Patent Abstract:
There is provided an optical apparatus that provides quick and precise TTL phase difference detection and pupil slicing focus detection. An optical apparatus includes a first optical element for splitting a first polarized light component contained in light that passes an exit pupil of a first optical system and directs to a photoelectric conversion element so that the first polarized light component direct to different light-receiving areas on the photoelectric conversion element. The optical apparatus may further include a second optical element for separating a second polarized light component contained in the light from said first optical element, from the first polarized light component.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an optical apparatus having an autofocus (“AF”) control using a photoelectric conversion element (“PCE”).  
         [0002]     A single lens reflex type digital still camera or digital single lens reflex (“DSLR”) camera adopts the so-called pupil slicing focus detection for precise focusing upon a quickly moving subject.  
         [0003]      FIG. 7  shows a schematic structure of a conventional DSLR camera system. When a photographer observes a subject through an eyepiece  104 , part of light  110  from the subject that transmits an image-taking lens  120  is reflected on a main mirror  101  and forms a subject image on a focusing glass  102  in a camera body  100 . The subject image formed on the focusing glass  102  is guided to the photographer&#39;s eye via a penta prism  103  and the eyepiece  104 .  
         [0004]     Part of the light  110  from the subject passes through the main mirror  101 , and is reflected on a sub-mirror  105  and guided to a focus detection unit  106 . The focus detection unit  106  includes a field lens, a mirror, a stop mask, a secondary imaging lens, and a light-receiving sensor. The light-receiving sensor receives the light that passes different pupil areas on the image-taking lens  120 , and an image signal is output from each of a pair of or plural pairs of line sensors in the light-receiving sensor. A focusing state (such as a defocus direction and a defocus amount) of the image-taking lens  120  can be detected based on a phase difference of this image signal. In addition, a driving direction and driving amount of a focus lens  123  in the image-taking lens  120  are calculated from the detected focusing state, and focus is obtained by driving the focus lens  123 .  
         [0005]     In the image-taking time, both the main mirror  101  and sub-mirror  105  retreat from the optical path and the light from the subject which has passed the image-taking lens  120  is guided to the image sensor  108 .  
         [0006]     The pupil slicing focus detection method of a through the taking lens (“TTL”) phase difference detection (“PDD”) requires a sensor dedicated for a focus detection and a secondary imaging optical system, and thus has difficulties in reducing a size and cost of the camera.  
         [0007]     Accordingly, a digital still camera has recently been proposed which utilizes a subject taking image sensor for the TTL PDD and pupil slicing focus detection. For example, Japanese Patent Application, Publication No. (“JP”) 9-43507 inserts, near a pupil of the image-taking lens, a mask that transmits the light from part of the pupil, and detects focus using a signal from the image sensor corresponding to two images by switching an opening position of the mask. Another focus detection system, proposed in JP 2004-46132, uses part of the image sensor as an AF sensor area, and guides to the area two lights split by a split image prism provided in the imaging optical system.  
         [0008]     JP 4-147207 discloses another structure for the TTL PDD AF by arranging a holographic optical element closer to the object side.  
         [0009]     However, the focus detection method proposed in JP 9-43507 changes the mask opening position, reads twice an image of the light that passes different pupil areas in the image-taking lens as an output (image signal) of the image sensor, compares these image signals. It takes a relatively long time to switch the mask opening position and to read two image signals from the image sensor. This focus detection method is rather unsuitable for a quickly moving subject.  
         [0010]     A method that uses a split image prism proposed in JP 2004-46132 needs a larger image than the split image prism, different from the TTL PDD. In addition, unless an image on the boundary of the split image prism has a linear shape, this method determines that it is defocused even if it is focused. Therefore, this method has a limited focus detection capability or is inferior to the TTL PDD.  
         [0011]     A method that uses the holographic optical element proposed in JP 4-147207 is similar to the TTL PDD in principle, but the holographic optical element possesses large color dispersion that causes an image to contain aberration in forming two AF images in the pupil slicing direction, which are important to a determination of the focusing state. Thus, this method is not practicable for the focus detection.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     Accordingly, it is an exemplary object of the present invention to provide an optical apparatus that provides quick and precise TTL PDD and pupil slicing focus detection.  
