Patent Publication Number: US-8118500-B2

Title: Vibrating device and image equipment having the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Applications No. 2009-142635, filed Jun. 15, 2009; and No. 2009-261154, filed Nov. 16, 2009, the entire contents of both of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to image equipment having image forming elements such as an image sensor element or a display element, and also to a vibrating device designed to vibrate the dust-screening member that is arranged at the front of each image forming element of such an image equipment. 
     2. Description of the Related Art 
     As image equipment having image forming elements, there is known an image acquisition apparatus that has an image sensor element configured to produce a video signal corresponding to the light applied to its photoelectric conversion surface. Also known is an image projector that has a display element, such as liquid crystal element, which displays an image on a screen. In recent years, image equipment having such image forming elements have been remarkably improved in terms of image quality. If dust adheres surface of the image forming element such as the image sensor element or display element or to the surface of the transparent member (optical element) that is positioned in front of the image forming element, the image produced will have shadows of the dust particles. This makes a great problem. 
     For example, digital cameras of called “lens-exchangeable type” have been put to practical use, each comprising a camera body and a photographic optical system removably attached to the camera body. The lens-exchangeable digital camera is so designed that the user can use various kinds of photographic optical systems, by removing the photographic optical system from the camera body and then attaching any other desirable photographic optical system to the camera body. When the photographic optical system is removed from the camera body, the dust floating in the environment of the camera flows into the camera body, possibly adhering to the surface of the image sensor element or to the surface of the transparent member (optical element), such as a lens, cover glass or the like, that is positioned in front of the image sensor element. The camera body contains various mechanisms, such as a shutter and a diaphragm mechanism. As these mechanisms operate, they produce dust, which may adhere to the surface of the image sensor element as well. 
     Projectors have been put to practical use, too, each configured to enlarge an image displayed by a display element (e.g., CRT or liquid crystal element) and project the image onto a screen so that the enlarged image may be viewed. In such a projector, too, dust may adhere to the surface of the display element or to the surface of the transparent member (optical element), such as a lens, cover glass or the like, that is positioned in front of the display element, and enlarged shadows of the dust particles may inevitably be projected to the screen. 
     Various types of mechanisms that remove dust from the surface of the image forming element or the transparent member (optical element) that is positioned in front of the image sensor element, provided in such image equipment have been developed. 
     In an electronic image acquisition apparatus disclosed in, for example. US 2004/0169761 A1, a ring-shaped piezoelectric element (vibrating member) is secured to the circumferential edge of a glass plat shaped like a disc (dust-screening member). When a voltage of a prescribed frequency is applied to the piezoelectric element, the glass plat shaped like a disc undergoes a standing-wave, bending vibration having nodes at the concentric circles around the center of the glass plat shaped like a disc. This vibration removes the dust from the glass disc. The vibration produced by the voltage of the prescribed frequency is a standing wave having nodes at the concentric circles around the center of the disc. The dust-screening member is held by dust-screening member holding members that contact the dust-screening member at the nodes of standing waves that form concentric circles. The dust-screening member holding members maintain dust screening condition between the dust-screening member and the image sensor element. 
     Jpn. Pat. Appln. KOKAI Publication No. 2007-267189 discloses a rectangular dust-screening member and piezoelectric elements secured to the opposite sides of the dust-screening member, respectively. The piezoelectric elements produce vibration at a predetermined frequency, resonating the dust-screening member. Vibration is thereby achieved in such mode that nodes extend parallel to the sides of the dust-screening member. In order to remove dust from the nodes of vibration, the dust-screening member is resonated at different frequencies, accomplishing a plurality of standing-wave vibrational modes, thereby changing the positions of nodes. Any one of the vibrational modes achieves bending vibration having nodes extending parallel to the sides of the dust-screening member. 
     Further, Jpn. Pat. Appln. KOKAI Publication No. 2009-38741 discloses a configuration in which vibrating plates (piezoelectric elements) are provided at the opposite sides of a low-pass filter (dust-screening member) that is shaped like a rectangular plate. The vibrating plate provided at one side is vibrated. The voltage generated as the vibrating plate provided at the other side is detected. From the voltage detected, it is determined whether the vibration application function is a normal one or not. If the vibration application function is a normal one, both vibrating plates are vibrated to remove dust from the low-pass filter. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a vibrating device comprising: 
     a dust-screening member shaped like a plate as a whole, having front and back surfaces and having a light-transmitting region for transmitting light between the front surface and the back surface; 
     a support member configured to support the dust-screening member, thereby to render the back surface of the dust-screening member airtight; 
     a first vibrating member shaped almost like a rectangle, arranged at a first outer circumferential part of the dust-screening member and composed of a first vibration application part configured to expand and contract when supplied with an electrical signal for expanding and contracting and a first non-vibration application part configured not to supplied with the electrical signal for expanding and contracting; 
     a second vibrating member shaped almost like a rectangle, arranged at a second outer circumferential part of the dust-screening member, which opposes the first outer circumferential part of the dust-screening member, and composed of a second vibration application part configured to expand and contract when supplied with the electrical signal for expanding and contracting and a second non-vibration application part configured not to supplied with the electrical signal for expanding and contracting; 
     a connection member composed of a first circuit, a second circuit and a connection part, the first circuit configured to input an electrical signal to the first vibration application part, the second circuit configured to output, from the second vibration application part, an electrical signal generated in the second vibration application part based on a vibration of the first vibration application part when an electrical signal is input to the first circuit, and the connection part configured to connect the first and second circuits electrically; and 
     a drive unit configured to drive the first and second vibrating members while the first and second circuits remain connected by the connection part, 
     wherein the first and second vibrating members are shaped symmetrically to one another in weight balance with respect to a virtual symmetry axis at the same distance from the first and second vibrating members and also to an virtual centerline connecting gravity centers of the first and second vibrating members, and 
     the second vibrating member which receives the vibration has a vibrational axis and is shaped asymmetrically in vibrational amplitude with respect to the vibrational axis. 
     According to a second aspect of the present invention, there is provided an image equipment comprising: 
     an image forming element having an image surface on which an optical image is formed; 
     a dust-screening member shaped like a plate as a whole, having front and back surfaces and having a light-transmitting region for transmitting light between the front surface and the back surface; 
     a support member configured to support the dust-screening member, to space the light-transmitting region of the dust-screening member, apart from the image surface of the image forming element by a predetermined distance, and to render the back surface of the dust-screening member airtight; 
     a first vibrating member shaped almost like a rectangle, arranged at a first outer circumferential part of the dust-screening member and composed of a first vibration application part configured to expand and contract when supplied with an electrical signal for expanding and contracting and a first non-vibration application part configured not to supplied with the electrical signal for expanding and contracting; 
     a second vibrating member shaped almost like a rectangle, arranged at a second outer circumferential part of the dust-screening member, which opposes the first outer circumferential part of the dust-screening member, and composed of a second vibration application part configured to expand and contract when supplied with the electrical signal for expanding and contracting and a second non-vibration application part configured not to supplied with the electrical signal for expanding and contracting; 
     a connection member composed of a first circuit, a second circuit and a connection part, the first circuit configured to input an electrical signal to the first vibration application part, the second circuit configured to output, from the second vibration application part, an electrical signal generated in the second vibration application part based on a vibration of the first vibration application part when an electrical signal is input to the first circuit, and the connection part configured to connect the first and second circuits electrically; and 
     a drive unit configured to drive the first and second vibrating members while the first and second circuits remain connected by the connection part, 
     wherein the first and second vibrating members are shaped symmetrically to one another in weight balance with respect to a virtual symmetry axis at the same distance from the first and second vibrating members and also to an virtual centerline connecting gravity centers of the first and second vibrating members, and 
     the second vibrating member which receives the vibration has a vibrational axis and is shaped asymmetrically in vibrational amplitude with respect to the vibrational axis. 
     Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  is a block diagram schematically showing an exemplary system configuration, mainly electrical, of a lens-exchangeable, single-lens reflex electronic camera (digital camera) that is a first embodiment of the image equipment according to this invention; 
         FIG. 2A  is a vertical side view of an image sensor element unit of the digital camera, which includes a dust removal mechanism (or a sectional view taken along line A-A shown in  FIG. 2B ); 
         FIG. 2B  is a front view of the dust removal mechanism, as viewed from the lens side; 
         FIG. 3  is an exploded perspective view showing a major component (vibrator) of the dust removal mechanism; 
         FIG. 4  is a front view showing the structure of the flexible printed board connected to the piezoelectric elements of the vibrator; 
         FIG. 5A  is a front view of a dust filter, explaining how the dust filter is vibrated; 
         FIG. 5B  is a sectional view of the dust filter, taken along line B-B shown in  FIG. 5A ; 
         FIG. 5C  is a sectional view of the dust filter, taken along line C-C shown in  FIG. 5A ; 
         FIG. 5D  is a sectional view of the dust filter, taken along line D-D shown in  FIG. 5A ; 
         FIG. 6  is a diagram explaining the length of the long sides and that of the short sides of the dust filter; 
         FIG. 7A  is a diagram explaining the concept of vibrating the dust filter; 
         FIG. 7B  is a front view of the dust filter vibrated in such a mode that node areas, where vibration hardly occurs, form a lattice pattern; 
         FIG. 8  is a diagram explaining how the dust filter is vibrated in another mode; 
         FIG. 9  is a diagram explaining how the dust filter is vibrated in still another mode; 
         FIG. 10  is a diagram showing the relation between the aspect ratio of the dust filter shown in  FIG. 5A  and the vibration speed ratio of the center part of the dust filter; 
         FIG. 11  is a diagram showing the relation that the voltage detected by such a vibration detector as shown in  FIG. 12  has with the vibration speed ratio of the dust filter that vibrates in the vibrational mode of  FIG. 9  when the dust filter control circuit applies a signal voltage to the two piezoelectric elements; 
         FIG. 12  is a diagram illustrating the concept of a vibration detector; 
         FIG. 13  is a diagram showing another configuration the dust filter and a flexible printed board may have; 
         FIG. 14  is a diagram showing still another configuration the dust filter and the flexible printed board may have; 
         FIG. 15  is a conceptual diagram of the dust filter, explaining the standing wave that is produced in the dust filter; 
         FIG. 16A  is a diagram showing an electric equivalent circuit that drives the vibrator at a frequency near the resonance frequency; 
         FIG. 16B  is a diagram showing an electric equivalent circuit that drives the vibrator at the resonance frequency; 
         FIG. 17  is a circuit diagram schematically showing the configuration of a dust filter control circuit; 
         FIG. 18  is a timing chart showing the signals output from the components of the dust filter control circuit; 
         FIG. 19A  is the first part of a flowchart showing an exemplary camera sequence (main routine) performed by the microcomputer for controlling the digital camera body according to the first embodiment; 
         FIG. 19B  is the second part of the flowchart showing the exemplary camera sequence (main routine); 
         FIG. 20  is a flowchart showing the operating sequence of “silent vibration” that is a subroutine shown in  FIG. 19A ; 
         FIG. 21  is a flowchart showing the operation sequence of the “display process” performed at the same time Step S 201  of “silent vibration,” i.e. subroutine ( FIG. 20 ), is performed; 
         FIG. 22  is a flowchart showing the operating sequence of the “display process” performed at the same time Step S 203  of “silent vibration,” i.e., or subroutine ( FIG. 20 ), is performed; 
         FIG. 23  is a flowchart showing the operating sequence of the “display process” performed at the same time Step S 205  of “silent vibration,” i.e., subroutine ( FIG. 20 ), is performed; 
         FIG. 24  is a diagram showing the form of a resonance-frequency wave continuously supplied to vibrating members during silent vibration; 
         FIG. 25  is a flowchart showing the operating sequence of “silent vibration,” i.e., subroutine in the operating sequence of the digital camera that is a second embodiment of the image equipment according to the present invention; and 
         FIG. 26  is a diagram showing the configuration of the vibration detector used in a third embodiment of the image equipment according to this invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Best modes of practicing this invention will be described with reference to the accompanying drawings. 
     First Embodiment 
     An image equipment according to this invention, which will be exemplified below in detail, has a dust removal mechanism for the image sensor element unit that performs photoelectric conversion to produce an image signal. Here, a technique of improving the dust removal function of, for example, an electronic camera (hereinafter called “camera” will be explained. The first embodiment will be described, particularly in connection with a lens-exchangeable, single-lens reflex electronic camera (digital camera), with reference to  FIGS. 1 to 2B . 
     First, the system configuration of a digital camera  10  according to this embodiment will be described with reference to  FIG. 1 . The digital camera  10  has a system configuration that comprises body unit  100  used as camera body, and a lens unit  200  used as an exchange lens, i.e., one of accessory devices. 
     The lens unit  200  can be attached to and detached from the body unit  100  via a lens mount (not shown) provided on the front of the body unit  100 . The control of the lens unit  200  is performed by the lens-control microcomputer (hereinafter called “Lucom”)  201  provided in the lens unit  200 . The control of the body unit  100  is performed by the body-control microcomputer (hereinafter called “Bucom”  101  provided in the body unit  100 . By a communication connector  102 , the Lucom  210  and the Bucom  101  are electrically connected to each other, communicating with each other, while the lens unit  200  remains attached to the body unit  100 . The Lucom  201  is configured to cooperate, as subordinate unit, with the Bucom  101 . 
     The lens unit  200  further has a photographic lens  202 , a diaphragm  203 , a lens drive mechanism  204 , and a diaphragm drive mechanism  205 . The photographic lens  202  is driven by a DO motor (not shown) that is provided in the lens drive mechanism  204 . The diaphragm  203  is driven by a stepping motor (not shown) that is provided in the diaphragm drive mechanism  205 . The Lucom  201  controls these motors in accordance with the instructions made by the Bucom  101 . 
