Image sensor and image-capturing device

An image sensor includes: a plurality of imaging pixels; and a plurality of focus detection pixels constituted with a micro-lens and a photoelectric conversion element that receives a focus detection light flux transmitted through a photographic optical system and executes photoelectric conversion. The photoelectric conversion element in each of the focus detection pixels includes a light-receiving area where the focus detection light flux is received; light-receiving area images corresponding to each of the focus detection pixels are each formed as the light-receiving area is projected via the micro-lens onto a pupil plane of the photographic optical system; and a positional relationship of the micro-lens and the light-receiving area is determined in correspondence to an image height so that the light-receiving area images corresponding to all the focus detection pixels are superimposed on one another on the pupil plane.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor and an image-capturing device, used to capture a subject image.

2. Description of Related Art

Japanese Laid Open Patent Publication No. 2000-156823 discloses an image sensor that includes focus detection pixels disposed over part of a two-dimensional imaging pixel array, captures a subject image formed via a photographic optical system and detects the focusing condition of the photographic optical system through a split-pupil phase difference detection method.

SUMMARY OF THE INVENTION

There is still a challenge to be addressed in the image sensor in the related art described above in that a pair of focus detection light fluxes having passed through different areas of the exit pupil of the photographic optical system enter photoelectric conversion elements with less precision at focus detection pixels disposed at positions on the light-receiving surface of the image sensor further away from the point at which the optical axis of the photographic optical system intersects the image sensor, resulting in poorer focus detection accuracy.

According to the first aspect of the present invention, an image sensor comprises: a plurality of imaging pixels, each constituted with a first micro-lens and a first photoelectric conversion element that receives, via the first micro-lens, an image-capturing light flux transmitted through a photographic optical system and executes photoelectric conversion on the received image-capturing light flux; and a plurality of focus detection pixels disposed within a two-dimensional array of the plurality of imaging pixels, and made up with a plurality of first focus detection pixels each of which receives one light flux in a pair of focus detection light fluxes transmitted through the photographic optical system and a plurality of second focus detection pixels each of which receives the other light flux in the pair of focus detection light fluxes, the plurality of focus detection pixels each being constituted with a second micro-lens and a second photoelectric conversion element that receives, via the second micro-lens, a focus detection light flux transmitted through the photographic optical system and executes photoelectric conversion on the focus detection light flux. The second photoelectric conversion element in each of the plurality of first focus detection pixels includes a light-receiving area where the one focus detection light flux is received; the second photoelectric conversion element in each of the plurality of second focus detection pixels includes a light-receiving area where the other focus detection light flux is received; light-receiving area images corresponding to each of the plurality of focus detection pixels are each formed as the light-receiving area is projected via the second micro-lens onto a pupil plane of the photographic optical system; and a positional relationship of the second micro-lens and the light-receiving area is determined in correspondence to an image height so that the light-receiving area images corresponding to all the first focus detection pixels are superimposed on one another and that the light-receiving area images corresponding to all the second focus detection pixels are superimposed on one another on the pupil plane.

According to the second aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the plurality of first focus detection pixels each receive the one light flux and part of the other light flux and the plurality of second focus detection pixels each receive the other light flux and part of the one light flux.

According to the third aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the light-receiving area and the pupil plane achieve a substantially conjugate relationship in relation to the second micro-lens.

According to the fourth aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the second photoelectric conversion element includes a photoelectric conversion area; and the light-receiving area is defined by the photoelectric conversion area.

According to the fifth aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the second photoelectric conversion element includes a photoelectric conversion area; the plurality of focus detection pixels each include a shielding mask covering part of the photoelectric conversion area; and the light-receiving area is defined by the shielding mask.

According to the sixth aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the second micro-lens assumes a circular shape viewed from the front and the first micro-lens assumes a substantially rectangular shape viewed from the front.

According to the seventh aspect of the present invention, in the image sensor according to the first aspect, it is preferred that the second micro-lens assumes either a circular shape or a substantially circular shape in a sectional view.

According to the eighth aspect of the present invention, in the image sensor according to the first aspect, it is preferred that a positional relationship between the first micro-lens and a photoelectric conversion area in the first photoelectric conversion element is uniform regardless of the image height.

According to the ninth aspect of the present invention, an image-capturing device comprises: an image sensor according to the first aspect; an image generation unit that generates a subject image based upon outputs from the plurality of imaging pixels; a focus detection unit that detects a focusing condition for the photographic optical system based upon outputs from the plurality of focus detection pixels; and a focus adjustment unit that executes focus adjustment for the photographic optical system based upon detection results provided by the focus detection unit.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1is a front view of the image sensor achieved in an embodiment. The image sensor1in the embodiment includes imaging pixels disposed in a two-dimensional array within its effective pixel area1a, with focus detection pixels disposed over part of the array, i.e., over a central area and areas to the left and to the right of the central area within the effective pixel area1a.FIG. 2Ashows the central area A in the image sensor1shown inFIG. 1in an enlargement, whereasFIG. 2Bshows part ofFIG. 2Ain a further enlargement. In addition,FIG. 3Ashows the image sensor1inFIG. 2Bin a section taken through B-B, whereasFIG. 3Bshows the image sensor1inFIG. 2Bin a section taken through C-C. It is to be noted thatFIG. 2Bshows photoelectric conversion elements disposed under micro-lenses as well.

