Focus detection apparatus

A focus detection apparatus comprises a field mask which has a rectangular opening, a first focus detection optical system which divides, in a longitudinal direction of the opening or a shorter side direction of the opening, the light beam, a second focus detection optical system which divides, in an oblique direction, the light beam, a plurality of first focus detection regions which respectively extend in the dividing direction by the first focus detection optical system within a frame on an expected imaging surface of the imaging lens, and a plurality of second focus detection regions which respectively extend in the dividing direction by the second focus detection optical system within the frame that receives the light beam divided by the second focus detection optical system, the plurality of second focus detection regions being aligned in the longitudinal direction or the shorter side direction within the frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a focus detection apparatus.

2. Description of the Related Art

As image capturing apparatuses represented by a digital camera and a video camera prevail, there has been an increasing demand for higher quality and downsizing of the image capturing apparatuses. More specifically, it has been desired to achieve higher precision and downsizing of a focus detection apparatus which detects the focus state of an imaging lens (imaging optical system) of an image capturing apparatus. In recent years, a focus detection apparatus which utilizes a TTL phase difference detection method for detecting the focus state of the imaging lens based on a relative positional relationship between a plurality of optical images formed by light beams passing through a plurality of regions obtained by dividing the pupil of the imaging lens is becoming the mainstream.

Such a focus detection apparatus described above forms a pair of images on a pair of light-receiving element arrays using a pair of lens portions formed on the incident surface of a secondary imaging lens, and a pair of prism portions formed on the exit surface. As an imaging lens goes out of focus, the pair of images move in a correlation direction as a direction in which a pair of openings are aligned. It is, therefore, possible to provide a focus detection apparatus that can detect a focus in a wide range within a photographing frame by arranging a plurality of focus detection regions in a direction perpendicular to the correlation direction. In, for example, Japanese Patent Laid-Open No. 2007-139935, a plurality of focus detection regions are arranged in the horizontal and vertical directions with respect to a photographing frame.

Japanese Patent Laid-Open No. 11-281877 describes a focus position detection apparatus, in which the first and second cross sensors which form an angle of about 45° with respect to each other are provided in the center of a frame.

As in Japanese Patent Laid-Open No. 2007-139935, arranging a plurality of focus detection regions in a direction perpendicular to the correlation direction as a direction in which the pair of openings are aligned has been conventionally considered. However, arranging a plurality of focus detection regions in an oblique direction as described in Japanese Patent Laid-Open No. 11-281877 has not been considered.

With reference to Japanese Patent Laid-Open No. 2007-139935, to densely arrange a plurality of focus detection regions in an oblique direction as described in Japanese Patent Laid-Open No. 11-281877, a plurality of focus detection regions are preferably arranged in a direction perpendicular to the correlation direction.

On the other hand, objects such as buildings, artificial objects, or eyebrows often have contrast in the horizontal or vertical direction. It is, therefore, easier to recognize the contrast of an object when a focus detection region extends in the horizontal or vertical direction as compared with an oblique direction. A field mask limiting a light beam for focus detection generally has a shape suitable for focus detection in the horizontal or vertical direction.

SUMMARY OF THE INVENTION

In the present invention, focus detection regions in an oblique direction are efficiently arranged within a frame.

According to a first aspect of the present invention, there is provided a focus detection apparatus for dividing a light beam which has passed through an imaging lens, and detecting a focus state of the imaging lens in accordance with a relative positional relationship between a plurality of images formed by the divided light beams, comprising: a field mask which has a rectangular opening for limiting the light beam that has passed through the imaging lens; a first focus detection optical system which divides, in a longitudinal direction of the opening or a shorter side direction of the opening, the light beam limited by the opening; a second focus detection optical system which divides, in an oblique direction different from the longitudinal direction or the shorter side direction, the light beam limited by the opening; a plurality of first focus detection regions which respectively extend in the dividing direction by the first focus detection optical system within a frame on an expected imaging surface of the imaging lens that receives the light beam divided by the first focus detection optical system; and a plurality of second focus detection regions which respectively extend in the dividing direction by the second focus detection optical system within the frame that receives the light beam divided by the second focus detection optical system, the plurality of second focus detection regions being aligned in the longitudinal direction or the shorter side direction within the frame.

According to a second aspect of the present invention, there is provided a focus detection apparatus for dividing a light beam which has passed through an imaging lens, and detecting a focus state of the imaging lens in accordance with a relative positional relationship between a plurality of images formed by the divided light beams, comprising: a field mask which has a rectangular opening for limiting the light beam that has passed through the imaging lens; a first focus detection optical system which divides, in a first direction of the opening or a second direction of the opening, the light beam limited by the opening; a second focus detection optical system which divides, in a third direction different from the first direction or the second direction, the light beam limited by the opening; a plurality of first focus detection regions which respectively extend in the dividing direction by the first focus detection optical system within a frame on an expected imaging surface of the imaging lens that receives the light beam divided by the first focus detection optical system; and a plurality of second focus detection regions which respectively extend in the dividing direction by the second focus detection optical system within the frame that receives the light beam divided by the second focus detection optical system, the plurality of second focus detection regions being aligned in the first direction or the second direction within the frame.

DESCRIPTION OF THE EMBODIMENTS

The schematic arrangement of an image capturing apparatus1including a focus detection apparatus100according to an embodiment of the present invention will be explained usingFIG. 1.