     In the body unit  100 , a penta-prism  103 , a screen  104 , a quick return mirror  105 , an ocular lens  106 , a sub-mirror  107 , a shutter  108 , an AF sensor unit  109 , an AF sensor drive circuit  110 , a mirror drive mechanism  111 , a shutter cocking mechanism  112 , a shutter control circuit  113 , a photometry sensor  114 , and a photometry circuit  115  are arranged as shown in  FIG. 1 . The penta-prism  103 , the screen  104 , the quick return mirror  105 , the ocular lens  106 , and the sub-mirror  107  are single-lens reflex components that constitute an optical system. The shutter  108  is a focal plane shutter arranged on the photographic optical axis. The AF sensor unit  109  receives a light beam reflected by the sub-mirror  107  and detects the degree of defocusing. 
     The AF sensor drive circuit  110  controls and drives the AF sensor unit  109 . The mirror drive mechanism  111  controls and drives the quick return mirror  105 . The shutter cocking mechanism  112  biases the spring (riot shown) that drives the front curtain and rear curtain of the shutter  108 . The shutter control circuit  113  controls the motions of the front curtain and rear curtain of the shutter  108 . The photometry sensor  114  detects the light beam coming from the penta-prism  103 . The photometry circuit  115  performs a photometry process on the basis of the light beam detected by the photometry sensor  114 . 
     In the body unit  100 , an image acquisition unit  116  is further provided to perform photoelectric conversion on the image of an object, which has passed through the above-mentioned optical system. The image acquisition unit  116  is a unit composed of a CCD  117  that is an image sensor element as an image forming element, an optical low-pass filter (LPF)  118  that is arranged in front of the CCD  117 , and a dust filter  119  that is a dust-screening member. Thus, in this embodiment, a transparent glass plate (optical element) that has, at least at its transparent part, a refractive index different from that of air is used as the dust filter  119 . Nonetheless, the dust filter  119  is not limited to a glass plate (optical element). Any other member (optical element) that exists in the optical path and can transmit light may be used instead. For example, the transparent glass plate (optical element) may be replaced by an optical low-pass filter (LPF), an infrared-beam filter, a deflection filter, a half mirror, or the like. In this case, the frequency and drive time pertaining to vibration and the position of a vibration member (later described) are set in accordance with the member (optical element). The CCD  117  is used as an image sensor element. Nonetheless, any other image sensor element, such as CMOS or the like, may be used instead. 
     As mentioned above, the dust filter  119  can be selected from various devices including an optical low-pass filter (LPF). However, this embodiment will be described on the assumption that the dust filter is a glass plate (optical element). 
     To the circumferential edge of the dust filter  119 , two piezoelectric elements  120   a  and  120   b  are attached, opposing each other across the center of the dust filter  119 . The piezoelectric elements  120   a  and  120   b  constitute a vibrating member and are almost rectangular. (The words “almost rectangular” mean a rectangular shape or a shape similar thereto.) The piezoelectric elements  120   a  and  120   b  have two electrodes each. A dust filter control circuit  121 , which is a drive unit, drives the piezoelectric elements  120   a  and  120   b  at the frequency determined by the size and material of the dust filter  119 . As the piezoelectric elements  120   a  and  120   b  vibrate, the dust filter  119  undergoes specific vibration. Dust can thereby be removed from the surface of the dust filter  119 . To the image acquisition unit  116 , an anti-vibration unit is attached to compensate for the motion the hand holding the digital camera  10 . 
     The digital camera  10  according to this embodiment further has a CCD interface circuit  122 , a liquid crystal monitor  123 , an SDRAM  124 , a Flash ROM  125 , and an image process controller  126 , thereby to perform not only an electronic image acquisition function, but also an electronic record/display function. The CCD interface circuit  122  is connected to the CCD  117 . The SDRAM  124  and the Flash ROM  125  function as storage areas. The image process controller  126  uses the SDRAM  124  and the Flash ROM  125 , to process image data. A recording medium  127  is removably connected by a communication connector (not shown) to the body unit  100  and can therefore communicate with the body unit  100 . The recording medium  127  is an external recording medium, such as one of various memory cards or an external HOD, and records the image data acquired by photography. As another storage area, a nonvolatile memory  128 , e.g., EEPROM, is provided and can be accessed from the Bucom  101 . The nonvolatile memory  128  stores prescribed control parameters that are necessary for the camera control. 
     To the Bucom  101 , there are connected an operation display LCD  129 , an operation display LED  130 , a camera operation switch  131 , and a flash control circuit  132 . The operation display LCD  129  and the operation display LED  130  display the operation state of the digital camera  10 , informing the user of this operation state. The operation display LED  129  or the operation display LED  130  has, for example, a display unit configured to display the vibration state of the dust filter  119  as long as the dust filter control circuit  121  keeps operating. The camera operation switch  131  is a group of switches including, for example, a release switch, a mode changing switch, a power switch, which are necessary for the user to operate the digital camera  10 . The flash control circuit  132  drives a flash tube  133 . 
     In the body unit  100 , a battery  134  used as power supply and a power-supply circuit  135  are further provided. The power-supply circuit  135  converts the voltage of the battery  134  to a voltage required in each circuit unit of the digital camera  10  and supplies the converted voltage to the each circuit unit. In the body unit  100 , too, a voltage detecting circuit (not shown) is provided, which detects a voltage change at the time when a current is supplied from an external power supply though a jack (not shown). 
     The components of the digital camera  10  configured as described above operate as will be explained below. The image process controller  126  controls the CCD interface circuit  122  in accordance with the instructions coming from the Bucom  101 , whereby image data is acquired from the CCD  117 . The image data is converted to a video signal by the image process controller  126 . The image represented by the video signal is displayed by the liquid crystal monitor  123 . Viewing the image displayed on the liquid crystal monitor  123 , the user can confirm the image photographed. 
     The SDRAM  124  is a memory for temporarily store the image data and is used as a work area in the process of converting the image data. The image data is held in the recording medium  127 , for example, after it has been converted to JPEG data. 
     The mirror drive mechanism  111  is a mechanism that drives the quick return mirror  105  between an up position and a down position. While the quick return mirror  105  stays at the down position, the light beam coming from the photographic lens  202  is split into two beams. One beam is guide to the AF sensor unit  109 , and the other beam is guided to the penta-prism  103 . The output from the AF sensor provided in the AF sensor unit  109  is transmitted via the AF sensor drive circuit  110  to the Bucom  101 . The Bucom  101  performs the distance measuring of the known type. In the meantime, a part of the light beam, which has passed through the penta-prism  103 , is guided to the photometry sensor  114  that is connected to the photometry circuit  115 . The photometry circuit  115  performs photometry of the known type, on the basis of the amount of light detected by the photometry sensor  114 . 
     The image acquisition unit  116  that includes the CCD  117  will be described with reference to  FIGS. 2A and 2B . Note that the hatched parts shown in  FIG. 2B  show the shapes of members clearly, not to illustrating the sections thereof. 
     As described above, the image acquisition unit  116  has the CCD  117 , the optical LPF  118 , the dust filter  119 , and the piezoelectric elements  120   a  and  120   b . The CCD  117  is an image sensor element that produces an image signal that corresponds to the light applied to its photoelectric conversion surface through the photographic optical system. The optical LPF  118  is arranged at the photoelectric conversion surface of the CCD  117  and removes high-frequency components from the light beam coming from the object through the photographic optical system. The dust filter  119  is a dust-screening member arranged in front of the optical LPF  118  and facing the optical LPF  118 , spaced apart therefrom by a predetermined distance. The piezoelectric elements  120   a  and  120   b  are arranged on the circumferential edge of the dust filter  119  and are vibrating members for applying specific vibration to the dust filter  119 . 
     The CCD chip  136  of the CCD  117  is mounted directly on a flexible substrate  137  that is arranged on a fixed plate  138 . From the ends of the flexible substrate  137 , connection parts  139   a  and  139   b  extend. Connectors  140   a  and  140   b  are provided on a main circuit board  141 . The connection parts  139   a  and  139   b  are connected to the connectors  140   a  and  140   b , whereby the flexible substrate  137  is connected to the main circuit board  141 . The CCD  117  has a protection glass plate  142 . The protection glass plate  142  is secured to the flexible substrate  137 , with a spacer  143  interposed between it and the flexible substrate  137 . 
     Between the CCD  117  and the optical LPF  118 , a filter holding member  144  made of elastic material is arranged on the front circumferential edge of the CCD  117 , at a position where it does not cover the effective area of the photoelectric conversion surface of the CCD  117 . The filter holding member  144  abuts on the optical LPF  118 , at a part close to the rear circumferential edge of the optical LPF  118 . The filter holding member  144  functions as a sealing member that maintains the junction between the CCD  117  and the optical LPF  118  almost airtight. A holder  145  is provided, covering seals the CCD  117  and the optical LPF  118  in airtight fashion. The holder  145  has a rectangular opening  146  in a part that is substantially central around the photographic optical axis. The inner circumferential edge of the opening  146 , which faces the dust filter  119 , has a stepped part  147  having an L-shaped cross section. Into the opening  146 , the optical LPF  118  and the CCD  117  are fitted from the back. In this case, the front circumferential edge of the optical LPF  118  contacts the stepped part  147  in a virtually airtight fashion. Thus, the optical LPF  118  is held by the stepped part  147  at a specific position in the direction of the photographic optical axis. The optical LPF  118  is therefore prevented from slipping forwards from the holder  145 . The level of airtight sealing between the CCD  117  and the optical LPF  118  is sufficient to prevent dust from entering to form an image having shadows of dust particles. In other words, the sealing level need not be so high as to completely prevent the in-flow of gasses. 
     On the front circumferential edge of the holder  145 , a dust-filter holding unit  148  is provided, covering the entire front circumferential edge of the holder  145 . The dust-filter holding unit  148  is formed, surrounding the stepped part  147  and projecting forwards from the stepped part  147 , in order to hold the dust filter  119  in front of the LPF  118  and to space the filter  119  from the stepped part  147  by a predetermined distance. The opening of the dust-filter holding unit  148  serves as focusing-beam passing area  149 . The dust filter  119  is shaped like a polygonal plate as a whole (a square plate, in this embodiment). The dust filter  119  is supported on a seal  150  (a support member), pushed onto the seal  150  by a pushing member  151  which is constituted by an elastic body such as a leaf spring and has one end fastened with screws  152  to the dust-filter holding unit  148 . More specifically, a cushion member  153  made of vibration attenuating material, such as rubber or resin, and adhered to the pushing member  151 , is interposed between the pushing member  151  and the dust filter  119 . On the other hand, at the back of the dust filter  119 , the seal  150  having an ring-shaped lip part  150   a  surrounding the center of the dust filter  119  is interposed between the circumferential part of the dust filter  119  and the dust-filter holding unit  148 . The pushing member  151  exerts a pushing force, which bends the lip part  150   a . The lip part  150   a  pushes the dust filter  119 . As a result, the space including the opening  146  is sealed airtight and the dust filter  119  is supported. 
     The dust filter  119  is positioned with respect to the Y-direction in the plane that is perpendicular to the optical axis, as that part of the pushing member  151  which is bent in the Z-direction, receive a force through a positioning member  154 . Or the other hand, the dust filter  119  is positioned with respect to the X-direction in the plane that is perpendicular to the optical axis, as a support part  155  provided on the holder  145  receive a force through the positioning member  154 , as is illustrated in  FIG. 2B . The positioning member  154  is made of vibration-attenuating material such as rubber or resin, too, not to impede the vibration of the dust filter  119 . The main body  150   b  of the seal  150  is pressed onto the outer circumferential edge of a ring-shaped projection  145   a  fitted at the rim of the opening  146  of the holder  145 , and is thereby set in place. 
     When the dust filter  119  receives gravitational acceleration G and an external force (such as an inertial force) as the camera is moved, the external force is applied to the pushing member  151  or the seal  150 . The pushing member  151  is a plate made of phosphor bronze or stainless steel, either for use as material of springs, and has high flexural rigidity. By contrast, the seal  150  is made of rubber and has small flexural rigidity. Thus, the seal  150  is deformed due to the external force. 
     Cushion members  156  (second support members) made of vibration attenuating material, such as rubber or soft resin, are provided on that surface of the dust-filter holding unit  148 , which faces the back of the dust filter  119 . At least two cushion members  156  (four cushion members in this embodiment) are positioned, almost symmetric with respect to the optical axis, and, are spaced by distance ΔZ from the dust filter  119  in the direction of the optical axis. When the seal  150  is deformed by the distance ΔZ, the dust filter  119  contacts the cushion member  156 . As a result, the external force tends to compress the cushion member  156  (at four parts). However, the cushion member  156  is scarcely deformed despite the external force, because its compression rigidity is higher than the flexural rigidity of the seal  150 . Therefore, the seal  150  is deformed, but very little. Note that the cushion member  156  is arranged, supporting the dust filter  119  at the nodes where the dust filter  119  scarcely undergoes vibration even if the dust filter  119  is pushed by the cushion member  156 . Since the cushion member  156  is arranged so, the vibration of the dust filter  119  is not much impeded. This helps to provide a dust-screening mechanism having that generate vibration at large amplitude and hence can remove dust at high efficiency. Moreover, the deformation of the seal  150 , caused by the external force, is as small as ΔZ (for example, 0.1 to 0.2 mm). Hence, an excessively large force will not applied to the seal  150  to twist the seal  150 , failing to maintain the airtight state, or the seal  150  will not contact the dust filter  119  with an excessive pressure when the external force is released from it. 
     The seal  150  may of course have its main body  150   b  secured to the holder  145  by means of, for example, adhesion. If the seal  150  is made of soft material such as rubber, it may be secured to the dust filter  119 . In this case, the seal  150  only needs to apply a pressing force large enough to support the vibrator configured by the dust filter  119  and the piezoelectric elements  120   a  and  120   b . Assume that the vibrator has a mass of several grams (e.g., less than 10 g (=0.01 kg). Then, in order to support the vibrator even while the digital camera  10  remains directed in horizontal direction, upward in vertical direction or downward in vertical direction, the vibrator must withstand at least 2 G, where G is the gravitational acceleration (=9.81 m/s 2 ). It is sufficient for the vibrator to withstand several times to ten times 2 G. In this case, the pushing force the seal  150  should exert is as small as 0.01×10×9.81≈1 N (Newton). If the pushing force is so small, the seal  150  would not suppress the vibration of the dust filter  119 . 
     Moreover, as shown in  FIG. 2B , the lip part  150   a  of the seal  150  is shaped like a ring, arced at the four corners and having no inflection points. So shaped, the lip part  150   a  is not locally deformed when it receives an external force. 
     The image acquisition unit  116  is thus configured as an airtight structure that has the holder  145  having a desired size and holding the CCD  117 . The level of airtight sealing between the dust filter  119  and the dust-filter holding unit  148  is sufficient to prevent dust from entering to form an image having shadows of dust particles. The sealing level need not be so high as to completely prevent the in-flow of gasses. 
     As shown in  FIG. 3  and  FIG. 4 , the piezoelectric element  120   a  has two signal electrodes  157   a  and  158   a , and the piezoelectric element  120   b  has two signal electrodes  157   b  and  158   b.    