         [0013]     An optical apparatus according to one aspect of the present invention includes a first optical element for splitting a first polarized light component contained in light that passes an exit pupil of a first optical system and directs to a photoelectric conversion element so that the first polarized light component direct to different light-receiving areas on the photoelectric conversion element. The optical apparatus may further include a second optical element for separating a second polarized light component contained in the light from said first optical element, from the first polarized light component.  
         [0014]     An image-taking system according to another aspect of the present invention includes a lens unit that includes a first optical system, and the above optical apparatus that serves as an image-taking apparatus, onto which said lens unit is mounted.  
         [0015]     Other objects and further features of the present invention will become readily apparent from the following description of the preferred embodiments with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic view of a structure of a camera system according to a first embodiment of the present invention.  
         [0017]      FIG. 2  is a schematic view of an optical path according to the first embodiment.  
         [0018]      FIGS. 3A  to  3 D are views for explaining a structure and manufacturing method of an optical deflector according to the first embodiment.  
         [0019]      FIGS. 4A and 4B  are schematic views of optical paths for a focus detection and for finder observation according to the first embodiment.  
         [0020]      FIG. 5  is an explanatory view showing a focus detection principle according to the first embodiment.  
         [0021]      FIG. 6  is a schematic view of a structure of a camera system according to a second embodiment of the present invention.  
         [0022]      FIG. 7  is a schematic view of a structure of a conventional camera system.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Referring now to the accompanying drawings, a description will be given of a preferred embodiment of the present invention.  
       First Embodiment  
       [0024]      FIG. 1  shows a schematic structure of a DSLR camera system according to an embodiment of the present invention. Those elements in  FIG. 1 , which are the corresponding elements in  FIG. 7 , are designated by the same reference numerals.  
         [0025]      300  denotes a camera body (optical apparatus), onto which an image-taking or interchangeable lens  120  is mounted via a lens mount  121 .  
         [0026]     The image-taking lens  120  includes an image-taking optical system  125 , and a lens controller, such as a MPU (not shown) etc. The image-taking optical system  125  serves as a first optical system that includes plural lenses including a focus lens  123  and a stop  124 . The lens controller can communicate with a camera controller  309 , such as an MPU, provided in the camera body  300 , via a lens-side contact  122  and a camera-side contact  112 . The camera controller  309  has an image processing function that generates a subject image based on a signal from an image-pickup device  108 , which will be described later, and a focus detection function that detects a focusing state of the imaging-taking optical system  125  based on the signal from the image-pickup device  108 , and operates a driving amount of the focus lens  123 .  
         [0027]     In the camera body  300 ,  200  denotes an optical deflector that serves as a first optical element, and its concrete structure will be described later.  108  denotes an image-pickup device (or an image sensor) that serves as a PCE, such as a CCD sensor and a CMOS sensor. A polarization beam splitter (“PBS”)  301  that serves as a second optical element is provided between the optical deflector  200  and the image-pickup device  108 . An infrared extinction and low-pass filter  107  is provided just prior to the image-taking element  108 .  
         [0028]      102  denotes a focusing glass.  103  denotes a penta prism that introduces a subject image formed on the focusing glass  102  to the eyepiece  104 . The focusing glass  102 , the penta prism  103  and the eyepiece  104  form a finder optical system that serves as a second optical system.  
         [0029]      FIG. 2  denotes an optical path in detecting the focusing state of the image-taking optical system  125  or at the focus detection time in the above camera system.  
         [0030]     The light  110  from the subject passes the image-taking optical system  125 , and is incident upon the optical deflector  200 . This light  110  is a non-polarized light. The optical deflector  200  deflects and transmits, in the +y direction part, (referred to as “plus deflected light” hereinafter)  200  of a first polarized light component that has a polarization plane parallel to the zx plane among the light  110 . The PBS  301  serves to transmit the first polarized light component and reflect a second polarized light component having a polarization plane orthogonal to the zx plane (or parallel to the yz plane).  