The image capturing apparatus1has an imaging lens10and a camera main body, as shown inFIG. 1. The camera main body is configured such that the imaging lens10is detachable via a mount unit (not shown). The image capturing apparatus1serves as, for example, a single-lens reflex camera.

The imaging lens10is an exchangeable imaging lens for capturing objects, and has an imaging optical system including a focus adjustment lens (not shown). The focus (state) of imaging lens10is adjusted by a control unit80(to be described later) through the focus adjustment lens based on the result of focus detection processing by the focus detection apparatus100(to be described later). A lens barrel LB so as to be movable in the direction of an optical axis OA supports the imaging lens10. Note that the imaging lens10is not a component of the image capturing apparatus1when it is detached from the camera main body, but the imaging lens10is considered herein as a component of the image capturing apparatus1since it is essential to attach the imaging lens10to the camera main body when the focus detection apparatus100detects a focus.

The camera main body has a main mirror20, a finder optical system30, a sub-mirror40, an image sensor50, the focus detection apparatus100, and the control unit80.

The main mirror20is a semitransparent half mirror, or a movable mirror partly having a half mirror surface. The main mirror20reflects part of light which has passed through the imaging lens10, and guides the reflected light to the finder optical system30(to be described later) along an optical axis OA″. Also, the main mirror20transmits part of the light which has passed through the imaging lens10, and guides the transmitted light to the sub-mirror40(to be described later) along the optical axis OA.

The finder optical system30is used for observing an object to be captured. In other words, the finder optical system30provides a user with a pseudo image for observation equivalent to an image of an object to be captured. As shown inFIG. 1, the finder optical system30has a focusing glass32, a pentaprism34, and an eyepiece lens36.

The light from the imaging lens10, which has been reflected by the main mirror20, converges near the focusing glass32. The focusing glass32has a mat surface and Fresnel surface with a finder field formed thereon. The focusing glass32diffuses object light, and outputs the diffused light to the pentaprism34. The pentaprism34serves as an optical path transforming element which reflects, with a plurality of surfaces, the light diffused by the focusing glass32, and guides the reflected light to the eyepiece lens36. The eyepiece lens36is also simply called an eyepiece. The eyepiece lens36is configured such that the user can observe the finder field formed on the focusing glass32through the eyepiece lens36.

The sub-mirror40is arranged downstream of the main mirror20along the optical axis OA. The sub-mirror40reflects the light (transmitted light) which has been transmitted by the main mirror20, and guides the reflected light to the focus detection apparatus100along an optical axis OA′. The optical axis OA′ is deflected from the optical axis OA by the sub-mirror40. The sub-mirror40is configured to be able to be inserted/removed in/from an image capturing optical path (the optical axis OA). The sub-mirror40is arranged at a predetermined position on the image capturing optical path (the optical axis OA) in observing the finder, and is removed from the image capturing optical path (the optical axis OA) in capturing an image.

The image sensor50has a pixel arrangement with a plurality of regularly arranged pixels. The image sensor50converts an image of an object formed on an imaging plane (pixel arrangement) by the imaging lens10into an image signal. The image sensor50includes, for example, an area (two-dimensional) sensor which performs photoelectric conversion on received light for each pixel, accumulates charges corresponding to the received light amount, and reads out the charges. The image sensor50may include, for example, a CMOS image sensor or CCD image sensor. Note that a signal output from the image sensor50undergoes predetermined processing in an image processing circuit (not shown) to be image data, and is then converted into image data for recording. After that, the image data for recording is recorded in a recording medium (not shown) such as a semiconductor memory, optical disk, and magnetic tape.

The focus detection apparatus100detects the focus state of the imaging lens10using a phase difference detection method. In other words, the focus detection apparatus100divides light which has passed through the imaging lens10and has been reflected by the sub-mirror40, and detects the focus state of the imaging lens10according to the relative positional relationship between a plurality of images formed by the divided light beams. That is, the focus detection apparatus100detects the focus state of the imaging lens10based on signals obtained by forming a plurality of pairs of images and performing photoelectric conversion on each pair of images.

More specifically, as shown inFIGS. 2 and 3, the focus detection apparatus100includes a field mask110, a field lens111, a filter113, a multi-aperture stop114, a reimaging lens unit (secondary optical system)115, and a focus detection sensor116along the optical axis OA′ in the order named.

As shown inFIG. 4, the field mask110has, at its center, a rectangular opening110afor limiting a light beam which has passed through the imaging lens10. The field mask110is arranged near the expected imaging surface of the imaging lens10.

The field lens111is arranged downstream of the field mask110along the optical axis OA′. The field lens111includes a lens portion111awith an optical effect. The lens portion111acorresponds to the opening110aof the field mask110in the field lens111.

The filter113blocks light having a wavelength longer than that of near infrared light. The filter113is adapted for detection of the focus of the imaging lens10which has undergone aberration compensation with respect to visible light, and prevents unwanted infrared light from entering the focus detection sensor116(to be described later).

The multi-aperture stop114includes a thin plate, and is arranged downstream of the filter113along the optical axis OA′ to be adjacent to the filter113. As shown inFIG. 5, the multi-aperture stop114has, at its center, a pair of openings114av1and114av2aligned in the longitudinal direction (Y direction) of the opening110a, and a pair of openings114ah1and114ah2aligned in the shorter side direction (X direction) of the opening110a. The multi-aperture stop114also has, at its center, a pair of openings114as1and114as2aligned in the 45° left oblique direction, and a pair of openings114ad1and114ad2aligned in the 45° right oblique direction. Note that an opening114acollectively represents the openings114av1,114av2,114ah1,114ah2,114as1,114as2,114ad1, and114ad2.