     The signal electrodes  158   a  and  159   b , which constitute a non-vibrating part, are electrically connected to the electrodes (back electrodes) provided respectively on the hacks of the piezoelectric elements  120   a  and  120   b , which face the dust filter  119 . The back electrodes are electrically connected to the ground of the dust filter control circuit  121 , too. Therefore, no electrical signals are supplied to the back electrodes to contracting and expanding the piezoelectric elements  120   a  and  120   b . The back electrodes electrically connected to the ground may not be electrically connected to the signal electrodes  158   a  and  158   b . That is, the signal electrodes  158   a  and  158   b  may not be electrically connected to the ground. (i.e., same potential), making it possible to supply electrical signals to the signal, electrodes  159   a  and  159   b . In this case, the signal electrodes  158   a  and  159   b  can function as a non-vibrating part if they are not supplied with electrical signals. This is why the above-mentioned configuration is possible. Even if the signal electrodes  158   a  and  158   b  can receive electrical signals, those parts of the piezoelectric elements  120   a  and  120   b , which correspond to the signal electrodes  158   a  and  158   b , will not function as vibrators unless they are “polarized.” This configuration is also possible. 
     On the other hand, the signal electrodes  157   a  and  157   b , which constitute vibration application parts, receive a voltage (electrical signal for expanding and contracting the piezoelectric elements) of a frequency determined by the dimensions and materials of the dust filter  119  and piezoelectric elements  120   a  and  120   b , from the dust filter control circuit  121  that is a drive unit. The vibration application parts of the piezoelectric elements  120   a  and  120   b , which are interposed between the signal electrodes  157   a  and  157   b , one the one hand, and the back electrodes, on the other, expand and contract, vibrating the dust filter  119 . Dust is thereby removed from the surface of the dust filter  119 . As shown in  FIG. 2B , the signal electrodes  157   a  and  157   b , which constitute the vibration application unit of the vibrator  159 , are arranged, having unbalance along the long sides of the piezoelectric elements  120   a  and  120   b , each shaped like a long and thin rectangle. 
     The signal electrodes  157   a ,  157   b ,  158   a  and  158   b  are electrically connected to the electrode terminals  160   a ,  160   b ,  160   c  and  160   d , which are provided on a flexible printed board  160  (hereinafter referred to as “flex”). When a specific electrical signal is input from the dust filter control circuit  121  to the vibration application parts of the piezoelectric elements  120   a  and  120   b , the piezoelectric elements  120   a  and  120   b  vibrate, which are arranged symmetrically with respect to the symmetry axis of the dust filter  119  (the symmetry axis of the dust filter  119  is a virtual axis at almost the same distance from the piezoelectric elements  120   a  and  120   b ). The piezoelectric elements  120   a  and  120   b  are arranged symmetrically to the symmetry axis of the dust filter  119  and so shaped to vibrate at a large amplitude as will be explained later. 
     The flex  160  further has test terminals  160   e ,  160   f  and  160   f , lead terminals  160   h  and  160   i , and connection terminals  160   j  and  160   k . The connection terminals  160   j  and  160   k  have yet to be connected to each other at the time of testing the vibrator  159 . The connection terminals  160   j  and  160   k  are connected to each other by soldering after the vibrator  156  is evaluated as good and before the dust filter  119  is incorporated into the product. Before the connection terminals  160   j  and  160   k  are connected to each other, either the conductive pattern provided between the connection terminal  160   j  and the electrode terminal  160   a  or the conductive pattern provided between the connection terminal  160   k  and the electrode terminal  160   c  constitutes a first circuit that inputs an electrical signal to one of the two vibration application units. The other conductive pattern constitutes a second circuit that outputs an electrical signal that the other vibration application unit generates from the vibration generated when an electrical signal is input to the first circuit. Hence, the connection terminals  160   j  and  160   k  constitute a connection unit which electrically connects the first and second circuits. The dust filter control circuit  121  cooperates with the Bucom  101 , and can function as a drive unit that drives both vibration application units while the first and second circuits remain connected by the connection terminals  160   j  and  160   k  as the connection unit. 
     The vibration application units of the piezoelectric elements  120   a  and  120   b  are asymmetrical with respect to the nodes of standing-wave bending vibration as will be specifically described later. If a specific electrical signal is applied between the test terminals  160   e  and  160   f , standing-wave bending vibration occurs, generating a voltage. The vibration (i.e., voltage equivalent to the vibrational frequency and amplitude) can therefore be detected. 
     The electrode terminals  160   a  and  160   b  of the flex  160  are made of resin and supper etc., and have flexibility. Therefore, they little attenuate the vibration of the vibrator  159  including the piezoelectric elements  120   a  and  120   b . The piezoelectric elements  120   a  and  120   b  are provided at positions where the vibrational amplitude is small (at the nodes of vibration, which will be described later), and can therefore suppress the attenuation of vibration. The piezoelectric elements  120   a  and  120   b  move relative to the body unit  100  if the camera  10  has such a hand-motion compensating mechanism as will be later described. Hence, if the dust filter control circuit  121  is held by a holding member formed integral with the body unit  100 , the electrode terminals  160   a  and  160   b  of the flex  160  and lead lines  161   a  and  161   b  connected to the flex  160  are deformed and displaced as the hand-motion compensating mechanism operates. The electrode terminals  160   a  and  160   b  of the flex  160  do not hinder the operation of the hand-motion compensating mechanism, because they are flexible and thin as described above. Moreover, the lead lines  161   a  and  161   b  do not hinder the operation of the hand-motion compensating mechanism, either, because they can flex in any directions. 
     In the present embodiment, the flex  160  has a simple structure, having electrode terminals  160   a  and  160   b  formed integral with, and led from, the piezoelectric elements and  120   b , respectively. The flex  160  is simple also in that its lead terminals  160   h  and  160   i  formed integral are connected to the dust filter control circuit  121  by the lead lines  161   a  and  161   b , respectively. The flex  160  can therefore be made small and light, and is therefore best fit for use in cameras having a hand-motion compensating mechanism. 
     The dust removed from the surface of the dust filter  119  falls onto the bottom of the body unit  100 , by virtue of the vibration inertia and the gravity. In this embodiment, a base  162  is arranged right below the dust filter  119 , and holding members  163   a  and  163   b  made of, for example, adhesive tape, is provided on the base  162 . The holding members  163   a  and  163   b  reliably trap the dust fallen from the dust filter  119 , preventing the dust from moving back to the surface of the dust filter  119 . 
     The hand-motion compensating mechanism will be explained in brief. As shown in  FIG. 1 , the hand-motion compensating mechanism is composed of an X-axis gyro  164 , a Y-axis gyro  165 , a vibration control circuit  166 , an X-axis actuator  167 , a Y-axis actuator  168 , an X-frame  169 , a Y-frame  170  (holder  145 ), a position sensor  172 , and an actuator drive circuit  173 . The X-axis gyro  164  detects the angular velocity of the camera when the camera moves, rotating around the X axis. The Y-axis gyro  165  detects the angular velocity of the camera when the camera rotates around the Y axis. The vibration control circuit  166  calculates a value by which to compensate the hand motion, from the angular-velocity signals output from the X-axis gyro  164  and Y-axis gyro  165 . In accordance with the hand-motion compensating value thus calculated, the actuator drive circuit  173  moves the CCD  117  in the X-axis direction and Y-axis direction, which are first and second directions orthogonal each other in the XY plane that is perpendicular to the photographic optical axis, thereby to compensate the hand motion, if the photographic optical axis is taken as Z axis. More precisely, the X-axis actuator  167  drives the X-frame  169  in the X-axis direction upon receiving a drive signal from the actuator drive circuit  173 , and the Y-axis actuator  168  drives the Y-frame  170  in the Y-axis direction upon receiving a drive signal from the actuator drive circuit  173 . That is, the X-axis actuator  167  and the Y-axis actuator  168  are used as drive sources, the X-frame  169  and the Y-frame  170  (holder  145 ) which holds the CCD  117  of the image acquisition unit  116  are used as objects that are moved with respect to the frame  171 . Note that the X-axis actuator  167  and the Y-axis actuator  168  are each composed of an electromagnetic motor, a feed screw mechanism, and the like. Alternatively, each actuator may be a linear motor using a voice coil motor, linear piezoelectric motor or the like. The position sensor  172  detects the position of the X-frame  169  and the position of the Y-frame  170 . On the basis of the positions the position sensor  172  have detected, the vibration control circuit  166  controls the actuator drive circuit  173 , which drives the X-axis actuator  167  and the Y-axis actuator  168 . The position of the CCD  117  is thereby controlled. 
     In the hand-motion compensating mechanism so configured as described above, the dust filter  119  is driven together with the CCD  117 . The dust filter  119  should therefore have a small mass. Further, the electrical connection member connecting the flex  160  and the vibration control circuit  166  should have a small mass, too, and should have but a small load while operating. In this embodiment, the flex  160  has connection terminals (i.e., lead terminals  160   h  and  160   i ) in the smallest number possible. The number of lead lines  161   a  and  161   b , which electrically connect the flex  160  the vibration control circuit  166 , can therefore be reduced to a minimum. This helps to reduce the mass of the electrical connection members, ultimately decreasing the load generated as the terminals are deformed while the hand-motion compensating mechanism is operating. 
     The dust removal mechanism of the first embodiment will be described in detail, with reference to  FIGS. 3 to 15 . The dust filter  119  has at least one side symmetric with respect to a certain symmetry axis, and is a glass plate (optical element) of a polygonal plate as a whole (a square plate, in this embodiment). The dust filter  119  has a region flaring in the radial direction from the position at which maximum vibrational amplitude is produced. This region forms a transparent part. Alternatively, the dust filter  119  may be D-shaped, formed by cutting a part of a circular plate, thus defining one side. Still alternatively, it may formed by cutting a square plate, having two opposite sides accurately cut and having upper and lower sides. The above-mentioned fastening mechanism fastens the dust filter  119 , with the transparent part opposed to the front of the LPF  118  and spaced from the LPF  118  by a predetermined distance. To one surface of the dust filter  119  (i.e., back of the filter  119 , in this embodiment), the piezoelectric elements  120   a  and  120   b , which are vibrating members, are secured at the upper and lower edges of the filter  119 , by means of adhesion using adhesive. The piezoelectric elements  120   a  and  120   b , which are arranged on the dust filter  119 , constitute the vibrator  159 . The vibrator  159  undergoes resonance when a voltage of a prescribed frequency is applied to the piezoelectric elements  120   a  and  120   b . The resonance achieves such two-dimensional bending vibration of a large amplitude, as illustrated in  FIGS. 5A to 5D ,  FIG. 7B ,  FIG. 8  and  FIG. 9 . 
     As shown in  FIG. 3 , signal electrodes  157   a  and  158   a  are formed on the piezoelectric element  120   a , and signal electrodes  157   b  and  158   b  are formed on the piezoelectric element  120   b . Note that the hatched parts shown in  FIG. 3  show the shapes of the signal electrodes clearly, not to illustrating the sections thereof. The signal electrodes  158   a  and  158   b  are provided on the back opposing the signal electrodes  157   a  and  157   b , and are bent toward that surface of the piezoelectric element  120   a , on which the signal electrodes  157   a  and  157   b  are provided. The flex  160  having the above-mentioned conductive pattern is electrically connected to the signal electrodes  157   a  and  158   a  and to the signal electrodes  157   b  and  158   b . To the signal electrodes  157   a ,  157   b ,  158   a  and  158   b , a drive voltage of the prescribed frequency is applied form the dust filter control circuit  121  through flex  160 . When this drive voltage is applied to the vibration application units of the piezoelectric elements  120   a  and  120   b  expand and contract in accordance with the drive voltage. The dust filter  119  is thereby forcedly vibrated. The bending, propagating wave transiently generated by the forced vibration is reflected at the edges of the dust filter  119 , and is eventually superimposed, for a predetermined time, on the propagating wave continuously generated. As a result, there can be generated such a two-dimensional, standing-wave bending vibration as is shown in  FIGS. 5A to 5D . The flex  160  shown in  FIG. 4  has more terminals than the flex shown in  FIG. 2B . Therefore, connection terminals  174   a  and  174   b  and a test terminal  175  are provided on the conductive pattern that connects the electrode terminals  160   a  and  160   b.    
     The dust filter  119  is dimensioned such that the long sides are of length LA and the short sides are of length LB orthogonal to the long sides. (This size notation accords with the size notation used in  FIG. 6 .) Since the dust filter  119  shown in  FIG. 5A  is rectangular, it is identical in shape to the “virtual rectangle” according to this invention (later described). Hence, the long sides LA of the dust filter  119  are identical to the sides LF of the virtual rectangle that include the sides LA. The bending vibration shown in  FIGS. 5A to 5D  is standing wave vibration. As seen from  FIG. 5A , the vibrational amplitude is not perfectly zero (0) in the node area (i.e., area where the vibrational amplitude is small)  176  indicated by a thin solid line. Rather, the vibrational amplitude is small at any position where the nodes, for example, intersect with one another. This characterizes the present embodiment. Note that the meshes shown in  FIG. 5A  are division meshes usually used in the final element method. 
     If the node areas  176  are at short intervals as shown in  FIG. 5A  when the vibration speed is high, in-plane vibration (vibration along the surface) will occur in the node areas  176 . This vibration induces a large inertial force in the direction of the in-plane vibration (see mass point Y 2  in  FIG. 15 , described later, which moves over the node along an arc around the node, between positions Y 2  and Y 2 ′) to the dust at the node areas  176 . If the dust filter  119  is inclined to become parallel to the gravity so that a force may act along the dust receiving surface, the inertial force and the gravity can remove the dust from the node areas  176 . 
     The area between the nodes shown in  FIG. 5A  is an antinode area having a large vibrational amplitude. In this antinode area, the peaks and valleys of waves alternately appear at different times. In  FIG. 5A , thin broken lines indicate the ridges  177  of the wave peaks. The dust adhering to the antinode area is removed because of the inertial force exerted by the vibration. The dust can be removed from the node areas  176 , too, by producing vibration in another mode (for example, the vibrational mode illustrated in  FIG. 9 ), at similar amplitude at each node area  176  in  FIG. 5A . 
     In the  FIGS. 5A to 5D , reference number  178  indicates a vibration application part, reference numbers  179  indicates the centerline of the vibration application unit  178 , reference number  180  indicates a non-vibrating part, and reference number  181  indicates a non-electrode part, and reference number  182  indicates a maximum amplitude region where the maximum amplitude is attained. 
     The bending vibrational mode shown in  FIGS. 5A to 5D  is achieved by synthesizing the bending vibration of the X-direction and the bending vibration of the Y-direction. The fundamental state of this synthesis is shown in  FIG. 7A . If the vibrator  159  is put on a member that little attenuates vibration, such as a foamed rubber block, and then made to vibrate freely, a vibrational mode of producing such lattice-shaped node areas  176  as shown in  FIG. 7B  will be usually attained easily. In the front view included in  FIG. 7A , the broken lines define the centers of the node areas  176  shown in  FIG. 7B  (more precisely, the lines indicate the positions where the vibrational amplitude is minimal in the widthwise direction of lines). In this case, a standing wave, bending vibration at wavelength λ x  occurs in the X-direction, and a standing wave, bending vibration at wavelength λ y  occurs in the Y-direction. These standing waves are synthesized as shown in  FIG. 7B . With respect to the origin (x=0, y=0), the vibration Z(x, y) at a given point P(x, y) is expressed by Equation 1, as follows:
 