         [0031]     Consequently, the plus deflected light  211  of the first polarized light component passes through the PBS  301 , and condenses in an upper light-receiving area of the image-pickup device  108  in  FIG. 2 . Part (referred to as “minus deflected light” hereinafter)  212  of the first polarized light component is deflected in the −y direction, passes through the PBS  301 , and condenses in a lower light-receiving area of the image-pickup device  108  in  FIG. 2 .  
         [0032]     The plus and minus deflected lights  211  and  212  among the first polarized light component which have reached the image-pickup device  108  are lights that have passed different areas of the exit pupil of the image-taking optical system  125 . Therefore, the plus and minus deflected lights  211  and  212  form a pair of images on the image-pickup device  108 , providing the pupil slicing focus detection based on the output or image signals from the image-pickup device  108  corresponding to the pair.  
         [0033]     On the other hand, the second polarized light component  210  contained in the light  110  travels straight and passes the optical deflector  200 , is reflected on the PBS  301  as the second optical element, and condenses on the focusing glass  102 , forming the subject image. Thereby, even in detecting focus using the first polarized light component including the plus and minus deflected lights  211  and  212 , a photographer can observe the subject image via the finder optical system.  
         [0034]     Referring now to  FIGS. 3A  to  3 D, a description will be given of a structure and manufacture method of the optical deflector  200 . The optical deflector  200  includes in order from a light incident side, as shown in  FIG. 3D , a resin substrate  201  having a blazed diffraction grating, liquid crystal  202  filled in the grooves or concaves of the diffraction grating, a polarization film  204  having a pair of openings  204   a  and  204   b  as shown in  FIG. 3C , and a glass substrate  203  adhered to the polarization film  204 .  
         [0035]     The resin substrate  201  includes, as shown in  FIG. 3A , first and second diffraction gratings  201   a  and  201   b  having different blazed directions from each other, and these blazed diffraction gratings  201   a  and  201   b  are manufactured by a molding method that uses the resin substrate  201  and a mold.  
         [0036]     The uniaxial light-transmitting liquid crystal  202  is filled, as shown in  FIG. 3B , in the grating grooves of each of both the diffraction gratings  201   a  and  201   b  in the resin substrate  201 . The material of the liquid crystal  202  is selected so that the ordinary index “no” of the liquid crystal  202  is approximately equal to the refractive index “ng” of the resin substrate  201 .  
         [0037]     The diffraction gratings  201   a  and  201   b  are oriented by applying an orientation film made of polyimide, etc. onto the surfaces of the diffraction gratings  201   a  and  201   b . Next, the liquid crystal  202  is cured by irradiating the ultraviolet (“UV”) light, after the solvent is vaporized by filling and heating the UV curing liquid crystal  202  in the grating grooves. The molecular axis of the liquid crystal polymer orientates approximately parallel to the grating groove direction (or x direction) of each of the diffraction gratings  201   a  and  201   b .  FIG. 3B  shows a molecular axis of the liquid crystal polymer in a stick shape. The molecular axis direction of the liquid crystal polymer approximately accords with the optical-axis direction relative to the polarized light.  
         [0038]     The glass substrate  203  pasted with the polarization film  204  is adhered to the side of the liquid crystal  202  of the resin substrate  201  in which the liquid crystal  202  is filled and cured in the grating grooves. The openings  204   a  and  204   b  in the polarization film  204  are arranged in areas corresponding to the diffraction gratings  201   a  and  201   b . The openings  204   a  and  204   b  are arranged symmetrical with respect to the yz plane including the optical axis (or z axis). The polarization film  204  is arranged so that its polarization or optical axis is approximately orthogonal to the optical-axis direction of the liquid crystal  202  filled in the grating grooves.  
         [0039]     Referring now to  FIGS. 4A and 4B , a description will be given of the pupil slicing focus detection while the finder optical system observes the subject image.  
