JP2004032497A - Digital camera - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress deterioration of the image quality of an object image taken with a small aperture, and at the same time, to adjust the depth of field by aperture adjustment regarding a digital camera, in which image data expressing the object image is obtained by forming an image of the object image on a solid-state image pickup element with an image pickup lens and by making the solid-state image pickup element to read the object image. <P>SOLUTION: The digital camera is provided with an aperture 16 to adjust the brightness of the object image, in which the diameter of the opening can be changed, a polarizing element 18a to polarize the object image, a filter element 17 to double the object image in a prescribed direction due to double refraction, and an interlocking mechanism 19 for interlocking a relative angle between the polarizing direction by the polarizing element 18a and the doubling direction in the filter element 17 with the diameter of the opening at the aperture 16. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a digital camera that photographs a subject with a solid-state imaging device.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in a digital camera, a subject image is formed on a solid-state image sensor by a photographing lens, and the subject is photographed by reading the subject image with the solid-state image sensor. In addition, like a silver salt camera, a digital camera is often provided with an aperture whose aperture diameter can be changed in order to adjust the brightness of a subject image.
[0003]
The imaging lens has a spatial frequency characteristic (MTF) of the imaging ability, which generally shows a property that the imaging ability decreases as the spatial frequency increases. In many cases, it has sufficient imaging ability up to a spatial frequency.
[0004]
[Problems to be solved by the invention]
However, when the aperture diameter of the stop is small, the influence of diffraction is large, so that the imaging ability is greatly reduced. As in a high-end model of a digital camera, a CCD (Charge Coupled Device) having a small pixel pitch is a solid-state image sensor In some cases, the deterioration of the image quality of the subject image due to such a decrease in the imaging ability cannot be ignored. That is, in order to prevent moiré and false color, a digital camera having an optical low-pass filter (OLPF) that overlaps subject images by birefringence and cancels a spatial frequency component equal to or higher than the Nyquist frequency is connected by diffraction as described above. When the image capability is lowered, coupled with the effect of the optical low-pass filter, the image quality of the subject image is degraded to a level that cannot be ignored.
[0005]
As a technique for avoiding such a decrease in imaging ability due to diffraction, Japanese Patent Application Laid-Open No. 5-323264 includes a pair of polarizing plates instead of a diaphragm that adjusts the amount of light by changing the aperture diameter. A technique for adjusting the amount of light by adjusting the relative angle of the polarization direction of the plate is disclosed. However, the aperture for changing the aperture diameter is used not only for adjusting the brightness of the subject image but also for the purpose of photographing such as changing the depth of field by adjusting the aperture. The technology of Japanese Patent No. 323264 cannot reflect such shooting intention.
[0006]
In view of the above circumstances, an object of the present invention is to provide a digital camera capable of suppressing image quality degradation of a subject image at a small aperture and adjusting the depth of field by adjusting the aperture.
[0007]
[Means for Solving the Problems]
The digital camera of the present invention that achieves the above object is a digital camera that obtains image data representing a subject image by forming a subject image on a solid-state image sensor with a photographing lens and causing the solid-state image sensor to read the subject image. In
An aperture that adjusts the brightness of the subject image, the aperture diameter can be changed,
A polarizing element for polarizing the subject image;
A filter element that doubles the subject image in a predetermined direction by birefringence;
And an interlocking mechanism for interlocking a relative angle between a polarization direction of the polarizing element and the predetermined direction of the filter element with an aperture diameter of the diaphragm.
[0008]
According to the digital camera of the present invention, since the aperture can be changed, the depth of field can be adjusted by adjusting the aperture. Furthermore, the digital camera according to the present invention can interlock the relative angle between the polarizing element and the filter element, which affects the ability to duplicate the subject image in the filter element, with the aperture diameter by the interlocking mechanism. It is possible to reduce the duplication capability of the filter element at the time of aperture and to suppress the deterioration of the image quality of the subject image at the time of small aperture.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0010]
FIG. 1 is an external view showing an embodiment of a digital camera of the present invention.