 Z ( x,y )= A·W   mn ( x,y )·cos(γ)+ A·W   nm ( x,y )·sin(γ)  (1)
 
where A is amplitude (a fixed value here, but actually changing with the vibrational mode or the power supplied to the piezoelectric elements); m and n are positive integers including 0, indicating the order of natural vibration corresponding to the vibrational mode; γ is a given phase angle;
 
     
       
         
           
             
               
                 
                   W 
                   mn 
                 
                 ⁡ 
                 
                   ( 
                   
                     x 
                     , 
                     y 
                   
                   ) 
                 
               
               = 
               
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       
                         n 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           π 
                           · 
                           x 
                         
                       
                       + 
                       
                         π 
                         2 
                       
                     
                     ) 
                   
                 
                 · 
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       
                         m 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           π 
                           · 
                           y 
                         
                       
                       + 
                       
                         π 
                         2 
                       
                     
                     ) 
                   
                 
               
             
             ; 
             
                 
             
             ⁢ 
             and 
           
         
       
       
         
           
             
               
                 W 
                 nm 
               
               ⁡ 
               
                 ( 
                 
                   x 
                   , 
                   y 
                 
                 ) 
               
             
             = 
             
               
                 sin 
                 ⁡ 
                 
                   ( 
                   
                     
                       m 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         π 
                         · 
                         x 
                       
                     
                     + 
                     
                       π 
                       2 
                     
                   
                   ) 
                 
               
               · 
               
                 
                   sin 
                   ⁡ 
                   
                     ( 
                     
                       
                         n 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           π 
                           · 
                           y 
                         
                       
                       + 
                       
                         π 
                         2 
                       
                     
                     ) 
                   
                 
                 . 
               
             
           
         
       
     
     Assume that the phase angle γ is 0 (γ=0). Then, Equation 1 changes to: 
     
       
         
           
             
               
                 
                   
                     Z 
                     ⁡ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     A 
                     · 
                     
                       
                         W 
                         mn 
                       
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     A 
                     · 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               n 
                               · 
                               π 
                               · 
                               x 
                             
                             
                               λ 
                               x 
                             
                           
                           + 
                           
                             π 
                             2 
                           
                         
                         ) 
                       
                     
                     · 
                     
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 m 
                                 · 
                                 π 
                                 · 
                                 y 
                               
                               
                                 λ 
                                 y 
                               
                             
                             + 
                             
                               π 
                               2 
                             
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
     Further assume that λ x =λ y =λ=1 (x and y are represented by the unit of the wavelength of bending vibration). Then: 
     
       
         
           
             
               
                 
                   
                     Z 
                     ⁡ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     A 
                     · 
                     
                       
                         W 
                         mn 
                       
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                   
                     A 
                     · 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             n 
                             · 
                             π 
                             · 
                             x 
                           
                           + 
                           
                             π 
                             2 
                           
                         
                         ) 
                       
                     
                     · 
                     
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               m 
                               · 
                               π 
                               · 
                               y 
                             
                             + 
                             
                               π 
                               2 
                             
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
             
           
         
       
     