The reimaging lens unit115includes a first focus detection optical system FD1and a second focus detection optical system FD2. The first focus detection optical system FD1divides a light beam limited by the opening110aof the field mask110in at least one of the longitudinal direction (Y direction) of the opening110aand the shorter side direction (X direction) of the opening110a. The second focus detection optical system FD2divides the light beam limited by the opening110aof the field mask110in an oblique direction which intersects the longitudinal direction of the opening110awithin a plane perpendicular to the optical axis of the light beam at an acute angle (for example, 45°). The reimaging lens unit115forms, on each element arrangement of a plurality of pairs of element arrangements of the focus detection sensor116arranged downstream of the optical axis OA′, an image (secondary image) obtained by reimaging an object image on the expected imaging surface, which has been formed by the imaging lens10. In each element arrangement of each pair of element arrangements, a plurality of focus detection elements are arranged in a predetermined direction, as will be described later. The reimaging lens unit115has prism portions and lens portions which correspond to four pairs of openings held by the multi-aperture stop114. Note thatFIG. 6Ashows the incident side of the reimaging lens unit andFIG. 6Bshows the exit side of the reimaging lens unit.

The reimaging lens unit115has the prism portions corresponding to the openings of the multi-aperture stop114on the incident side, as shown in FIG.6A. The reimaging lens unit115has, at its center, a pair of prism portions1151av1and1151av2aligned in the longitudinal direction (Y direction) of the opening110a, and a pair of prism portions1151ah1and1151ah2aligned in the shorter side direction (X direction) of the opening110a. The pair of the prism portions1151av1and1151av2divide the light beam limited by the opening110ain the longitudinal direction (Y direction) of the opening110a. The pair of the prism portions1151ah1and1151ah2divide the light beam limited by the opening110ain the shorter side direction (X direction) of the opening110a. That is, the above first focus detection optical system FD1includes the pair of the prism portions1151av1and1151av2, and the pair of the prism portions1151ah1and1151ah2.

The reimaging lens unit115also has, at its center, a pair of prism portion1151as1and1151as2aligned in the 45° left oblique direction, and a pair of prism portions1151ad1and1151ad2aligned in the 45° right oblique direction. The 45° left oblique direction (oblique direction) is obtained by rotating the longitudinal direction of the opening110acounterclockwise by 45° when viewed from upstream to downstream of the optical axis OA′. The 45° right oblique direction (the second oblique direction) is obtained by rotating the longitudinal direction of the opening110aclockwise by 45° when viewed from upstream to downstream of the optical axis OA′, and is perpendicular to the 45° left oblique direction within a plane normal to the optical axis OA′. The pair of the prism portions1151as1and1151as2divide the light beam limited by the opening110ain the 45° left oblique direction. The pair of the prism portions1151ad1and1151ad2divide the light beam limited by the opening110ain the 45° right oblique direction. That is, the above second focus detection optical system FD2includes the pair of the prism portions1151as1and1151as2, and the pair of the prism portions1151ad1and1151ad2.

A prism portion1151acollectively represents the prism portions1151av1,1151av2,1151ah1,1151ah2,1151as1,1151as2,1151ad1, and1151ad2.

As shown inFIG. 6B, the reimaging lens unit115has the lens portions corresponding to the above-described prism portions on the exit side. Note that each lens portion has a spherical surface. The reimaging lens unit115has, at its center, a pair of lens portions1152av1and1152av2aligned in the longitudinal direction (Y direction) of the opening110a, and a pair of lens portions1152ah1and1152ah2aligned in the shorter side direction (X direction) of the opening110a. The pair of the lens portions1152av1and1152av2guide the light beams divided in the longitudinal direction (Y direction) of the opening110ato the focus detection sensor116. The pair of lens portions1152ah1and1152ah2guide the light beams divided in the shorter side direction (X direction) of the opening110ato the focus detection sensor116. That is, the above first focus detection optical system FD1includes the pair of the lens portions1152av1and1152av2, and the pair of the lens portions1152ah1and1152ah2.

The reimaging lens unit115also has, at its center, a pair of lens portions1152as1and1152as2aligned in the 45° left oblique direction, and a pair of lens portions1152ad1and1152ad2aligned in the 45° right oblique direction. The pair of the lens portions1152as1and1152as2guide the light beams divided in the 45° left oblique direction to the focus detection sensor116. The pair of the lens portions1152ad1and1152ad2guide the light beams divided in the 45° right oblique direction to the focus detection sensor116. That is, the above second focus detection optical system FD2includes the pair of the lens portions1152as1and1152as2, and the pair of the lens portions1152ad1and1152ad2.

SinceFIG. 6Ashows the incident side of the reimaging lens unit115andFIG. 6Bshows the exit side of the reimaging lens unit115, the above-described lens portions are shown by reversing the left and right. A lens portion1152acollectively represents the lens portions1152av1,1152av2,1152ah1,1152ah2,1152as1,1152as2,1152ad1, and1152ad2.

Next, the focus detection operation of the focus detection apparatus100will be explained. Note that subscripts1and2of the reference numerals inFIGS. 6A and 6Bindicate elements for forming a pair of object images in the focus detection apparatus using the phase difference detection method.