[0011]
The digital camera 1 includes a photographic lens 11 corresponding to an example of a photographic lens according to the present invention, and includes a CCD (Charge Coupled Device) corresponding to an example of an individual imaging device according to the present invention. Thus, a subject image is formed on the CCD, and the subject image is read by the CCD, thereby obtaining image data representing the subject image.
[0012]
The digital camera 1 also includes a finder window 12 for determining the shooting range, a flash light emitting device 13 that emits a flash when the subject is dark, and a shutter button 14 that instructs the CCD to start reading the subject image. Yes.
[0013]
FIG. 2 is a diagram illustrating an imaging optical system in a digital camera.
[0014]
FIG. 2 shows two lenses 11a and 11b constituting the above-described photographing lens, the above-described CCD 15 for reading a subject image, an iris diaphragm 16 for adjusting the brightness (light quantity) of the subject image, and a subject by birefringence. An optical low-pass filter 17 that doubles the image and cancels the high frequency region of the spatial frequency, a polarizing member 18 incorporating a polarizing plate 18a that polarizes the subject image, and a gear 19 that links the iris diaphragm 16 and the polarizing member 18 are shown. Has been.
[0015]
As described above, the CCD 15 is an example of a solid-state image sensor according to the present invention.
[0016]
The iris diaphragm 16 is an example of the diaphragm according to the present invention, and is rotated as indicated by an arrow f2 in FIG. 2 so that the aperture diameter is adjusted and the amount of light of the subject image is adjusted. Since such an iris diaphragm 16 is provided, the digital camera according to the present embodiment can reflect a shooting intention of changing the depth of field by adjusting the diaphragm.
[0017]
The optical low-pass filter 17 is an example of a filter element according to the present invention, and specifically, a quartz plate is used.
[0018]
3A and 3B are a front view and a side view illustrating the operation of the optical low-pass filter.
[0019]
Here, the operation will be described by taking as an example the case where the light of the point image 20 is incident on the optical low-pass filter 17.
[0020]
The physical property called birefringence possessed by the optical low-pass filter 17 (quartz plate) is to separate incident light into two polarization components and advance these two polarization components in two traveling directions divided in a predetermined separation direction. It is. Specifically, among the light of the point image 20 incident on the optical low-pass filter 17, the component light polarized in the left-right direction in FIG. 3 goes straight through the optical low-pass filter 17 to become a point image 21. On the other hand, of the light of the point image 20, the light of the component polarized in the vertical direction of FIG. The distance between these two point images 21 and 22 is a distance that exactly cancels the spatial Nyquist frequency of the CCD 15.
[0021]
Since the optical low-pass filter 17 shows such birefringence, in the case of a general subject image, these two points are separated in the separation direction of the two point images 21 and 22 separated as shown in FIG. The double image is shifted by the distance between the images 21 and 22.
[0022]
As shown in FIG. 2, the digital camera of this embodiment includes a polarizing plate 18a corresponding to an example of the polarizing element according to the present invention, and the subject image is polarized by the polarizing plate 18a. In addition, the polarizing member 18 in which the polarizing plate 18a is fitted rotates on the supporting member 18b together with the polarizing plate 18a as indicated by the arrow f2, and when the polarizing member 18 rotates in this way, the polarization direction of the subject image changes. To do. Furthermore, the gear 19 is an example of an interlocking mechanism according to the present invention, and rotates the polarizing member 18 in conjunction with the opening diameter being adjusted by the rotation of the iris diaphragm 16.
[0023]
Thus, in the digital camera of this embodiment, the polarizing member 18 rotates in conjunction with the aperture diameter of the iris diaphragm 16, and the polarization direction of the subject changes. As a result, the image duplication capability in the optical low-pass filter 17 changes as described below.
[0024]
FIG. 4 is a diagram showing a state in which the image duplication capability in the optical low-pass filter changes. In the following description, the angle between the polarization direction by the polarizing plate 18a shown in FIG. 2 and the image separation direction by the optical low-pass filter 17 is expressed as “θ”.