     Similarly, if γ=π/2, too, the front term of the equation (1) will be zero. Hence, a similar standing wave is generated.  FIG. 7A  shows the vibrational mode that is applied if m=n (since the X-direction vibration and the Y-direction vibration are identical in terms of order and wavelength, the dust filter  119  has a square shape). In this vibrational mode, the peaks, nodes and valleys of vibration appear at regular intervals in both the X-direction and the Y-direction, and vibration node areas  176  appear as a checkerboard pattern (conventional vibrational mode). In the vibrational mode where m=0, n=1, the vibration has peaks, nodes and valleys parallel to a side (LB) that extends parallel to the Y-direction. In the vibrational mode described above, the X-direction vibration and the Y-direction vibration are generated, independent of each other. Even if the X-direction vibration and the Y-direction vibration are synthesized, the amplitude of vibration (or vibration speed) will have the same value as in the case where only X-direction vibration is generated (forming nodes and peaks and valleys, all parallel to the side LB) or the case where only Y-direction vibration is generated (forming nodes and peaks and valleys, all parallel to the side LA). This takes place, also in the vibration mode shown in  FIG. 7B . In these vibrational modes, the phase angle γ is k×π/2 (γ=k×π/2) as pointed out before, if k is 0 or an integer (either positive or negative). That is, in these vibrational modes, cos γ and sin γ are 0. 
     A vibrational mode in which the phase angle γ has a different value will be explained. In view of this, the dust filter  119  may be elongated a little, shaped like a rectangle, and may be vibrated at a specific frequency, or in a mode where m=3 and n=2. In this vibrational mode, the phase angle γ is +π/4 or ranges from −π/4 to −π/8. This vibrational mode is a mode in which the present embodiment will have very large vibrational amplitude (the maximum amplitude at the same level as at the conventional circular dust filter). If γ=+π/4, the vibrational mode will be the mode shown in  FIGS. 5A to 5D . In this vibrational mode, a closed curve is defined by the peak ridges  177  of the vibrational amplitude, which is plane-symmetrical with respect to the midpoint on the optical axis (i.e., point at which the above-mentioned virtual symmetry axis intersects with a virtual, centerline later described), though the dust filter  119  is rectangular. Consequently, a reflected wave coming from a side extending in the X-direction and a reflected wave coming from a side extending in the Y-direction are efficiently combined, forming a standing wave.  FIG. 8  shows a vibrational mode in which γ=−π/4 and which is achieved by changing the vibrational frequency of the dust filter  119  of  FIGS. 5A to 5D . In this vibrational mode, peak ridges  177  of vibrational amplitude are formed, surrounding the midpoint of each side. That is, the center of the dust filter  119  becomes a node area  176  where vibrational amplitude is scarcely observed. Peak ridges  177  of vibrational amplitude are formed, surrounding the midpoint of each side. 
       FIG. 9  shows the vibrational mode shown in  FIGS. 5A to 5D , in which vibration is generated almost in the conventional vibrational mode wherein the peaks of vibration are parallel to the sides of the piezoelectric elements. The vibrational mode shown in  FIG. 9  can be achieved by changing the dust filter  119  and piezoelectric elements  120   a  and  120   b  in configuration (e.g., aspect ratio of the dust filter  119 , as described later) in order to increase or decrease the phase angle from +π/4. 
     The dust filter  119  of the vibrator  159 , shown in  FIG. 5A , is a glass plate (optical element) having a size of 30.8 mm (X-direction: LA, LF)×28.5 mm (Y-direction: LB)×0.65 mm (thickness). The dust filter  119  is rectangular, having long sides LA (30.8 mm, extending in the X-direction) and short sides LB (28.5 mm). Therefore, the dust filter  119  is identical to the “virtual rectangle” according to this invention, which has the same area as the dust filter  119 . The long sides LA of the dust filter  119  are arranged are thus identical to the sides LF of the virtual rectangle that includes the sides LA. The piezoelectric elements  120   a  and  120   b  are made of lead titanate-zirconate ceramic and have a size of 21 mm (X-direction: LP)×3 mm (Y-direction)×0.8 mm (thickness). The piezoelectric elements  120   a  and  120   b  are adhered with epoxy-based adhesive to the dust filter  119 , extending along the upper and lower sides of the filter  119  (optical element), respectively. More specifically, the piezoelectric elements  120   a  and  120   b  extend in the X-direction and arranged symmetric in the left-right direction, with respect to the centerline of the dust filter  119 , which extends in the Y-direction. In this case, the resonance frequency in the vibrational mode of  FIG. 5A  is in the vicinity of 91 kHz. At the center of the dust filter  119 , a maximal vibration speed and vibrational amplitude can be attained if the dust filter is shaped like a circle in which the rectangular dust filter  119  is inscribed. The vibration-speed ratio has such a value as shown in  FIG. 10 , the maximum value of which is 1.000. In the graph of  FIG. 10 , the line curve pertains to the case where the piezoelectric elements  120   a  and  120   b  are arranged parallel to the long sides of the dust filter  119 , and the dots pertain to the case where the  120   a  and  120   b  are arranged parallel to the short sides of the dust filter  119 . In this vibrational mode, the piezoelectric elements  120   a  and  120   b  should better be arranged at the longer sides of the dust filter  119 . A higher vibration speed can be achieved than otherwise. 
     As described above, the phase angle γ is +π/4 or ranges from −π/4 to −π/8. Nevertheless, the phase angle need not have such a precise value. If the phase angle γ differs a little from such value, the vibrational amplitude can be increased. Even in the vibrational mode of  FIG. 9 , in which the phase angle γ is a little smaller than +π/4, the peak ridges  177  of vibrational amplitude form closed loops around the optical axis, too, and the vibration speed decreases in the Z-direction at the center of the vibrator  159 . This dust filter  119  is a glass plate (optical element) that has a size of 30.8 mm (X-direction: LA, LF)×28.5 mm (Y-direction: LB)×0.65 mm (thickness). The dust filter  119  is rectangular, having long sides LA (30.8 mm, extending in the X-direction) and short sides LB (28.5 mm). Therefore, the dust filter  119  is identical to the “virtual rectangle” according to this invention, which has the same area as the dust filter  119 . The piezoelectric elements  120   a  and  120   b  have a size of 30 mm (X-direction)×3 mm (Y-direction) 0.8 mm (thickness), having a length almost equal to the length LF (in the X-direction) of the dust filter  119 , and are made of lead titanate-zirconate ceramic. The piezoelectric elements  120   a  and  120   b  are adhered with an epoxy-based adhesive to the dust filter  119 , extending along the upper and lower sides of the filter  119 , respectively, and positioned symmetric in the X-direction with respect to the centerline of the dust filter  119 . In this case, the resonance frequency in the vibrational mode shown in  FIG. 9  is in the vicinity of 68 kHz. As in  FIGS. 2A and 2B , in this case, too, the dust filter  119  is supported by the lip part  150   a  of the seal  150 , and the holder  145  has four cushion members  156 , which act as second support members if an external force is applied to the seal  150 . 
     The vibrator  159  of this configuration may not achieve a target vibration speed. The vibrator  159  may therefore fail to achieve a sufficient dust removal efficiency, depending on the material, shape and assembling deviation of the vibrator  159  and the material, shape, supporting position and supporting force of the supporting unit. To test the vibrator  159  during the manufacture or repairing process, the dust filter control circuit  121  may be used to vibrate the dust filter  119  and a laser Doppler speedometer may be used to measure the vibration speed of the dust filter  119 . However, the vibration speed cannot be measured unless a light reflecting tape, for example, is adhere to the dust filter  119 , because the vibration surface of the dust filter  119  is transparent. Further, the vibrational amplitude greatly changes in accordance with the vibrational mode and the position of the dust filter  119 , as shown in  FIGS. 5A to 5D . Therefore, the dust filter  119  must be precisely positioned to measure its vibration speed. Therefore, the vibration speed cannot easily be measured with the laser Doppler speedometer. The use of the laser Doppler speedometer during the manufacture requires a high cost and much labor. Hence, the laser Doppler speedometer can hardly be employed. 
     This is why a piezoelectric element may be provided, on the dust filter to detect the vibration as described above, or two piezoelectric elements may be provided on a rectangular dust filter as in this embodiment. In the latter case, an electrical signal is supplied to one piezoelectric element, and the vibration of the dust filter is detected by the other piezoelectric element. 
     A detection piezoelectric element, if provided on a part of either piezoelectric element, hinders the vibration of the dust filter  119  and renders the vibrator large. If the detection piezoelectric element is small in order not to make the vibrator large, it cannot accurately detect the vibration of the dust filter, depending on the position where it is arranged. In addition, if the detection piezoelectric element has a trouble, it will erroneously determine that abnormality has developed in the vibrator. 
     By contrast, the technology of using two piezoelectric elements cannot detect the vibrational amplitude that reflects the vibrational state of the dust filter if the vibrational state is symmetrical to the dust filter and complex as in the present embodiment. 
     In the present embodiment, the piezoelectric elements  120   a  and  120   b  are configured to assume weight balance with respect to not only the virtual symmetry axis at the same distance from the piezoelectric elements  120   a  and  120   b , but also the virtual centerline connecting the gravity centers of the piezoelectric elements  120   a  and  120   b . The vibration application part  178  is arranged asymmetrical to the virtual centerlines of the piezoelectric elements  120   a  and  120   b . Therefore, the vibrational amplitude of the vibration application part  178  will be asymmetrical to the centerline of the vibration application part if vibration symmetrical to the virtual centerline is generated. (The centerline of the vibration application part is perpendicular to the virtual symmetry axis, bisects the surface area of the vibration application part  178  into equal halves, and is the axis of vibration.) 
     More specifically, in the case of  FIGS. 5A to 5D , X 3 , X 4  and X 5  are vibrational regions in the signal electrodes  157   a  and  157   b , respectively, which correspond to the vibration application parts  179 . In the vibrational regions X 3  and X 5 , the vibration application part vibrates in the same phase. X 4  is a vibrational region, in which the vibration application part vibrates in the opposite phase. If one of the piezoelectric elements detects the vibration of the dust filter  119 , a positive electrical charge is generated in the vibrational regions X 3  and X 5  and a negative electrical charge is generated in the vibrational region X 4 , or other way around. Nonetheless, these charges generated do not completely cancel out one another, because the vibration application part  178  is so formed that its vibrational amplitude is asymmetrical to its own centerline as described above. Thus, the electrical charge remains in part, resulting in a voltage that corresponds to the vibrational amplitude in the vibrational regions X 3 , X 4  and X 5 . 