         [0040]      FIGS. 4A and 4B  show a separation of the second polarized light component  210  used for the finder observation from the first polarized light component (plus deflected light  211  and minus deflected light  212 ) used for the focus detection.  
         [0041]      FIG. 4A  shows an optical path of a plane orthogonal to the zx plane that passes the center of the opening  204   a  in the polarization film  204  provided in the optical deflector  200 . The opening  204   a  in the polarization film  204  transmits a part (or first area) of the exit pupil in the image-taking optical system  125 , and receives the light that has passed the diffraction grating  201   a , i.e., the first and second polarized light components. The polarization axis direction of the polarization film  204  is set so that it absorbs the first polarized light component having the polarization plane parallel to the zx plane.  
         [0042]     The molecular axis of the liquid crystal  202  filled in the grating grooves in the diffraction grating  201   a  orientates approximately parallel to the grating groove direction or the x direction. An extraordinary index “ne” of the liquid crystal  202  and the refractive index “ng” of the resin substrate  201  have the following relationship: 
 
ne&gt;ng  (1) 
 
 The first polarized light component having the polarization plane approximately parallel to the molecular axis of the liquid crystal  202  is deflected in the +y direction as illustrated. Part of the first polarized light component that has transmitted the liquid crystal  202  is absorbed in the polarization film  204 , but the plus deflected light  211  of the first polarized light component that has passed the opening  204   a  in the polarization film  204  and the glass substrate  203  transmits the PBS  301  and reaches the upper light-receiving area in the image-pickup device  108  as illustrated. 
 
         [0043]     For example, when the resin substrate  201  has a refractive index ng of 1.5, the liquid crystal  202  has an extraordinary index ne of 1.7, and a necessary deflecting angle Φ is 8° for the plus deflected light  211 , a desired deflecting angle Φ is obtained by setting the grating pitch p of each of the diffraction gratings  201   a  and  201   b  to 4 μm, and an inclination angle θ of the resin substrate  201  to 35°.  
         [0044]      FIG. 4B  shows an optical path of a plane orthogonal to the zx plane that passes the center of the opening  204   b  in the polarization film  204  provided in the optical deflector  200 . The opening  204   b  in the polarization film  204  receives the light that has passed another part (or second area) of the exit pupil in the image-taking optical system  125 , i.e., the first and second polarized light components.  
         [0045]     The molecular axis of the liquid crystal  202  filled in the grating grooves in the diffraction grating  201   b  orientates approximately parallel to the grating groove direction or the x direction, and the refractive index ng of the resin substrate  201  and the extraordinary index ne of the liquid crystal  202  have the above relationship defined in Equation (1). Therefore, the first polarized light component having the polarization plane (parallel to the zx plane) approximately parallel to the molecular axis of the liquid crystal  202  is deflected in the −y direction in  FIG. 4A . Part of the first polarized light that transmits the liquid crystal  202  is absorbed in the polarized film  204 , but the minus deflected light  212  of the first polarized light component that has passed the opening  204   b  in the polarization film  204  and the glass substrate  203  transmits the PBS  301  and reaches the lower light-receiving area in the image-pickup device  108  as illustrated.  
         [0046]     The ordinary index “no” of the liquid crystal  202  and the refractive index “ng” of the resin substrate  201  have the following relationship: 
 
no≈ng  (2) 
 
 Therefore, the second polarized light component  210  having the polarization plane (parallel to the yz plane) orthogonal to the molecular axis of the liquid crystal  202  travels straight without being subject to the deflections by the diffraction gratings  201   a  and  201   b . The first polarized light component  210  is reflected by the PBS  301 , condenses on the focusing glass  102 , and forms the subject image. A photographer observes the subject image formed on the focusing glass  102  via the penta prism  103  and the eyepiece  104 . 