[0025]
FIG. 4A shows the separation ability in the case of θ = 45 °, and shows that the two point images 21 and 22 shown in FIG. 3 become point images having the same brightness. The electric field of the light of each of the point images 21 and 22 is an electric field having an intensity of “1 / √2” with the electric field in the original point image 20 being “1”.
[0026]
Similarly, FIG. 4B, FIG. 4C, and FIG. 4D show the separation capability when θ = 30 °, θ = 15 °, and θ = 0 °, respectively. In the case of θ = 30 °, the electric fields of the two point images 21 and 22 shown in FIG. 3 are √3 / 2 times and 0.5 times the electric fields of the original point image 20, respectively. When θ = 15 °, the electric fields of the two point images 21 and 22 are cos 15 ° and sin 15 °, respectively, with the electric field in the original point image 20 being “1”. When θ = 0 °, the point images are not separated, and only the point image 21 made of light traveling straight through the optical low-pass filter is obtained.
[0027]
Thus, the separation ability that changes according to θ can be expressed as a change in spatial frequency characteristics (MTF).
[0028]
FIG. 5 is a graph showing the spatial frequency characteristics (MTF) of the optical low-pass filter.
[0029]
This graph 31 shows the spatial frequency characteristics when θ = 45 °, θ = 30 °, θ = 15 °, and θ = 0 °, and the horizontal axis of the graph is normalized by the sampling frequency. The vertical axis represents the spatial frequency characteristic.
[0030]
As shown in this graph 31, when θ = 45 °, the spatial frequency characteristic approaches the value “0” as the spatial frequency approaches the value “0.5” corresponding to the Nyquist frequency. When θ = 0 °, the spatial frequency characteristic is a value “1” regardless of the spatial frequency. Thus, the spatial frequency characteristic of the optical low-pass filter changes according to θ.
[0031]
On the other hand, the spatial frequency characteristics in the combination of the photographing lens 11 and the iris diaphragm 16 described above change according to the aperture diameter of the iris diaphragm 16.
[0032]
FIG. 6 is a graph showing the spatial frequency characteristic (MTF) in the combination of the photographing lens and the iris diaphragm. The horizontal axis of the graph represents the spatial frequency normalized by the sampling frequency, and the vertical axis represents the spatial frequency characteristic. Represents.
[0033]
Here, an F number is used instead of the aperture diameter of the iris diaphragm, and the larger the F number, the smaller the aperture diameter.
[0034]
The graph 32 in FIG. 6 shows the spatial frequency characteristics corresponding to the F numbers of F5.6, F8, F11, and F16, and the imaging ability increases as the F number increases and the iris diaphragm aperture diameter decreases. Has been shown to decrease. When the F number is F16, the spatial frequency characteristic reaches approximately “0” in the vicinity of the value “0.5” corresponding to the Nyquist frequency. It is also shown that will occur. Therefore, if an optical low-pass filter is simply inserted after the combination of the taking lens and the iris diaphragm, image quality degradation that cannot be ignored occurs in the subject image.
[0035]
Therefore, in the present embodiment, the gear 19 shown in FIG. 2 links the aperture diameter of the iris diaphragm 16 and the polarization direction of the polarizing plate 18a, thereby suppressing image quality deterioration at the time of small diaphragm as described below. It has been.
[0036]
FIG. 7 is a diagram illustrating the interlocking relationship between the polarization direction and the aperture diameter.
[0037]
The horizontal axis of the graph 33 indicates the F number corresponding to the aperture diameter of the diaphragm, and the vertical axis indicates the angle θ described above. A broken line 33a of the graph 33 represents the relationship between the polarization direction and the aperture diameter, and indicates that as the aperture diameter is smaller, θ approaches 0 ° and the separation capability of the optical low-pass filter decreases. .
[0038]
FIG. 8 is a graph showing the overall spatial frequency characteristics (MTF) of the optical low-pass filter, the imaging lens, and the iris diaphragm when the polarization direction and the aperture diameter are linked in the relationship shown in FIG. The horizontal axis of represents the spatial frequency normalized by the sampling frequency, and the vertical axis represents the spatial frequency characteristic.