The light beam which has passed through the opening110aof the field mask110(the light beams limited by the opening110a) passes through the lens portion111aof the field lens111, and then enters the multi-aperture stop114via the filter113. The opening114aof the multi-aperture stop114is configured to undergo back projection near the exit pupil of the imaging lens10using the lens portion111aof the field lens111. Part of the light beam which has entered the opening110aof the field mask110surely reaches the opening114aof the multi-aperture stop114.

The openings114av1,114av2,114ah1, and114ah1of the multi-aperture stop114are arranged so as to be inscribed in substantially the same circle. The openings114as1,114as2,114ad1, and114ad2of the multi-aperture stop114are arranged so as to be inscribed in substantially the same circle which has the same center as that of the above-mentioned inscribed circle, and a diameter larger than that of the inscribed circle. With this arrangement, a light beam of the imaging lens10with higher brightness (a smaller F-number) reaches the openings114as1,114as2,114ad1, and114ad2, as compared with the other openings114av1,114av2,114ah1, and114ah1.

The light beam which has passed through the opening110aof the field mask110(the light beam limited by the opening110a) is guided to each prism portion and lens portion of the reimaging lens unit115arranged downstream of the multi-aperture stop114along the optical axis OA′. The opening110aof the field mask110is provided for a focus detection optical system having the lens portion111aof the field lens111, the opening114aof the multi-aperture stop114, and the prism portion1151aand the lens portion1152aof the reimaging lens unit115.

The light beam which has exited from the reimaging lens unit115enters the focus detection sensor116located downstream of the optical axis OA′. Four pairs of secondary images (that is, eight images) obtained by using the opening110aof the field mask110as an object image (optical image) are formed on the focus detection sensor116.

FIG. 7Ais a schematic plan view showing the focus detection sensor116on which object images have been formed. Referring toFIG. 7A, reference numerals117av1to117ad2denote optical images which are formed by the opening110aof the field mask110when the imaging lens10is in focus. Two optical images are formed for each opening of the field mask110by the action of each pair of openings (with subscripts1and2inFIG. 5) of the multi-aperture stop114, and the action of each pair of prism portions (with subscripts1and2inFIG. 6A) and each pair of lens portions (with subscripts1and2inFIG. 6B) of the reimaging lens unit115.

A pair of light beams which have been divided in the longitudinal direction (Y direction) of the opening110aby the first focus detection optical system FD1form a pair of optical images117av1and117av2. A pair of light beams which have been divided in the shorter side direction (X direction) of the opening110aby the first focus detection optical system FD1form a pair of optical images117ah1and117ah2. A pair of light beams which have been divided in the 45° left oblique direction by the second focus detection optical system FD2form a pair of optical images116as1and116as2. A pair of light beams which have been divided in 45° right oblique direction by the second focus detection optical system FD2form a pair of optical images116ad1and116ad2.

A pair of element arrangements116av1and116av2are arranged within the pair of the optical images117av1and117av2, respectively. In the pair of the element arrangements116av1and116av2, six pairs of focus detection elements116av1-1to116av1-6and116av2-1to116av2-6respectively extend in the longitudinal direction of the opening110a. Note that focus detection elements having the same number after a hyphen are paired. In each of the pair of the element arrangements116av1and116av2, a plurality of focus detection elements are aligned in the shorter side direction of the opening110a. This makes it possible to define a plurality of third focus detection areas115av-1to115av-6(seeFIG. 8A), which are aligned in the shorter side direction of the opening110awithin a frame on the expected imaging surface of the imaging lens10, and respectively extend in the longitudinal direction of the opening110a.

Similarly, a pair of element arrangements116ah1and116ah2are aligned within the pair of the optical images117ah1and117ah2, respectively. In the pair of the element arrangements116ah1and116ah2,10pairs of focus detection elements116ah1-1to116ah1-10and116ah2-1to116ah2-10respectively extend in the shorter side direction of the opening110a. Note that focus detection elements having the same number after a hyphen are paired. In each of the pair of the element arrangements116ah1and116ah2, a plurality of focus detection elements are aligned in the longitudinal direction of the opening110a. This makes it possible to define a plurality of fourth focus detection areas115ah-1to115ah-10(seeFIG. 8A) which are aligned in the longitudinal direction of the opening110awithin the frame on the expected imaging surface of the imaging lens10, and respectively extend in the shorter side direction of the opening110a.

That is, the focus detection sensor116includes the plurality of first focus detection elements116av1-1to116av1-6,116av2-1to116av2-6,116ah1-1to116ah1-10, and116ah2-1to116ah2-10shown inFIGS. 7B and 7C. The plurality of first focus detection elements receive the light beams divided by the first focus detection optical system FD1. Each of the first focus detection elements generates a signal (charge) for focus detection by performing photoelectric conversion on the received light. Each of the first focus detection elements is, for example, a photodiode. The plurality of first focus detection elements define a plurality of first focus detection regions which respectively extend in the dividing direction by the first focus detection optical system FD1within the frame on the expected imaging surface of the imaging lens, and are aligned at a predetermined interval. The plurality of first focus detection regions include a plurality of third focus detection areas115av-1to115av-6and a plurality of fourth focus detection areas115ah-1to115ah-10(seeFIG. 8A). This makes it possible to define, on the expected imaging surface of the imaging lens10, a plurality of focus detection regions (focus detection areas) which extend in a nearly checkerboard pattern. As a result, it becomes easy to improve the processing accuracy of focus detection independently of (the direction of) the spatial pattern of an object.