 
         [0047]     Referring now to  FIGS. 5A and 5B , a description will be given of a focus detection principle using the plus and minus deflected lights  211  and  212 .  FIG. 5A  shows an optical path when the image-taking optical system  125  is focused on a predetermined subject. Among the first polarized light component having a polarization plane parallel to the zx plane in  FIG. 5A  out of the light that passes the image-taking optical system  125  and enters the optical deflector  200 , the plus and minus deflected lights  211  and  212  that have passed the openings  204   a  and  204   b  in the polarization film  204  transmit the PBS  301 , which is omitted in  FIG. 5A , and image on the image-pickup device  108 .  FIG. 5D  shows image signals from predetermined two lines extending in the x direction in the image-pickup device  108 , which correspond to two images formed by the plus and minus deflected lights  211  and  212 .  
         [0048]     Although the plus and minus deflected lights  211  and  212  pass different areas on the exit pupil in the image-taking optical system  125 , an image (signal) A formed by the plus deflected light  211  that passes the opening  204   a  accords in the x direction on the image-pickup device  108  with an image (signal) B formed by the minus deflected light  212  that passes the opening  204   b , because the image-taking optical system  125  is focused on the subject.  
         [0049]      FIG. 5B  shows an optical path when the image-taking optical system  125  is in the front focus state to the subject or focused on a position before the subject. The plus and minus deflected lights  211  and  212  once image before the image-pickup device  108 , diverge, and reach the image-pickup device  108 .  FIG. 5E  shows an image signal from the above two lines corresponding to two images formed by the plus and minus deflected lights  211  and  212 .  
         [0050]     Since the image-taking optical system  125  is in the front focus state, the image (signal) A formed by the plus deflected light  211  that passes the opening  204   a  shifts from in the −x direction on the image-pickup device  108  with the image (signal) B formed by the minus deflected light  212  that passes the opening  204   b . Thus, the camera controller  309  shown in  FIG. 1  detects a defocus amount and a defocus direction (or front focus direction) of the image-taking optical system  125  based on the shifting direction and a positional relationship (or phase difference) between the images A and B.  
         [0051]      FIG. 5C  shows an optical path when the image-taking optical system  125  is in the so-called back focus state to the subject or focused on a position after the subject. The plus and minus deflected lights  211  and  212  divergently enter the image-pickup device  108 .  FIG. 5F  shows an image signal from the above two lines corresponding to two images formed by the plus and minus deflected lights  211  and  212 .  
         [0052]     Since the image-taking optical system  125  is in the back focus state, the image (signal) A formed by the plus deflected light  211  that passes the opening  204   a  shifts from in the +x direction on the image-pickup device  108  with the image (signal) B formed by the minus deflected light  212  that passes the opening  204   b . Thus, the camera controller  309  shown in  FIG. 1  detects a defocus amount and a defocus direction (or front focus direction) of the image-taking optical system  125  based on the shifting direction and a positional relationship (or phase difference) between the images A and B.  
         [0053]     Based on the detected defocus direction and defocus amount, the camera controller  309  operates a driving direction and a driving amount necessary for focusing of the focus lens  123  shown in  FIG. 1 , and controls driving of the focus lens  123  via the lens controller (not shown).  
         [0054]     The optical deflector  200  of this embodiment that has a polarization characteristic is affected by the subject when the subject has a polarization characteristic, the optical deflector  200  is affected. In order to cancel the polarization characteristic of the subject, it is preferable to provide a ½ wave plate at an incident side of the optical deflector  200 , and to rotate the polarization direction or the polarization plane of the incident light upon the optical deflector  200  if necessity arises.  
         [0055]     After all of the finder observation, the focus detection, and the AF control are thus completed, the optical deflector  200  and the PBS  301  are retreated from the optical path between the image-taking optical system  125  and the image-pickup device  108  in photographing the subject with the image-pickup device  108 .  
         [0056]     As discussed above, this embodiment detects focus using the image-pickup device while confirming the subject through the finder optical system, and dispenses with the sensor and optical system dedicated for detecting the focus, promoting the miniaturization and cost reduction of the camera.  