[0039]
In the graph 34 of FIG. 8, when the F number is “F5.6” and θ = 45 °, the F number is “F8” and θ = 30 °, and the F number is “F11”. ”And θ = 15 °, and when the F number is“ F16 ”and θ = 0 °, the respective spatial frequency characteristics are shown. The spatial frequency characteristics in each case are very similar to each other, and even when the F number is “F16”, the imaging ability is maintained up to a value “0.5” corresponding to the Nyquist frequency. It is shown.
[0040]
Thus, in the digital camera of the present embodiment, image quality deterioration at the time of small aperture is suppressed.
[0041]
In the above description, a photographing lens composed of two lenses is shown as an example of the photographing lens according to the present invention. However, the photographing lens according to the present invention may be a single lens, or It may be composed of three or more lenses. The photographing lens according to the present invention may be a lens having a fixed focal length, a zoom lens, an autofocus lens, or the like.
[0042]
In the above description, a CCD is shown as an example of the solid-state imaging device according to the present invention. However, the solid-state imaging device according to the present invention may be a CMOS image sensor.
[0043]
In the above description, an optical low-pass filter made of a quartz plate is shown as an example of the filter element according to the present invention. However, the filter element according to the present invention is made of another optical material exhibiting birefringence. May be.
[0044]
In the above description, a polarizing plate is shown as an example of the polarizing element according to the present invention. However, the polarizing element according to the present invention may be a polarizing prism or the like.
[0045]
In the above description, a gear is shown as an example of the interlocking mechanism according to the present invention, but the interlocking mechanism according to the present invention may be a cam mechanism, a crank mechanism, or the like. Further, although the gear described above rotates the polarizing plate in conjunction with the aperture diameter of the iris diaphragm, the interlocking mechanism according to the present invention may rotate the CCD and the optical low-pass filter. The polarization direction may be changed between the polarizing plate and the optical low-pass filter.
[0046]
【The invention's effect】
As described above, according to the digital camera of the present invention, it is possible to suppress the deterioration of the image quality of the subject image when the aperture is small and to adjust the depth of field by adjusting the aperture.
[Brief description of the drawings]
FIG. 1 is an external view showing an embodiment of a digital camera of the present invention.
FIG. 2 is a diagram illustrating an imaging optical system in a digital camera.
FIGS. 3A and 3B are a front view and a side view for explaining the operation of the optical low-pass filter. FIGS.
FIG. 4 is a diagram illustrating a state in which the image duplication capability in the optical low-pass filter changes.
FIG. 5 is a graph showing spatial frequency characteristics (MTF) of an optical low-pass filter.
FIG. 6 is a graph showing spatial frequency characteristics (MTF) in a combination of a photographing lens and an iris diaphragm.
FIG. 7 is a diagram illustrating the interlocking between the polarization direction and the aperture diameter.
8 is a graph showing a total spatial frequency characteristic (MTF) of an optical low-pass filter, a photographing lens, and an iris diaphragm when the polarization direction and the aperture diameter are linked in the relationship shown in FIG.
[Explanation of symbols]
1 Digital Camera 11 Shooting Lenses 11a and 11b Lens 12 Finder Window 13 Flash Light Emitting Device 14 Shutter Button 15 CCD
16 Iris diaphragm 17 Optical low-pass filter 18 Polarizing member 18a Polarizing plate 19 Gear

Claims (1)

In a digital camera that obtains image data representing a subject image by forming a subject image on a solid-state image sensor with a photographing lens and causing the solid-state image sensor to read the subject image.
A diaphragm that adjusts the brightness of the subject image, the aperture diameter of which can be changed,
A polarizing element for polarizing the subject image;
A filter element that doubles the subject image in a predetermined direction by birefringence;
A digital camera comprising an interlocking mechanism for interlocking a relative angle between a polarization direction of the polarizing element and the predetermined direction of the filter element with an aperture diameter of the diaphragm.

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