which a principal ray with a maximum image height of 80% at the spatial frequency u enters the optical low-pass filter. When M2 (u), the following conditions:
0.9 <(M1 (u) × M2 (u)) / (M10 (u) × M20 (u)) <1.3
However, u = 1 / 4p
An optical low-pass filter characterized by satisfying 2. The optical low-pass filter according to claim 1, wherein either one of the incident side and the exit side is a flat surface. The optical low-pass filter according to claim 1 or 2, wherein either one of the incident side and the exit side is formed as an aspherical surface. An imaging device unit comprising the optical low-pass filter according to claim 1. A digital camera comprising the imaging device unit according to claim 4. A portable information terminal device comprising the imaging unit according to claim 4.

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