Likewise, a pair of element arrangements116as1and116as2are arranged within the pair of the optical images117as1and117as2, respectively. In the pair of the element arrangements116as1and116as2,10pairs of focus detection elements116as1-1to116as1-10and116as2-1to116as2-10respectively extend in the 45° left oblique direction. Note that focus detection elements having the same number after a hyphen are paired. In each of the pair of the element arrangements116as1and116as2, a plurality of focus detection elements are aligned in the longitudinal direction of the opening110a. This makes it possible to define a plurality of first focus detection areas115as-1to115as-10(seeFIG. 8B) which are aligned in the longitudinal direction of the opening110awithin the frame on the expected imaging surface of the imaging lens10, and respectively extend in the 45° left oblique direction.

A pair of element arrangements116ad1and116ad2are arranged within the pair of the optical images117ad1and117ad2, respectively. In the pair of the element arrangements116ad1and116ad2,10pairs of focus detection elements116ad1-1to116ad1-10and116ad2-1to116ad2-10respectively extend in the 45° right oblique direction. Note that focus detection elements having the same number after a hyphen are paired. In each of the pair of the element arrangements116ad1and116ad2, a plurality of focus detection elements are aligned in the longitudinal direction of the opening110a. This makes it possible to define a plurality of second focus detection areas115ad-1to115ad-10(seeFIG. 8B) which are aligned in the longitudinal direction of the opening110awithin the frame on the expected imaging surface of the imaging lens10, and respectively extend in the 45° right oblique direction.

That is, the focus detection sensor116includes the plurality of second focus detection elements116as1-1to116as1-10,116as2-1to116as2-10,116ad1-1to116ad1-10, and116ad2-1to116ad2-10shown inFIGS. 7D and 7E. The plurality of second focus detection elements receive the light beams divided by the second focus detection optical system FD2. Each of the second focus detection elements generates a signal (charge) for focus detection by performing photoelectric conversion on the received light. Each of the second focus detection elements is, for example, a photodiode. The plurality of second focus detection elements define a plurality of second focus detection regions which respectively extend in the dividing direction by the second focus detection optical system FD2within the frame on the expected imaging surface of the imaging lens. The plurality of second focus detection regions include the plurality of first focus detection areas115as-1to115as-10and the plurality of second focus detection areas115ad-1to115ad-10(seeFIG. 8B). This makes it possible to define a plurality of focus detection regions (focus detection areas) which extend in the 45° left and right oblique directions that intersect with each other, and are aligned in the longitudinal direction of the opening110a. By using the focus detection regions in combination with the above plurality of focus detection regions extending in a nearly grid shape, it becomes easier to improve the processing accuracy of focus detection independently of (the direction of) the spatial pattern of an object.

Each of the first and second focus detection elements may include an accumulation-type photoelectric conversion element. Each of the first and second focus detection elements extends almost linearly, as shown inFIGS. 7B to 7E. However, it may extend in a shape according to optical image distortion in order to correct distortion of a focus detection optical system. Furthermore, a pair of focus detection elements need not be arranged separately, and a linear array of focus detection elements may be used and divided into a plurality of groups in processing a signal.

In the focus detection apparatus100, with a change in position in the optical axis direction of an actual object image with respect to the expected imaging surface, optical images on the focus detection sensor116of an object within an optical image of the field mask110move closer to or away from each other. If, for example, the imaging lens10images light beams upstream of the expected imaging surface along the optical axis OA′, the pair of optical images formed on the focus detection sensor116move closer to one another. Alternatively, if the imaging lens10images light beams downstream of the expected imaging surface along the optical axis OA′, the pair of optical images formed on the focus detection sensor116move in the opposite directions, that is, away from one another. In this case, by arranging a plurality of focus detection elements in a direction in which the optical images move, the movement of the optical images is detected. Based on a detection result (an output from the focus detection sensor116), a known correlation calculation means calculates a relative position interval between the light amount distributions of the optical images.

If it is possible to calculate a change amount of the interval of a pair of optical images, it is also possible to obtain the defocus amount of the imaging lens10by approximating the relationship between the change amount and the defocus amount of the imaging lens10using a polynomial having the change amount as a variable, or the like. This allows to detect the focus (focus state) of the imaging lens10. In other words, calculating the interval of a pair of optical images enables to detect the focus (focus state) of the imaging lens10with respect to a plurality of focus detection regions defined as back projection images on the expected imaging surfaces of a plurality of focus detection elements.

Since the pair of the element arrangements116av1and116av2are aligned in the longitudinal direction of the opening110a, they are suitable for focus detection of an object having a contrast component in the longitudinal direction of the opening110a. On the other hand, since the pair of the element arrangements116ah1and116ah2are aligned in the shorter side direction of the opening110a, they are suitable for focus detection of an object having a contrast component in the shorter side direction of the opening110a. Combining the above element arrangements allows so-called cross-type focus detection. This makes a focus detection result less susceptible to a direction in which the object has a contrast component. As a result, it becomes easy to perform highly accurate focus detection.

In this embodiment, as described above, light beams which have passed through the openings114av1,114av2,114ah1, and114ah2of the multi-aperture stop114reach the element arrangements116av1,116av2,116ah1, and116ah2. As compared with focus detection using light beams which have passed through the other openings of the multi-aperture stop114, the brightness (F-number) of the imaging lens10has a smaller adverse effect, and a relatively large number of imaging lens can execute focus detection.