       FIG. 11  shows the relation between the voltages detected in a plurality of dust filters, each generated at piezoelectric element  120   a , and the vibration speed ratio detected at the center part of any dust filter  119  when an electrical signal is supplied from the dust filter control circuit  121  to both piezoelectric elements  120   a  and  120   b . (The vibration speed ratio is a ratio of vibration speed V at arbitrary state to vibration speed V 0  at which dust filter must be vibrated to remove dust from it.) Note that the voltage detected pertains to the vibration generated in each dust filter  119  and detected by a vibration detector  184  of  FIG. 12 , according to this embodiment, in the vibrational state of  FIG. 9 . As shown in  FIG. 12 , the vibration detector  184  has a dust filter drive circuit  184 A and a voltage detection circuit  184 B. The dust filter drive circuit  184 A is connected to the first circuit and configured to supply an electrical signal to the piezoelectric element of the first circuit, i.e., piezoelectric element  120   b  in this case. The voltage detection circuit  184 B is connected to the second circuit and configured to detect the voltage generated across the piezoelectric element of the second circuit, i.e., piezoelectric element  120   a  in this case. 
     As described above and as shown in  FIG. 2A  and  FIG. 2B , the piezoelectric elements  120   a  and  120   b  are arranged symmetrically with respect to the dust filter  119 . (The piezoelectric elements  120   a  and  120   b  assume weight balance with respect to not only the virtual symmetry axis, but also the virtual centerline.) Further, the piezoelectric elements  120   a  and  120   b  are identical in shape. The vibration generated at the dust filter  119  is therefore symmetrical (with respect to the plane containing the virtual centerline and being perpendicular to the virtual symmetry axis). 
     Vibration is generated in the vibrational regions X 3  and X 5  in the same phase, and vibration is generated in the vibrational region X 4  in the opposite phase. The vibrational regions X 3 , X 4  and X 5  have the same area. A positive electrical charge is generated in the vibrational regions X 3  and X 5  and a negative electrical charge is generated in the vibrational region X 4 , or other way around. Therefore, the electrical charges seem to cancel out each other to generate no voltage at all. Nonetheless, the vibrational amplitude is the largest in the vibrational region X 3 , has a medium value in the vibrational region X 4 , and is the smallest in the vibrational region X 5 . The total electrical charge generated because of the vibration does not become zero. Thus, the voltage detected at the piezoelectric elements is not zero also in the embodiment of  FIG. 9 . This is because the vibrational amplitude is asymmetrical with respect the centerline of the vibration application part. The amplitude of the vibration generated can therefore be accurately detected. 
     Vibrational nodes may be generated, which, are parallel to each side of the rectangular dust filter  119  (see  FIG. 7A  and  FIG. 7B ). In this case, the total electrical charge the vibration generates at the vibration application part will become zero if the centerline  179  of the vibration application part  178  is aligned with a node of vibration. As a result, the vibration of the dust filter  119  cannot be detected at all. If the piezoelectric element vibrated or the piezoelectric element used to detect vibration has a trouble, the electrical signal detected has a low level. Thus, the detected electrical signal correctly reflects the amplitude of the vibration generated when electrical signals are input to both piezoelectric elements of the product. 
     In the vibrational state (vibrational mode) of  FIG. 11 , resonation occurs at a frequency near 93 kHz. The voltage detected is collated with the vibration speed ratio. The speed of the vibration generated at the dust filter  119  when an electrical signal is supplied to both piezoelectric elements in the detection method according to this embodiment can therefore be accurately predicted. Hence, the vibration detector  184  can easily determine whether the vibrator  159  composed of the dust filter  119 , piezoelectric elements  120   a  and  120   b  and flex  160  is a good one or a defective one. Moreover, the product shown in  FIG. 25  can be inspected to be a good one or a defective one with regard to whether the dust filter  119  is duly held in the product. 
     The dust filter  119  has a vibrational mode at a frequency near 82 kHz, too. The voltage detected in the vibrational mode of 93 kHz is collated with the vibration speed ratio in the vibrational mode of 82 kHz, in the same manner as in the case shown in  FIG. 11 . Therefore, once the vibration speed is detected in one vibrational mode, the vibration speed in any other vibrational mode can be predicted. 
     In the vibrational state of  FIG. 7B , vibrational regions X 1  and X 2  are opposite in terms of vibrational phase and are almost identical in terms of vibrational amplitude. Also in the vibrational state of  FIG. 7B , the vibrational amplitude is asymmetrical with respect to the centerline of the vibration application part  178 . The vibrational state, including the amplitude of the vibration generated at the dust filter  119 , can accurately detected from the voltage detected at the vibration application part  178 . In the vibrational state of  FIG. 8B , too, the vibrational regions X 2  and X 3  are opposite in terms of vibrational phase, and the vibrational amplitude in the vibrational region X 2  is larger than that in the vibrational region X 3 . Also in this vibrational state, the vibrational amplitude is asymmetrical, with respect to the centerline of the vibration application part  178 , and can be accurately detected, along with the amplitude of the vibration generated at the dust filter  119 . 
       FIG. 13  shows a modification of the vibrator  159 . The modified vibrator  159  has a dust filter  119  that is D-shaped, formed by cutting a part of a plate shaped like a disc, thus defining one side. That is, the modified vibrator  159  uses a D-shaped dust filter  119  that has a side symmetric with respect to the symmetry axis extending in the Y-direction. The piezoelectric element  120   a  is arranged on the surface of the dust filter  119 , extending parallel to that side and positioned symmetric with respect to the midpoint of the side (or to a symmetry axis extending in the Y-direction). On the other hand, the piezoelectric element  120   b  is substantially inscribed in the outer circumference of the dust filter  119  and extends parallel to that side of the dust filter  119 . So shaped, the dust filter  119  is more symmetric with respect to its center (regarded as the centroid), and can more readily vibrate in a state shown in  FIGS. 5A to 5D . In addition, the dust filter  119  can be smaller than the circular one. 
     This embodiment has two piezoelectric elements  120   a  and  120   b , which are identical in shape. The piezoelectric, elements  120   a  and  120   b  are connected by a flex  160 . The flex  160  has a conductive pattern. The conductive pattern is so formed to be electrically connected to the piezoelectric elements  120   a  and  120   b , or to electrically connect the piezoelectric elements  120   a  and  120   b  in parallel (see  FIG. 3  and  FIG. 4 ). 
     Further, the dust filter  119  may have a shape asymmetrical (to vibration), as shown in  FIG. 13 . The dust filter  119  is so shaped, by cutting a part of a disc, defining one side. The piezoelectric elements  120   a  and  120   b  are arranged parallel to the side, are made rigid and symmetrical, and can achieve a desirable vibrational state. 
     The dust filter  119  shown in  FIG. 13  is not rectangular, having neither long sides nor short sides. Imagine a virtual rectangle  185 , one side of which is the side formed by cutting a part of the disc. A side opposite to the side thus formed extends along the outer lateral edge of the piezoelectric element  120   b . The virtual rectangle  185  has two other sides parallel and opposite to each other, so that it has the same area as the dust filter  119 . The long sides and short sides of the virtual rectangle  185  are set as the long sides and short sides of the dust filter  119  shown in  FIG. 13 . 
     In the configuration of  FIG. 13 , the piezoelectric elements  120   a  and  120   b  are not arranged symmetrically to each other with respect of the symmetry axis of the dust filter  119 . Nonetheless, the piezoelectric elements  120   a  and  120   b  are arranged symmetrically with respect to the symmetry axis of the virtual rectangle  185 . The same advantages can therefore be attained as in the case where the piezoelectric elements  120   a  and  120   b  are arranged symmetrically to each other with respect of the symmetry axis of the dust filter  119 . 
     The dust filter  119  is supported as follows. The seal  150  so shaped as shown in  FIG. 2B  (shaped like, as it were, a deformed athletic track) is interposed between the dust filter  119  and the holder  145 . Such a pushing member  151  as shown in  FIG. 2A  pushes the dust filter  119 . The dust filter  119  is thereby held in position. The seal  150  has the lip part  150   a . The lip part  150   a  contacts the dust filter  119 , sealing the space defined by the dust filter  119 , holder  145 , optical LPF  118  and seal  150 . Further, cushion members  156  are provided on the holder  145  at three points. They support the dust filter  119  when an external force is exerted on the seal  150 . The seal  150  contacts the dust filter  119 , at its track-shaped lip part  150   a . The seal  150  therefore extends along the node area of vibration generated in the dust filter  119  and surrounding the center of the dust filter  119 . Hence, the seal  150  less impeding the vibration of the dust filter  119  than otherwise. Since the corners (and lip part  150   a ) of the seal  150  are obtuse-angled, they will be scarcely deformed when the seal  150  receives an external force. This is why the corners of the seal  150  are not arced as in the configuration of  FIG. 2B . 
       FIG. 14  shows another modification of the vibrator  159 . This modified vibrator  159  has a dust filter  119  is formed by cutting a circular plate along two parallel lines, forming two parallel sides. That is, the modified vibrator  159  uses a dust filter  119  that has two sides symmetric with respect to the symmetry axis extending in the Y-direction. In this case, actuate piezoelectric elements  120   a  and  120   b  are arranged not on the straight sides, but on the curved parts defining a circle. Since the dust filter  1  is so shaped, the piezoelectric elements  120   a  and  120   b  are arranged, efficiently providing a smaller vibrator  159 . The dust filter  119  of  FIG. 1  is not rectangular, either. It has two long sides, but not two short sides. Therefore, a virtual rectangle  185  is set for the dust filter  119 , in the same mariner as for the dust filter  119  of  FIG. 13 . That is, the two parallel, sides of the dust filter  119 , which have been defined by cutting a disc, are used as two opposite sides of the virtual rectangle  185 . The virtual rectangle  185  has two other sides so that it may have the same area as the dust filter  119 . The long sides of the virtual rectangle  185  are set as long sides of the dust filter  119 , and the short sides of the virtual rectangle  185  are set as short sides of the dust filter  119 . The piezoelectric elements  120   a  and  120   b  and the flex  160  are electrically connected, and the dust filter  119  is supported, in the same mariner as explained with reference to  FIG. 13 . 
     A method of removing dust will be explained in detail, with reference to  FIG. 15 .  FIG. 15  shows a cross section identical to that shown in  FIG. 5B . Assume that the piezoelectric elements  120   a  and  120   b  are polarized in the direction of arrow  186  as shown in  FIG. 15 . If a voltage of a specific frequency is applied to the piezoelectric elements  120   a  and  120   b  at a certain time t 0 , the vibrator  159  will be deformed as indicated by solid lines. At the mass point Y existing at given position y in the surface of the vibrator  159 , the vibration z in the Z-direction is expressed by Equation 2, as follows:
 
 z=A ·sin( Y )·cos(ω t )  (2)
 
where ω is the angular velocity of vibration, A is the amplitude of vibration in the Z-direction, and Y=2πy/λ (λ: wavelength of bending vibration).
 