         [0057]     In addition, since two lights deflected by the optical deflector form two images on the image-pickup device, a single readout of an image signal from the image-pickup device provides focus detection, maintaining the precision of the focus detection even to a quickly moving the subject.  
         [0058]     Moreover, since the optical deflector deflects, in different directions, the lights that enter different pupil areas on the image-taking optical system, this embodiment can provide pupil slicing and TTL PDD focus detection based on the image formed by each defected light.  
         [0059]     The optical deflector is configured to split and deflect the first polarized light component to form the focus detection images while transmitting the second polarized light component. Thus, the second polarized light component forms a non-distorted finder image.  
         [0060]     The second polarized light component that transmits the optical deflector, is reflected on the PBS, and forms the finder image prevents drop of the light intensity of the finder image.  
         [0061]     The configuration that allows the PBS and the optical deflector to retreat from the optical path between the image-taking optical system and the image-pickup device eliminates from a shot subject image distortion or lowed light intensity caused by the PBS and optical deflector.  
         [0062]     The optical deflector includes the first and second blazed diffraction gratings having different blaze directions, uniaxial light-transmitting material member, and polarization film, and provides an inexpensive optical element used for the pupil slicing focus detection.  
       Second Embodiment  
       [0063]     While the first embodiment discusses the camera body that includes both the optical deflector  200  and the PBS  301 , the optical deflector  200  may be provided at or near the pupil position (a position of the stop  124 ) in the image-taking or interchangeable lens  420  as shown in  FIG. 6 .  
         [0064]     While the first and second embodiments discuss an interchangeable lens SLR camera system, the present invention is applicable to other camera systems, such as an integrated lens camera system.  
         [0065]     While the above embodiments discuss the optical deflector  200  that deflects the minus part in the +y direction and minus part in the −y direction among the first polarized light component, the present invention may deflect only one of parts and introduce the other to the image-pickup device  108  without deflecting the other. In this case, a pair of images are formed, for example, at the top and center of the image-pickup device.  
         [0066]     While the above embodiments discuss deflections of the plus and minus deflected lights  211  and  212  in the same direction as the slicing direction in the pupil area in the image-taking optical system which these lights pass, a deflecting direction of each deflected light is not limited to these embodiments.  
         [0067]     While the above embodiments discuss two images formed on the image-pickup device  108  using two deflected lights  211  and  212 , the number of deflected lights and deflecting directions may increase so as to form four or more images.  
         [0068]     While the above embodiments discuss use of the optical deflector  200  having a pair of blazed diffraction gratings, another element that serves to deflect the light and generate less aberration may form the optical deflector, in addition to the blazed diffraction grating. While the above embodiments discuss the optical deflector that includes the liquid crystal filled in the grating grooves in the diffraction grating, the inventive arrangement between the diffraction grating and the liquid crystal is not limited to these embodiments. For example, a member that encloses the liquid crystal between glass substrates may be arranged adjacent to the diffraction grating (or resin substrate).  
         [0069]     While the above embodiments discuss the optical deflector  200  that splits part of the incident light using the polarization characteristic and forms two images on the image-pickup device  108 , the inventive first optical element may use an optical deflector that splits part of the incident light by using an optical characteristic, such as a wavelength characteristic, other than the polarization characteristic.  
         [0070]     As discussed, the above embodiments split the first polarized light component incident upon the first optical element, and form a pair of images on the PCE, more quickly providing an image signal than switching the mask opening position. In addition, a pair of images have less aberration suitable for the focus detection than the split image prism and the holographic optical element. Thus, a fast and highly precise, TTL PDD and pupil slicing focus detection is available.  
         [0071]     A separation of a second light component, such as the second polarized light component, contained in the light from a first optical element, from a first light component, such as the first polarized light component, directing to the PCE, only a pair of images can be formed on the PCE while the second light component can be used for other applications, such as a finder observation.  
         [0072]     This application claims foreign priority benefits based on Japanese Patent Application No. 2004-294182, filed on Oct. 6, 2004, which is hereby incorporated by reference herein in its entirety as if fully set forth herein.

Technology Classification (CPC): 6