Since the pair of the element arrangements116as1and116as2are aligned in the 45° left oblique direction, they are suitable for focus detection of an object having a contrast component in the 45° left oblique direction. On the other hand, since the pair of the element arrangements116ad1and116ad2are aligned in the 45° right oblique direction, they are suitable for focus detection of an object having a contrast component in the 45° right oblique direction. Performing cross-type focus detection in an oblique direction in addition to the above-described cross-type focus detection makes a focus detection result less susceptible to a direction in which the object has a contrast component. As a result, it becomes easier to perform highly accurate focus detection.

In this embodiment, as described above, light beams which have passed through the openings114as1,114as2,114ad1, and114ad2of the multi-aperture stop114reach the element arrangements116as1,116as2,116ad1, and116ad2. As compared with focus detection using light beams which have passed through the other openings of the multi-aperture stop114, the imaging lens10needs to have higher brightness (a smaller F-number). However, since a change amount of the interval of a pair of optical images is large with respect to a defocus amount, it is possible to perform focus detection with higher accuracy.

Referring toFIGS. 8A and 8B, focus detection regions in the finder field will be explained.FIG. 8Ashows the relationship between the focus detection regions and the finder field when observing the surface of the focusing glass32of the image capturing apparatus1via the pentaprism34and the eyepiece lens36. The focusing glass32is arranged near the expected imaging surface of the imaging lens10, and can thus be considered as an expected imaging surface.

Referring toFIGS. 8A and 8B, reference numeral151denotes a finder field as an erect image within an image capturing range. The finder field roughly corresponds to the image capturing range. The image capturing range corresponds to a frame in the attached claims. The image capturing apparatus is generally configured so that the finder field has an almost rectangular shape whose long sides are parallel to the shorter side direction of the opening110aand whose short sides are parallel to the longitudinal direction of the opening110a. If a pair of element arrangements in the focus detection sensor116undergo to back projection on the expected imaging surface of the imaging lens10, their back projection images nearly coincide with other.

FIG. 8Ashows the finder field including a plurality of first focus detection regions which extend in the longitudinal and shorter side directions of the opening110a. In the finder field151, reference numerals115av-1to115ah-10denote a plurality of first focus detection regions which are defined as back projection images corresponding to the two pairs of the element arrangements (116av1,116av2,116ah1, and116ah2) of the focus detection sensor116. Since the focus detection regions115av-1to115ah-10are defined as back projection images of the focus detection elements on the focus detection sensor116, it is possible to detect the light amount distribution of an object having a shape extending in the longitudinal or shorter side direction of the opening110a.

Note that a focus detection region group115avcollectively represents the focus detection regions115av1to115av6. A focus detection region group115ahcollectively indicates the focus detection regions115ah1to115ah10.

In this embodiment, by paring two focus detection regions, the defocus amount of the focus detection regions is calculated. In this case, it is possible to perform a more highly accurate focus detection by shifting each of the two focus detection regions by half the pixel pitch of an element arrangement, that is, shifting the two focus detection regions to have a staggered arrangement, and calculating the interval of optical images based on the light amount distributions of an object obtained from the two focus detection regions.

The subscripts av to ah given to the focus detection regions correspond to those given to the element arrangements of the focus detection sensor116shown inFIGS. 7A to 7E. For example, the focus detection region115av-1is defined by back projection of the focus detection element116av1-1.

In the finder field151, the intersection of a pair of focus detection regions extending in the shorter side direction (represented by the subscript h) of the opening110aand a pair of focus detection regions extending in the longitudinal direction (represented by the subscript v) of the opening110ais defined as a focus detection point. As shown inFIG. 8A, reference symbols P1to P15denote focus detection points. A defocus amount at each focus detection point is calculated based on the light amount distributions of an object in the longitudinal and shorter side directions of the opening110a. In other words, the focus detection points P1to P15serve as a plurality of intersections in cross-shaped focus detection regions. That is, the focus detection points P1to P15serve as a plurality of intersections of a plurality of third focus detection areas extending in the longitudinal direction of the opening110aand a plurality of fourth focus detection area extending in the shorter side direction of the opening110a.

In this embodiment, the cross-shaped focus detection regions in the longitudinal and shorter side directions of the opening110ahave a total of 15 focus detection points in five rows and three columns in the center of the finder field151. It is, thus, possible to perform focus detection of an object corresponding to the focus detection regions. To perform focus detection at each focus detection point, a defocus amount is calculated using at least part of a corresponding focus detection region (focus detection area). That is, the interval of optical images is calculated by cutting and using at least part of an output from each element arrangement.

FIG. 8Bshows the finder field including a plurality of second focus detection regions extending in the 45° left and right oblique directions. In the finder field151, reference numerals115as-1to115ad-10denote a plurality of second focus detection regions which are defined as back projection images corresponding to the two pairs of the element arrangements (116as1,116as2,116ad1,116ad2) of the focus detection sensor116. Since the focus detection regions115as-1to115ad-10are defined as back projection images of the focus detection elements in the focus detection sensor116, it is possible to detect the light amount distribution of an object having a shape extending in the 45° left or right oblique direction.

Note that a focus detection region group115as collectively represents the focus detection regions115as1to115as10. A focus detection region group115adcollectively indicates the focus detection regions115ad1to115ad10.

In this embodiment, by paring two focus detection regions, the defocus amount of the focus detection regions is calculated.