     The Equation 2 represents the standing-wave vibration shown in  FIG. 5A . Thus, if y=s·λ/2 (here, s is an integer), then Y=sπ, and sin(Y)=0. Hence, a node  187 , at which the amplitude of vibration in the Z-direction is zero irrespective of time, exists for every π/2. This is standing-wave vibration. The state indicated by broken lines in  FIG. 15  takes place if t=kπ/ω (k is odd), where the vibration assumes a phase opposite to the phase at time t 0 . 
     Vibration z(Y 1 ) at point. Y 1  on the dust filter  119  is located at an antinode  188  of standing wave, bending vibration. Hence, the vibration in the Z-direction has amplitude A, as expressed in Equation 3, as follows:
 
 z ( Y   1 )= A ·cos(ω t )  (3)
 
     If Equation 3 is differentiated with time, the vibration speed Vz(Y 1 ) at point Y 1  is expressed by Equation 4, below, because ω=2πf, where f is the frequency of vibration: 
     
       
         
           
             
               
                 
                   
                     Vz 
                     ⁡ 
                     
                       ( 
                       
                         Y 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ⅆ 
                         
                           ( 
                           
                             z 
                             ⁡ 
                             
                               ( 
                               
                                 Y 
                                 1 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                       
                         ⅆ 
                         t 
                       
                     
                     = 
                     
                       
                         - 
                         2 
                       
                       ⁢ 
                       π 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         f 
                         · 
                         A 
                         · 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               t 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     If Equation 4 is differentiated with time, vibration acceleration αz(Y 1 ) is expressed by Equation 5, as follows: 
     
       
         
           
             
               
                 
                   
                     αz 
                     ⁡ 
                     
                       ( 
                       
                         Y 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ⅆ 
                         
                           ( 
                           
                             Vz 
                             ⁡ 
                             
                               ( 
                               
                                 Y 
                                 1 
                               
                               ) 
                             
                           
                           ) 
                         
                       
                       
                         ⅆ 
                         t 
                       
                     
                     = 
                     
                       
                         - 
                         4 
                       
                       ⁢ 
                       
                         π 
                         2 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
                           f 
                           2 
                         
                         · 
                         A 
                         · 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               ω 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               t 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     Therefore, a dust  189  adhering at point Y 1  receives the acceleration of Equation 5. The inertial force Fk the dust  189  receives at this time is given by Equation 6, as follows:
 
 Fk=αz ( Y   1 )· M=− 4π 2   f   2   ·A ·cos(ω t )· M   (6)
 
where M is the mass of the dust  189 .
 
     As can be seen from Equation 6, the inertial force Fk increases as frequency f is raised, in proportion to the square of f. However, the inertial force cannot be increased if amplitude A is small, no matter how much frequency f is raised. Generally, kinetic energy of vibration can be produced, but in a limited value, if the piezoelectric elements  120   a  and  120   b  that produce the kinetic energy have the same size. Therefore, if the frequency is raise in the same vibrational mode, vibrational amplitude A will change in inverse proportion to the square of frequency f. Even if the resonance frequency is raised to achieve a higher-order resonance mode, the vibrational frequency will fall, not increasing the vibration speed or the vibration acceleration. Rather, if the frequency is raised, ideal resonance will hardly be accomplished, and the loss of vibrational energy will increase, inevitably decreasing the vibration acceleration. That is, the mode cannot attain large amplitude if the vibration is produced in a resonance mode that uses high frequency only. The dust removal efficiency will be much impaired. In order to increase the vibration speed, it is therefore necessary not only to raise the vibrational frequency, but also to generate efficient resonance at the dust filter  119  (that is, to increase the vibrational amplitude). 
     Further, how large a vibrational amplitude the vibrator  159  including the dust filter  119  produced has achieved is detected. From the vibrational amplitude thus detected and from the vibrational frequency (i.e., frequency input from the circuit), the vibration speed and vibrational acceleration can be calculated and the dust removal efficiency can be detected. If the dust removal efficiency is detected before the camera is assembled, it can be determined whether the vibrator  159  produced is a good one, and whether the vibrator  159  is appropriately pushed and supported. 
     Although the dust filter  119  is rectangular, the peak ridges  177  of vibrational amplitude form closed loops around the optical axis in the vibrational mode of the embodiment, which is shown in  FIG. 5A . In the vibrational mode of the embodiment, which is shown in  FIG. 8 , the peak ridges  177  of vibrational amplitude form curves surrounding the midpoint of each side. The wave reflected from the side extending in the X-direction and the wave reflected from the side extending in the Y-direction are efficiently synthesized, forming a standing wave. The method of supporting the dust filter  119  in this vibrational mode is identical to the method explained with reference to  FIG. 5A .  FIG. 8  shows a seal contact area  190  and support areas  191 . In the seal contact area  190 , the seal  150  contacts the dust filter  119 . In the support areas  191 , the cushion members  156  support the dust filter  119  when an external force acts on the dust filter  119 . The seal contact area  190  and the support areas  191  are located near vibration nodes area  176  and are small areas in which the vibrational amplitude is small. Hence, they scarcely impede the vibration generated in the dust filter  119 . 
     The shape and size of the dust filter  119  greatly contribute, to efficient generation of this synthesized standing wave. As seen from  FIG. 10 , it is better to set the aspect ratio (short side/long side, i.e., ratio of the length of the short sides to that of the long sides of the dust filter  119 ) to a value smaller than 1, than to 1 (to make the dust filter  119  square). If the aspect ratio is smaller than 1, the speed of vibration at the center of the dust filter  119 , in the Z-direction will be higher (the vibration speed ratio is 0.7 or more), no matter how the piezoelectric elements  120   a  and  120   b  are arranged. In  FIG. 10 , the ratio (V/V max ) of the vibration speed V to the maximum vibration speed V max  possible in this region is plotted on the ordinate. The maximum aspect ratio (i.e., short side/long side) is, of course, 1. At the aspect ratio of 0.9 or less, the vibration speed abruptly decreases. Therefore, the dust filter  119  preferably has an aspect ratio (short side/long side) of 0.9 to 1, but less than 1. The two dots in  FIG. 10 , which pertain to the case where the  120   a  and  120   b  are arranged parallel to the short sides of the dust filter  119 , indicate vibration speed ratios, which are smaller than the vibration speed ratios attainable if the piezoelectric elements  120   a  and  120   b  are arranged parallel to the long sides of the dust filter  119 . It is therefore advisable to arrange the piezoelectric elements  120   a  and  120   b  at the long sides of the dust filter  119 , not at the short sides thereof. If the elements  120   a  and  120   b  are so arranged, the vibration speed ratio will increase to achieve a high dust removal ability. The maximum vibration speed ratio is attained in  FIG. 10  in the case where the vibrational mode is that of  FIG. 5A  and γ=+π/4 in the equation (1). 
     In vibration wherein the peak ridges  177  of vibrational amplitude form closed loops around the optical axis or the peak ridges  177  form curves surrounding the midpoint of each side, the dust filter  119  can undergo vibration of amplitude a similar to that of concentric vibration that may occur if the dust filter  119  has a disc shape. In any vibrational mode in which the amplitude is simply parallel to the side, the vibration acceleration is only 10% or more of the acceleration achieved in this embodiment. 
     In the vibration wherein the peak ridges  177  of vibrational amplitude form closed loops or curves surrounding the midpoint of each side, the vibrational amplitude is the largest at the center of the vibrator  159  and small at the closed loop or the curve at circumferential edges. Thus, the dust removal capability is maximal at the center of the image. If the center of the vibrator  159  is aligned with the optical axis, the shadow of dust  189  will not appear in the center part of the image, which has high image quality. This is an advantage. The vibrational amplitude gradually decreases from the center of the dust filter  119  toward the circumference thereof. The vibrational amplitude region can therefore be easily made asymmetrical with respect to the centerline of the vibration application parts  178  of the piezoelectric elements  120   a  and  120   b . This simple configuration can serve to detect the vibration. 
     In the vibration node areas  176 , which exist in the focusing-beam passing area  149 , the nodes  187  may be changed in position by changing the drive frequencies of the piezoelectric elements  120   a  and  120   b . Then, the elements  120   a  and  120   b  resonate in a different vibrational mode, whereby the dust  189  can be removed, of course. The high vibrational acceleration generated by the vibrator  159  changes with the deviation of the material, shape, size and position of its components (i.e., dust filter  119  and piezoelectric elements  120   a  and  120   b ) from design specifications. In addition, the vibration speed of the vibrator  159  changes with the change in the manner of holding the flex  160 , with deviation the pushing force applied to the vibrator  159  from the specifications and with the deviation of the support members from the specifications. In view of this, it is very important to detect the vibrational acceleration. The vibrational acceleration can be detected easily and accurately in the present embodiment. (What can be detected in practice is the piezoelectric voltage generated by vibration. Nonetheless, the vibrational acceleration can be detected by detecting the frequency because the piezoelectric voltage detected is highly collated with the vibration speed as shown in  FIG. 11 .) Thus, the vibrator  139  or the vibrating device incorporating the vibrator  159  can be evaluated as a defective one if the vibrator  159  cannot achieve a prescribed vibration speed because of the deviation of the various parameters mentioned above. Moreover, the vibrator  159  can be made small, because additional vibration detecting members need not be used as in the conventional vibrating device. Furthermore, vibrating device that generates vibration at a higher efficiency can be provided, because the vibrator  159  has no superfluous components. 
     A vibration state that is attained if the piezoelectric elements  120   a  and  120   b  are driven at a frequency near the resonance frequency will be described with reference to  FIGS. 16A and 16B .  FIG. 16A  shows an equivalent circuit that drives the piezoelectric elements  120   a  and  120   b  at a frequency near the resonance frequency of the vibrator  159 . In  FIG. 16A , C 0  is the electrostatic capacitance attained as long as the piezoelectric elements  120   a  and  120   b  remain connected in parallel, and L, C and R are the values of a coil, capacitor and resistor that constitute an electric circuit equivalent to the mechanical vibration of the vibrator  159 . Naturally, these values change with the frequency. 
     When the frequency changes to resonance frequency f 0 , L and C achieve resonance as is illustrated in  FIG. 16B . As the frequency is gradually raised toward the resonance frequency from the value at which no resonance takes place, the vibration phase of the vibrator  159  changes with respect to the phase of vibration of the piezoelectric elements  120   a  and  120   b . When the resonance starts, the phase reaches π/2. As the frequency is further raised, the phase reaches π. If the frequency is raised even further, the phase starts decreasing. When the frequency comes out of the resonance region, the phase becomes equal to the phase where no resonance undergoes at low frequencies. In the actual situation, however, the vibration state does not become ideal. The phase does not change to π in some cases. Nonetheless, the drive frequency can be set to the resonance frequency. 
     Support areas  191  existing at the four corners, which are shown in  FIG. 5A  and  FIG. 8 , are areas in which virtually no vibration takes place. Therefore, when pushed in Z direction with an external force, these parts hold the dust filter  119  through the cushion members  156  that are made of vibration-attenuating material such as rubber. So held, the dust filter  119  can be reliably supported without attenuating the vibration, because the lip part  150   a  of the seal  150  displaces only a little and the pushing force of the seal  150  does not increase. Moreover, the lip part  150   a  reliably restores its initial shape it is released from the external force. Made of rubber or the like, the cushion members  153  can allow the dust filter  119  to vibrate in plane and never attenuate the in-plane vibration of the dust filer. The user may remove the exchange lens and may then remove fine dust particles from the surface of the dust filter  119 , using a cleaning device. While being so cleaned, the dust filter  119  may receive an external force. In this case, the external force would act directly on the seal  150 , twisting the seal  150 , if the supporting/pushing structure had not the configuration according to this embodiment. Even after released from the external force, the lip part  150   a  of the seal  150  should remain deformed, not restoring its initial shape. The dust filter  119  most be cleaned for the following reason. That is, fine dust particles and fine liquid particles cannot be removed by vibrating the dust filter  119 , as will be explained later. Many fine dust particles remaining on the dust filter  119  lower the transmittance the dust filter  119  has with respect to a focusing-beam, as will be explained later. Hence, the surface of the dust filter  119  must be cleaned if it is excessively unclean with fine dust particles or fine liquid particles. 
     On the other hand, the seal  150  must be provided in the area having vibrational amplitude, too. In the vibrational mode of the present invention, the peripheral vibrational, amplitude is small. In view of this, the lip part  150   a  of the seal  150  holds the circumferential part of the dust filter  119  and receives a small pressing force. As a result, the force does not greatly act in the amplitude direction of bending vibration. Therefore, the seal  156  attenuates, but very little, the vibration whose amplitude is inherently small. As shown in  FIG. 5A ,  FIG. 7B  and  FIG. 8 , as many seal-contact parts  190  as possible contact the node areas  176  in which the vibrational amplitude is small. This further reduces the attenuation of vibration. If this support configuration is employed, the hindrance of vibration will greatly change because of the material of the seal  150  and the deviation of the pushing force and size of the seal  150  from the specifications. Further, visual inspection can hardly determine whether the seal  150  has been distortedly assembled into the vibrating device. In the present embodiment, the voltage collated with the vibrational amplitude (or vibration speed) pertaining to the dust filter  119  can be easily detected. Any defective product can therefore be easily detected. 
     The prescribed frequency at which to vibrate the piezoelectric elements  120   a  and  120   b  is determined by the shape, dimensions, material and supported state of the dust filter  119 , which is one component of the vibrator  159 . In most cases, the temperature influences the elasticity coefficient of the vibrator  159  and is one of the factors that change the natural frequency of the vibrator  159 . Therefore, it is desirable to measure the temperature of the vibrator  159  and to consider the change in the natural frequency of the vibrator  159 , before the vibrator  159  is used. A temperature sensor (not shown) is therefore connected to a temperature measuring circuit (not shown), in the digital camera  10 . The value by which to correct the vibrational frequency of the vibrator  159  in accordance with the temperature detected by the temperature sensor is stored in the nonvolatile memory  128 . Then, the measured temperature and the correction value are read into the Bucom  101 . The Bucom  101  calculates a drive frequency, which is used as drive frequency of the dust filter control circuit  121 . Thus, vibration can be produced, which is efficient with respect to temperature changes, as well. 
     The dust filter control circuit  121  of the digital camera  10  according to this invention will be described below, with reference to  FIGS. 17 and 18 . The dust filter control circuit  121  has such a configuration as shown in  FIG. 17 . The components of the dust filter control circuit  121  produce signals (Sig 1  to Sig 4 ) of such waveforms as shown in the timing chart of  FIG. 18 . These signals will control the dust filter  119 , as will be described below. 
     More specifically, as shown in  FIG. 17 , the dust filter control circuit  121  comprises a N-scale counter  192 , a half-frequency dividing circuit  193 , an inverter  194 , a plurality of MOS transistors Q 00 , Q 01  and Q 02 , a transformer  195 , and a resistor R 00 . 
     The dust filter control circuit  121  is so configured that a signal (Sig 4 ) of the prescribed frequency is produced at the secondary winding of the transformer  195  when MOS transistors Q 01  and Q 02  connected to the primary winding of the transformer  195  are turned on and off. The signal of the prescribed frequency drives the piezoelectric elements  120   a  and  120   b , thereby causing the vibrator  159 , to which the dust filter  119  is secured, to produce a resonance standing wave. 
     The Bucom  101  has two output ports P_PwCont and D_NCnt provided as control ports, and a clock generator  196 . The output ports P_PwCont and P_NCnt and the clock generator  196  cooperate to control the dust filter control circuit  121  as follows. The clock generator  196  outputs a pulse signal (basic clock signal) having a frequency much higher than the frequency of the signal that will be supplied to the piezoelectric elements  120   a  and  120   b . This output signal is signal Sig 1  that has the waveform shown in the timing chart of  FIG. 18 . The basic clock signal is input to the N-scale counter  192 . 
     The N-scale counter  192  counts the pulses of the pulse signal. Every time the count reaches a prescribed value “N,” the N-scale counter  192  produces a count-end pulse signal. Thus, the basic clock signal is frequency-divided by N. The signal the N-scale counter  192  outputs is signal Sig 2  that has the waveform shown in the timing chart of  FIG. 18 . 
     The pulse signal produced by means of frequency division does not have a duty ratio of 1:1. The pulse signal is supplied to the half-frequency dividing circuit  193 . The half-frequency dividing circuit.  193  changes the duty ratio of the pulse signal to 1:1. The pulse signal, thus changed in terms of duty ratio, corresponds to signal Sig 3  that has the waveform shown in the timing chart of  FIG. 18 . 
     While the pulse signal, thus changed in duty ratio, is high, MOS transistor Q 01  to which this signal has been input is turned on. In the meantime, the pulse signal is supplied via the inverter  194  to MOS transistor Q 02 . Therefore, while the pulse signal (signal Sig 3 ) is low state, MOS transistor Q 02  to which this signal has been input is turned on. Thus, the transistors Q 01  and Q 02 , both connected to the primary winding of the transformer  195 , are alternately turned on. As a result, a signal Sig 4  of such frequency as shown in  FIG. 18  is produced in the secondary winding of the transformer  195 . 
     The winding ratio of the transformer  195  is determined by the output voltage of the power-supply circuit  135  and the voltage needed to drive the piezoelectric elements  120   a  and  120   b . Note that the resistor R 00  is provided to prevent an excessive current from flowing in the transformer  195 . 
     In order to drive the piezoelectric elements  120   a  and  120   b , MOS transistor Q 00  must be on, and a voltage must be applied from the power-supply circuit  135  to the center tap of the transformer  195 . In this case, MOS transistor Q 00  is turned on or off via the output port P_PwCont of the Bucom  101 . Value “N” can be set to the N-scale counter  192  from the output port D_NCnt of the Bucom  101 . Thus, the Bucom  101  can change the drive frequency for the piezoelectric elements  120   a  and  120   b , by appropriately controlling value “N.” 
     The frequency can be calculated by using Equation 7, as follows: 
                   fdrv   =     fpls     2   ⁢           ⁢   N               (   7   )               
where N is the value set to the N-scale counter  192 , fpls is the frequency of the pulse output from the clock generator  196 , and fdrv is the frequency of the signal supplied to the piezoelectric elements  120   a  and  120   b.  
 