The subscripts as to ad given to the focus detection regions correspond to those given to the element arrangements of the focus detection sensor116shown inFIGS. 7A to 7E. For example, the focus detection region115as-1is defined by back projection of the element arrangement116as1-1.

In the finder field151, the intersection of a pair of focus detection regions extending in the 45° right oblique direction (represented by the subscript s) and a pair of focus detection regions extending in the 45° left oblique direction (represented by the subscript d) is arranged so as to coincide with corresponding one of the focus detection points P6to P10. That is, the plurality of intersections P1to P15of the plurality of third focus detection areas and the plurality of fourth focus detection areas include the plurality of intersections P6to P10of the plurality of first focus detection areas and the plurality of second focus detection areas. A defocus amount at each focus detection point is calculated based on the light amount distributions of an object in the 45° right and left oblique directions. In other words, the focus detection points P6to P10serve as a plurality of intersections in cross-shaped focus detection regions in the 45° right and left oblique directions. The plurality of focus detection points P6to P10are aligned in the longitudinal direction of the opening110awithin the frame.

In this embodiment, the cross-shaped focus detection regions in the 45° right and left oblique directions have a total of five focus detection points in five rows and one column in the center of the finder field151. It is, thus, possible to perform focus detection of an object corresponding to the focus detection regions. To perform focus detection at each focus detection point, a defocus amount is calculated using at least part of a corresponding focus detection region (focus detection area). That is, the interval of optical images is calculated by cutting and using at least part of an output from each element arrangement.

In consideration of composition in capturing an image, it is necessary for the focus detection apparatus to have, in a wide range, a plurality of focus detection regions at positions where an object may be located at a high probability. It is also necessary for a focus detection region corresponding to each focus detection point to extend in more directions, and to acquire a light amount distribution irrespective of the direction of an object.

To satisfy these requirements, it is possible to make the frame larger by widening the opening110aof the field mask110shown inFIG. 2as much as possible, and thus to arrange the focus detection regions in a wider range. Within the image capturing range (the finder field151), however, as the opening of the finder field is widened, it becomes impossible to detect a focus since the optical images117av1to117ad2on the focus detection sensor116overlap with each other. It is possible to avoid this problem by providing an opening of the field mask in addition to the opening110a, and arranging it so as to have, as the center, a position as far away as possible from the intersection with the optical axis OA′. When light beams which have passed through the provided opening pass through an additionally provided subsequent focus detection optical system, it is possible to arrange the focus detection regions in a wider range. Alternatively, to avoid optical images from overlapping each other, the optical path of the focus detection optical system may be extended. With this method, however, downsizing is difficult in consideration of arranging a focus detection apparatus within an image capturing apparatus.

The size (dimensions) of the opening110aof the field mask110is limited. In this embodiment, under such limitation, the above requirements are satisfied by more densely arranging focus detection regions extending in more directions within the frame, as described above.

Another arrangement of the plurality of first focus detection regions according to this embodiment will now be explained.FIGS. 9A and 9Bshow an example in which cross-shaped focus detection regions are more densely arranged in the finder field151.FIG. 9Ashows the arrangement of the cross-shaped focus detection regions in the longitudinal and shorter side directions of the opening110a.FIG. 9Bshows the arrangement of the cross-shaped focus detection regions in the 45° left and right oblique directions.

Referring toFIG. 9A, the focus detection regions are densely arranged by respectively juxtaposing a plurality of first focus detection regions extending in the longitudinal and shorter side directions of the opening110ain directions perpendicular to the respective extending directions without any gap. Reference symbols Lv1to Lv6denote focus detection areas (third focus detection areas) extending in the longitudinal direction of the opening110a; and Lh1to Lh10, focus detection areas (fourth focus detection areas) extending in the shorter side direction of the opening110a. Each focus detection region (each focus detection area) is defined as the back projection image of a focus detection element, as described above. The intersection of a pair of focus detection regions extending in the shorter side direction (represented by the subscript h) of the opening110aof the finder field151and a pair of focus detection regions extending in the longitudinal direction (represented by the subscript v) of the opening110ais defined as a focus detection point, as described with reference toFIGS. 8A and 8B. In this case, it is possible to arrange a total of 15 focus detection points (black points inFIG. 9A) in a two-dimensional manner (in a matrix topology). A case in which the 15 focus detection points are arranged has been shown in this example. It is, however, possible to arrange more or less focus detection points in the cross-shaped focus detection regions in the longitudinal and shorter side directions of the opening110ain the same manner.

Referring toFIG. 9B, the focus detection regions are densely arranged by respectively juxtaposing a plurality of first focus detection regions extending in the 45° right and left oblique directions in directions perpendicular to the respective extending directions without any gap. Reference symbols Ld1to Ld6denote focus detection areas extending in the 45° left oblique direction; and Ls1to Ls10, focus detection areas extending in the 45° right oblique direction. Each focus detection region (each focus detection area) is defined as the back projection image of a focus detection element, as described above. The intersection of a pair of focus detection regions extending in the 45° left oblique direction (represented by the subscript d) of the finder field151and a pair of focus detection regions extending in the 45° right oblique direction (represented by the subscript s) is defined as a focus detection point, as described with reference toFIGS. 8A and 8B. In this case, it is possible to arrange a total of 15 focus detection points (black points inFIG. 9B) in a 45° oblique rectangle. A case in which the 15 focus detection points are arranged has been shown in this example. It is, however, possible to arrange more or less focus detection points in the cross-shaped focus detection regions in the 45° right and left oblique directions in the same manner.