     The calculation based on Equation 7 is performed by the CPU (control unit) of the Bucom  101 . 
     If the dust filter  119  is vibrated at a frequency in the ultrasonic region (i.e., 20 kHz or more), the operating state of the dust filter  119  cannot be aurally discriminated, because most people cannot hear sound falling outside the range of about 20 to 20,000 Hz. This is why the operation display LCD  129  or the operation display LED  130  has a display unit for showing how the dust filter  119  is operating, to the operator of the digital camera  10 . More precisely, in the digital camera  10 , the vibrating members (piezoelectric elements  120   a  and  120   b ) imparts vibration to the dust-screening member (dust filter  119 ) that is arranged in front of the CCD  117 , can be vibrated and can transmit light. In the digital camera  10 , the display unit is operated in interlock with the vibrating member drive circuit (i.e., dust filter control circuit  121 ), thus informing how the dust filter  119  is operating (later described in detail). 
     To explain the above-described characteristics in detail, the control the Bucom  101  performs will be described with reference to  FIGS. 19A to 23 .  FIGS. 19A and 19B  show the flowchart that relates to the control program, which the Bucom  101  starts executing when the power switch (not shown) provided on the body unit  100  of the camera  10  is turned on. 
     First, a process is performed to activate the digital camera  10  (Step S 101 ). That is, the Bucom  101  control the power-supply circuit  135 . So controlled, the power-supply circuit  135  supplies power to the other circuit units of the digital camera  10 . Further, the Bucom  101  initializes the circuit, components. 
     Next, the Bucom  101  calls a sub-routine “silent vibration,” vibrating the dust filter  119 , making no sound (that is, at a frequency falling outside the audible range) (Step S 102 ). The “audible range” ranges from about 200 to 20,000 Hz, because most people can hear sound falling within this range. 
     Steps S 103  to S 124 , which follow, make a group of steps that is cyclically repeated. That is, the Bucom  101  first detects whether an accessory has been attached to, or detached from, the digital camera  10  (Step S 103 ). Whether the lens unit  200  (i.e., one of accessories), for example, has been attached to the body unit  100  is detected. This detection, e.g., attaching or detaching of the lens unit  200 , is performed as the Bucom  101  communicates with the Lucom  201 . 
     If a specific accessory is detected to have been attached to the body unit  100  (YES in Step S 104 ), the Bucom  101  calls a subroutine “silent vibration” and causes the dust filter  119  to vibrate silently (Step S 105 ). 
     While an accessory, particularly the lens unit  200 , remains not attached to the body unit  100  that is the camera body, dust is likely to adhere to each lens, the dust filter  119 , and the like. It is therefore desirable to perform an operation of removing dust the time when it is detected that the lens unit  200  is attached to the body unit  100 . It is highly possible that dust adheres as the outer air circulates in the body unit  100  at the time a lens is exchanged with another. It is therefore advisable to remove dust when a lens is exchange with another. Then, it is determined that photography will be immediately performed, and the operation goes to Step S 106 . 
     If a specific accessory is not detected to have been attached to the body unit  100  (NO in Step S 104 ), the Bucom  101  goes to the next step, i.e., Step S 106 . 
     In Step S 106 , the Bucom  101  detects the state of a specific operation switch that the digital camera  10  has. 
     That is, the Bucom  101  determines whether the first release switch (not shown), which is a release switch, has been operated from the on/off state of the switch (Step S 107 ). The Bucom  101  reads the state. If the first release switch has not been turned on for a predetermined time, the Bucom  101  discriminates the state of the power switch (Step S 108 ). If the power switch is on, the Bucom  101  returns to Step S 103 . If the power switch is off, the Bucom  101  performs an end-operation (e.g., sleep). 
     On the other hand, the first release switch may be found to have been turned on in Step S 107 . In this case, the Bucom  101  acquires the luminance data about the object, from the photometry circuit  115 , and calculates from this data an exposure time (Tv value) and a diaphragm value (Av value) that are optimal for the image acquisition unit  106  and lens unit  200 , respectively (Step S 109 ). 
     Thereafter, the Bucom  101  acquires the detection data from the AF sensor unit  109  through the AF sensor drive circuit  110 , and calculates a defocus value from the detection data (Step S 110 ). The Bucom  101  then determines whether the defocus value, thus calculated, falls within a preset tolerance range (Step S 111 ). If the defocus value does not fall within the tolerance range, the Bucom  101  drives the photographic lens  202  (Step S 112 ) and returns to Step S 103 . 
     On the other hand, the defocus value may falls within the tolerance range. In this case, the Bucom  101  determines whether the second release switch (not shown), which is another release switch, has been operated (Step S 114 ). If the second release switch is on, the Bucom  101  goes to Step S 115  and starts the prescribed photographic operation (later described in detail). If the second release switch is off, the Bucom  101  returns to Step S 108 . 
     During the image acquisition operation, the electronic image acquisition is controlled for a time that corresponds to the preset time for exposure (i.e., exposure time), as in ordinary photography. 
     As the above-mentioned photographic operation, Steps S 115  to S 121  are performed in a prescribed order to photograph an object. First, the Bucom  101  transmits the Av value to the Lucom  201 , instructing the Lucom  201  to drive the diaphragm  203  (Step S 115 ). Thereafter, the Bucom  101  moves the quick return mirror  105  to the up position (Step S 116 ). Then, the Bucom  101  causes the front curtain of the shutter  108  to start running, performing open control (Step S 117 ). Further, the Bucom  101  makes the image process controller  126  perform “image acquisition operation” (Step S 118 ). When the exposure to the CCD  117  (i.e., photography) for the time corresponding to the Tv value ends, the Bucom  101  causes the rear curtain of the shutter  108  to start running, achieving CLOSE control (Step S 119 ). Then, the Bucom  101  drives the quick return mirror  105  to the down position and cocks the shutter  108  (Step S 120 ). 
     Then, the Bucom  101  instructs the Lucom  210  to move the diaphragm  203  back to the open position (Step S 121 ). Thus, a sequence of image acquisition steps is terminated. 
     Next, the Bucom  101  determines whether the recording medium  127  is attached to the body unit  100  (Step S 122 ). If the recording medium.  127  is not attached, the Bucom  101  displays an alarm (Step S 123 ). The Bucom  101  then returns to Step S 103  and repeats a similar sequence of steps. 
     If the recording medium  127  is attached, the Bucom  101  instructs the image process controller  126  to record the image data acquired by photography, in the recording medium  127  (Step S 124 ). When the image data is completely recorded, the Bucom  101  returns to Step S 103  again and repeats a similar sequence of steps. 
     In regard to the detailed relation between the vibration state and the displaying state will be explained in detail, the sequence of controlling the “silent vibration” subroutine will be explained with reference to  FIGS. 20 to 23 . The term “vibration state” means the state of the vibration induced by the piezoelectric elements  120   a  and  120   b , i.e., vibrating members.  FIG. 24  shows the form of a resonance-frequency wave that is continuously supplied to the vibrating members during silent vibration. The subroutine of  FIG. 20 , i.e., “silent vibration,” and the subroutine of  FIGS. 21 to 23 , i.e., “display process” are routines for accomplishing vibration exclusively for removing dust from the dust filter  119 . Vibrational frequency f 0  is set to a value close to the resonance frequency of the dust filter  119 . In the vibrational mode of  FIG. 5A , for example, the vibrational frequency is 91 kHz, higher than at least 20 kHz, and produces sound not audible to the user. 
     As shown in  FIG. 20 , when the “silent vibration.” is called, the Bucom  101  first reads the data representing the drive time (Toscf 0 ) and drive frequency (resonance frequency: Noscf 0 ) from the data stored in a specific area of the nonvolatile memory  128  (Step S 201 ). At this timing, the Bucom  101  causes the display unit provided in the operation display LCD  129  or operation display LED  130  to turn on the vibrational mode display, as shown in  FIG. 21  (Step S 301 ). The Bucom  101  then determines whether a predetermined time has passed (Step S 302 ). If the predetermined time has not passed, the Bucom  101  makes the display unit keep turning on the vibrational mode display. Upon lapse of the predetermined time, the Bucom  101  turns off the displaying of the vibrational mode display (Step S 303 ). 
     Next, the Bucom  101  outputs the drive frequency Noscf 0  from the output port D_NCnt to the N-scale counter  192  of the dust filter control circuit  121  (Step S 202 ). 
     In the following steps S 203  to S 205 , the dust is removed as will be described below. First, the Bucom  101  sets the output port P_PwCont to Nigh, thereby starting the dust removal (Step S 203 ). At this timing, the Bucom  101  starts displaying the vibrating operation as shown in  FIG. 22  (Step S 311 ). The Bucom  101  then determines whether or riot the predetermined time has passed (Step S 312 ). If the predetermined time has not passed, the Bucom  101  keeps displaying the vibrating operation. Upon lapse of the predetermined time, the Bucom  101  stops displaying of the vibrating operation (Step S 313 ). The display of the vibrating operation, at this time, changes as the time passes or as the dust is removed (how it changes is not shown, though). The predetermined time is almost equal to Toscf 0 , i.e., the time for which the vibration (later described) continues. 
     If the output port P_PwCont is set to High in Step S 203 , the piezoelectric elements  120   a  and  120   b  vibrate the dust filter  119  at the prescribed vibrational frequency (Noscf 0 ), removing the dust  189  from the surface of the dust filter  119 . At the same time the dust is removed from the surface of the dust filter  119 , air is vibrated, producing an ultrasonic wave. The vibration at the drive frequency Noscf 0 , however, does not make sound audible to most people. Hence, the user hears nothing. The Bucom  101  waits for the predetermined time Toscf 0 , while the dust filter  119  remains vibrated (Step S 204 ). Upon lapse of the predetermined time Toscf 0 , the Bucom  101  sets the output port P_PwCont to Low, stopping the dust removal operation (Step S 205 ). At this timing, the Bucom  101  turns on the display unit, whereby the displaying of the vibration-end display is turned on (Step S 321 ). When the Bucom  101  determines (in Step S 322 ) that the predetermined time has passed, the displaying of the vibration-end display is turned off (Step S 323 ). The Bucom  101  then returns to the step next to the step in which the “silent vibration” is called. 
     The vibrational frequency f 0  (i.e., resonance frequency Noscf 0 ) and the drive time (Toscf 0 ) used in this subroutine define such a waveform as shown in the graph of  FIG. 24 . As can be seen from this waveform, constant vibration (f 0 =91 kHz) continues for a time (i.e., Toscf 0 ) that is long enough to accomplish the dust removal. 
     That is, the vibrational mode adjusts the resonance frequency applied to the vibration application unit  178 , controlling the dust removal. 
     In the first embodiment described above, whether normal vibration has been generated can easily and reliably determined even if the dust-screening member is miniaturized. This can provide a small vibrating device having high dust removal ability, which can be incorporated in various products without the necessity of adding special components after the inspection. 
     Further, the dust-screening member can undergo standing-wave vibration that can increase the vibrational amplitude to a maximum. 
     Still further, an intensity of a voltage signal of high level corresponding to the amplitude of the standing-wave vibration can be reliably acquired, because the vibrating member assumes symmetrical weight balance and also because the second vibration application part receiving vibration is asymmetrical with respect to the nodes of standing-wave vibration. 
     Second Embodiment 
     The subroutine “silent vibration” called in the camera sequence (main routine) that the Bucom performs in a digital camera that is a second embodiment of the image equipment according to this invention will be described with reference to  FIG. 25 .  FIG. 25  illustrates a modification of the subroutine “silent vibration” shown an  FIG. 20 . The second embodiment differs from the first embodiment in the operating mode of the dust filter  119 . In the first embodiment, the dust filter  119  is driven at a fixed frequency, i.e., frequency f 0 , producing a standing wave. By contrast, in the second embodiment, the drive frequency is gradually changed, thereby achieving large-amplitude vibration at various frequencies including the resonance frequency, without strictly controlling the drive frequency. 
     If the aspect ratio shown in  FIG. 10  has changed from the design value of 0.9, during the manufacture, the vibrational mode will greatly change (that is, the vibration speed ratio will abruptly change). Therefore, a precise resonance frequency must be set in each product and the piezoelectric elements  120   a  and  120   b  must be driven at the frequency so set. This is because the vibration speed will further decrease if the piezoelectric elements are driven at any frequency other than the resonance frequency. An extremely simple circuit configuration can, nonetheless, drive the piezoelectric elements precisely at the resonance frequency if the frequency is controlled as in the second embodiment. A method of control can therefore be achieved to eliminate any difference in resonance frequency between the products. 
     In the subroutine “silent vibration” of  FIG. 25 , the vibrational frequency f 0  is set to a value close to the resonance frequency of the dust filter  119 . The vibrational frequency f 0  is 91 kHz in, for example, the vibrational mode of  FIG. 5A . That is, the vibrational frequency exceeds at least 20 kHz, and makes sound not audible to the user. 
     First, the Bucom  101  reads the data representing the drive time (Toscf 0 ), drive-start frequency (Noscfs), frequency change value (Δf) and drive-end frequency (Noscft), from the data stored in a specific area of the nonvolatile memory  128  (Step S 211 ). At this timing, the Bucom  101  causes the display unit to display the vibrational mode as shown in  FIG. 21 , in the same way as in the first embodiment. 
     Next, the Bucom  101  sets the drive-start frequency (Noscfs) as drive frequency (Noscf) (Step S 212 ). The Bucom  101  then outputs the drive frequency (Noscf) from the output port D_NCnt to the N-scale counter  192  of the dust filter control circuit  121  (Step S 213 ). 
     In the following steps S 203  et seq., the dust is removed as will be described below. First, the dust removal is started. At this time, the display of the vibrating operation is performed as shown in  FIG. 22 , as in the first embodiment. 
     First, the Bucom  101  sets the output port P_PwCont to High, to achieve dust removal (Step S 214 ). The piezoelectric elements  120   a  and  120   b  vibrate the dust filter  119  at the prescribed vibrational frequency (Noscf), producing a standing wave of a small amplitude at the dust filter  119 . The dust  189  cannot be removed from the surface of the dust filter  119 , because the vibrational amplitude is small. This vibration continues for the drive time (Toscf 0 ) (Step S 215 ). Upon lapse of this drive time (Toscf 0 ), the Bucom  101  determines whether the drive frequency (Noscf) is equal to the drive-end frequency (Noscft) (Step  216 ). If the drive frequency is not equal to the drive-end frequency (NO in Step S 216 ), the Bucom  101  adds the frequency change value (Δf) to the drive frequency (Noscf), and sets the sum to the drive frequency (Noscf) (Step S 211 ). Then, the Bucom  101  repeats the sequence of Steps S 212  to S 216 . 
     If the drive frequency (Noscf) is equal to the drive-end frequency (Noscft) (YES in Step S 216 ), the Bucom  101  sets the output port P_PwCont to Low, stopping the vibration of the piezoelectric elements  120   a  and  120   b  (Step S 218 ), thereby terminating the “silent vibration.” At this point, the display of vibration-end is performed as shown in  FIG. 23 , as in the first embodiment. 
     As the frequency is gradually changed as described above, the amplitude or the standing wave increases. In view of this, the drive-start frequency (Ncoscfs), the frequency change value (Δf) and the drive-end frequency (Noscft) are set so that the resonance frequency of the standing wave may be surpassed. As a result, a standing wave of small vibrational amplitude is produced at the dust filter  119 . The standing wave can thereby controlled, such that its vibrational amplitude gradually increases until it becomes resonance vibration, and then decreases thereafter. If the vibrational amplitude (corresponding to vibration speed) is larger than a prescribed value, the dust  189  can be removed. In other words, the dust  189  can be removed while the vibrational frequency remains in a prescribed range. This range is broad in the present embodiment, because the vibrational amplitude is large during the resonance. 
     If the difference between the drive-start frequency (Noscfs) and the drive-end frequency (Noscft) is large, the fluctuation of the resonance frequency, due to the temperature of the vibrator  159  or to the deviation in characteristic change of the vibrator  159 , during the manufacture, can be absorbed. Hence, the dust  189  can be reliably removed from the dust filter  119 , by using an extremely simple circuit configuration. 
     Further, this circuit configuration should better be applied to the dust filter drive circuit  184 A of the vibration detector  184  shown in  FIG. 12 . If the dust filter drive circuit  184 A has this configuration, the frequency at which the vibration detection voltage is maximal becomes resonance frequency even if the resonance frequency of vibrational mode changes because the components of the vibrator  159  deviate from the specifications, in terms of the materials, sizes and assembling manner. The resonance frequency and the voltage at the resonance frequency can therefore be detected easily. 
     The second embodiment so configured can achieve almost the same advantages as the first embodiment described above. Moreover, the second embodiment can generate vibration of a large amplitude, without the necessity of strictly controlling the drive frequency, thereby reliably removing the dust. Further, even if the drive frequency changes as the ambient temperature changes, the control method need not be altered. 
     Third Embodiment 
       FIG. 26  is a diagram showing the configuration of the vibration detector used in a third embodiment of this invention. More precisely,  FIG. 26  shows the circuit configuration of the vibration detector  184 . The components identical to those of the first and second embodiments are designated by the same reference numbers and will, not be described. Only the characterizing features of the second embodiments will be described. 
     The circuit configuration of  FIG. 26  pertains to a camera that incorporates a vibration detector  184 . The configuration of  FIG. 26  differs from that of  FIG. 12  in that a switch  197  is used as a connection unit in place of the connection terminals  160   j  and  160   k  of the flex  160 . The switch  197  may be a mechanical switch or an electrical switch such as a transistor. The switch  197  can connect and disconnect the first circuit and the second circuit more easily and reliably than in the first and second embodiments, in which the first and second circuits are connected by soldering and not connected by performing no soldering. 
     The configuration of  FIG. 26  can easily switch two states, from one to the other, whereby the dust filter control circuit  121  drives both piezoelectric elements  120   a  and  120 , or the voltage detection circuit  184 B detects the voltage across one piezoelectric element, while the dust filter control circuit  121  is driving the other piezoelectric element. Hence, the circuit according to the first embodiment can set the frequency for the camera, if the resonance frequency is inferred from the vibration voltage detected by the voltage detection circuit  184 B and if the resonance frequency thus inferred is utilized. Furthermore, from the voltage level detected, it can be determined whether the vibration is appropriate or whether the vibrating device normally operates or not. 
     The third embodiment can achieve almost the same advantages as the first and second embodiments described above, and can further inspect, the vibrator  159  easily and reliably even after the vibrating device has been incorporated into the camera. 
     The present invention has been explained, describing some embodiments. Nonetheless, this invention is not limited to the embodiments described above. Various changes and modifications can, of course, be made within the scope and spirit of the invention. 
     For example, a mechanism that applies an air flow or a mechanism that has a wipe may be used in combination with the dust removal mechanism having the vibrating member, in order to remove the dust  189  from the dust filter  119 . 
     In the embodiments described above, the vibrating members are piezoelectric elements. The piezoelectric elements may be replaced by electrostrictive members or super magnetostrictive elements. 
     In order to remove dust more efficiently from the member vibrated, the member may be coated with an indium-tin oxide (ITO) film, which is a transparent conductive film, indium-zinc film, poly 3,4-ethylenedioxy thiophene film, surfactant agent film that is a hygroscopic anti-electrostatic film, siloxane-based film, or the like. In this case, the frequency, the drive time, etc., all related to the vibration, are set to values that accord with the material of the film. 
     Moreover, the optical LPF  118 , described as one embodiment of the invention, may be replaced by a plurality of optical LPFs that exhibit birefringence. Of these optical LPFs, the optical LPF located closest to the object of photography may be used as a dust-screening member (i.e., a subject to be vibrated), in place of the dust filter  119  shown in  FIG. 2A . 
     Further, a camera may does not have the optical LPF  118  of  FIG. 2A  described as one embodiment of the invention, and the dust filter  119  may be used as an optical element such as an optical LPF, an infrared-beam filter, a deflection filter, or a half mirror. 
     Furthermore, the camera may not have the optical LPF  118 , and the dust filter  119  may be replaced by the protection glass plate  142  shown in  FIG. 2A . In this case, the protection glass plate  142  and the CCD chip  136  remain free of dust and moisture, and the structure of  FIG. 2A  that supports and vet vibrates the dust filter  119  may be used to support and vibrate the protection glass plate  142 . Needless to say, the protection glass plate  142  may be used as an optical element such as an optical LPF, an infrared-beam filter, a deflection filter, or a half mirror. 
     The image equipment according to this invention is riot limited to the image acquisition apparatus (digital camera) exemplified above. This invention can be applied to any other apparatus that needs a dust removal function. The invention can be practiced in the form of various modifications, if necessary. More specifically, a dust moving mechanism according to this invention may be arranged between the display element and the light source or image projecting lens in an image projector. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.