In general, to select a focus detection point, the user uses a 4-way selector or the like. In consideration of this, it is easier to select a focus detection point if focus detection regions are aligned in the longitudinal and shorter side directions of the opening110aas shown inFIG. 9A, than if focus detection regions are alternately aligned in oblique directions as shown inFIG. 9B. The focus detection regions are used to acquire the light amount distribution of an object and calculate a defocus amount. At this time, many general objects such as people and buildings have a remarkable light amount distribution, that is, a high contrast in the vertical or horizontal direction. As described above, in consideration of arranging focus detection regions in a wider range within the image capturing range, the shape of the opening110aof the field mask110can roughly be formed by straight lines in the vertical and horizontal directions. The image capturing range is generally a rectangle formed by straight lines in the horizontal and vertical directions. Therefore, when the shape of the opening of the field mask is also formed by straight lines in the vertical and horizontal directions, it is possible to more densely arrange focus detection regions without wasting space. If the first focus detection regions are arranged as shown inFIG. 9B, the opening of the field mask is a 45° oblique rectangle. In this case, when providing another opening of the field mask, an efficient arrangement is difficult to implement.

From the above viewpoint, the cross-shaped focus detection regions (the plurality of first focus detection regions) are preferably arranged in the longitudinal and shorter side directions of the opening110a. In this embodiment, as described with reference toFIG. 8B, the intersections of the oblique cross-shaped focus detection regions (the plurality of second focus detection regions) are aligned in the longitudinal direction of the opening110ato overlap with the points P6to P10. This makes it possible to efficiently arrange the oblique focus detection regions within the range of the focus detection regions in the longitudinal and shorter side directions of the opening110a. This enables to arrange focus detection points at high density in a wide range of the focus detection regions, thereby simultaneously implementing detection of the light amount distribution of an object in more directions.

Although in this embodiment, a focus detection region coincides with the back projection image of a focus detection element, a focus detection region may be part of the back projection image of a focus detection element. More specifically, although oblique cross-shaped focus detection regions are aligned in the longitudinal direction of the opening110asince the opening of the field mask is vertically oriented in general, the back projection images of focus detection elements are not limited to this. The back projection images may be arranged within a rectangle formed by 45° oblique sides, and some of them may be used as focus detection regions. As shown inFIG. 10, for example, oblique fields may be aligned in the shorter side direction.

In this embodiment, the imaging lens10is configured to deal with brighter light beams (with a smaller F-number) in the second focus detection optical system whose light dividing directions are the 45° oblique directions, as compared with the first focus detection optical system whose light dividing directions are the longitudinal and shorter side directions of the opening110a. With this configuration, the plurality of first focus detection elements perform focus detection using light beams with an F-number equal to or smaller than the first F-number of the imaging lens10. On the other hand, the plurality of second focus detection elements perform focus detection using light beams with an F-number equal to or smaller than the second F-number of the imaging lens10, which is smaller than the first F-number. This allows more lenses to execute cross-type focus detection in the longitudinal and shorter side directions of the opening110a. When the imaging lens10has higher brightness (a smaller F-number) and light beams have a shallow depth of an object, it is possible to perform cross-type focus detection in the 45° oblique directions which enables to execute focus detection with higher accuracy, in addition to the focus detection in the longitudinal and shorter side directions of the opening110a. This makes it possible to improve the accuracy of the focus detection processing.

Returning toFIG. 1, the control unit80controls the focus adjustment lens of the imaging lens10to adjust the focus of the imaging lens10, based on the focus state (an out-of-focus direction and a defocus amount) of the imaging lens10detected by the focus detection apparatus100. More specifically, the control unit80calculates the drive amount of the focus adjustment lens based on the out-of-focus direction and defocus amount of the imaging lens10, and transmits a calculation result to an imaging lens side control unit (not shown). The imaging lens side control unit drives the focus adjustment lens via a motor or the like based on the drive amount of the focus adjustment lens acquired from the control unit80.

The operation of the image capturing apparatus1will be explained next.

In observing a finder, light which has passed through the imaging lens10is reflected by the main mirror20, forms an image on the focusing glass32, and is observed by the user via the pentaprism34and the eyepiece lens36. Light which has passed through the main mirror20is reflected by the sub-mirror40, and enters the focus detection apparatus100. As described above, the focus detection apparatus100can detect the focus state of the imaging lens10with high accuracy. Based on a detection result, the control unit80and the imaging lens side control unit (not shown) drive a focus lens included in the imaging lens10, and obtain an in-focus state.

On the other hand, in capturing an image (in acquiring an image for recording, that is, actually capturing an image), the main mirror20and the sub-mirror40are removed from the image capturing optical path, and light which has passed through the imaging lens10is captured by the image sensor50. The image capturing apparatus1can detect the focus of the imaging lens10with high accuracy using the focus detection apparatus100, and also adjust the focus of the imaging lens10based on a detection result, thereby capturing a high-quality image. As described above, the focus detection apparatus100can prevent itself from becoming large, and keep the cost down. Therefore, the image capturing apparatus1using the focus detection apparatus100can also prevent itself from becoming large, and keep the cost down.

This application claims the benefit of Japanese Patent Application No. 2009-256541, filed Nov. 9, 2009, which is hereby incorporated by reference herein in its entirety.