As shown inFIGS. 3A and 3B, the image sensor1is formed by attaching a photoelectric conversion element array6, with a plurality of photoelectric conversion elements (2b,3b,4b) two-dimensionally disposed therein, in close contact with the rear surface of a micro-lens array5constituted with a plurality of micro-lenses (2a,3a,4a) two-dimensionally disposed therein. A unit made up with a micro-lens and a photoelectric conversion element corresponding to the particular micro-lens is referred to as a pixel. A micro-lens2aand the corresponding photoelectric conversion element2bconstitute an imaging pixel2, a micro-lens3aand the corresponding photoelectric conversion element3bconstitute a first focus detection pixel3and a micro-lens4aand the corresponding photoelectric conversion element4bconstitute a second focus detection pixel4. Namely, imaging pixels2are disposed in a two-dimensional array within the effective pixel area1awith focus detection pixels3and4disposed over three areas, i.e., the central area, the left area and the right area, in the effective pixel area1a, taking up part of the two-dimensional array at the image sensor1.

The micro-lenses2aof the imaging pixels2are formed so as to assume a rectangular shape when viewed from the front, as shown inFIGS. 2A and 2B, so as to maximize their performance as condensers by minimizing the areas separating adjacent micro-lenses. It is to be noted that they do not need to achieve a perfectly rectangular shape and that, depending upon certain manufacturing restrictions that may be imposed, they may assume a roughly rectangular shape instead. In addition, the shape of the micro-lenses2ais designed so that they assume a sectional shape disallowing clear image formation, in order to ensure that a large amount of light is received thereat by minimizing the extent of decrease in the amount of peripheral light attributable to the positional relationship between the pupil of the photographic optical system and the aperture. As are the micro-lenses2a, the photoelectric conversion elements2b, indicated as the hatched areas inFIG. 2B, are formed in a rectangular shape viewed from the front, so as to maximize their condensing performance. Color filters (not shown) assuming red (R), green (G) and blue (B) colors are disposed between the micro-lenses2aand the photoelectric conversion elements2bat the imaging pixels2, and the imaging pixels2equipped with the color filters are disposed in a Bayer array.

At the three areas, i.e., the central area, the left area and the right area, in the effective pixel area1aof the image sensor1, two types of focus detection pixels3and4are disposed alternately. The first focus detection pixels3each mainly receive a light flux having passed through the right-side area of the exit pupil of the photographic optical system, having a left-side area and a right-side area defined therein. The second focus detection pixels4each mainly receive a light flux having passed through the left-side area of the exit pupil of the photographic optical system. In order to assure the maximum level of imaging performance, the micro-lenses3aand4aat the focus detection pixels3and4are both formed in a circular shape viewed from the front. In addition, as shown inFIG. 3B, the micro-lenses3aand4aare designed to form circular or substantially circular curved surfaces in a sectional view, so as to maximize the imaging performance.

The photoelectric conversion elements3band4bat the focus detection pixels3and4are formed to achieve shapes different from each other viewed from the front. The photoelectric conversion elements3bat the focus detection pixels3, indicated as the hatched areas inFIG. 2B, are each formed in a rectangular shape viewed from the front, taking up an area corresponding to the left half of the corresponding micro-lens3a, so as to receive mainly a light flux having passed through the right-side area of the exit pupil of the photographic optical system via the micro-lens3a. The photoelectric conversion elements4bat the focus detection pixels4are each formed in a rectangular shape viewed from the front, taking up an area corresponding to the right half of the corresponding micro-lens4a, so as to receive mainly a light flux having passed through the left-side area of the exit pupil of the photographic optical system via the micro-lens4a.

It is to be noted that the micro-lens array5, which includes two types of micro-lenses, i.e., the micro-lens2aat the imaging pixels2and the micro-lenses3aand4aat the focus detection pixels3and4, may be manufactured by optimizing the shapes of the circular masks used to form the two types of micro-lenses in conjunction with a micro-lens-forming film assuming a uniform film thickness.

FIG. 4illustrates the method adopted in the image sensor1to detect the focusing condition via the focus detection pixels3and4. It is to be noted thatFIG. 4shows eight focus detection pixels3and4disposed at the center of the image sensor to facilitate the description of the focus detection method. EP inFIG. 4indicates the exit pupil of the photographic optical system, whereas PO inFIG. 4indicates the exit pupil distance measured from the micro-lens surfaces at the image sensor1to the exit pupil EP. In addition, the “central axis” in the figure is the vertical line passing through the center of the effective pixel area1a(seeFIG. 1) at the image sensor1and the embodiment is described by assuming that the central axis is in alignment with the optical axis of the photographic optical system.

As described earlier, at each first focus detection pixel3, a “right light flux” having passed through a right-side area EP1at the exit pupil EP having a pair of areas, i.e., the right-side area EP1and a left-side area EP2defined therein, is received at the photoelectric conversion element3bvia the micro-lens3a, whereas at each second focus detection pixel4, a “left light flux” having passed through the left-side area EP2at the exit pupil EP, is received at the photoelectric conversion element4bvia the micro-lens4a. Then, a relative shift quantity indicating the extent of shift manifested relative to each other by a first signal string a1, a2, a3, . . . made up with the outputs from the photoelectric conversion elements3bat the first focus detection pixels3set in sequence and a second signal string b1, b2, b3, . . . made up with the outputs from the photoelectric conversion elements4bat the second focus detection pixels4set in sequence, in the array of the focus detection pixels3and4, is detected and based upon the relative shift amount, the focusing condition of the photographic optical system and a defocus amount representing the focusing condition may be detected through a split-pupil phase difference detection method of the known art.

It is to be noted that focus detection pixels3and4are disposed with similar arrangements over the left area and the right area in the effective pixel area1aat the image sensor1and that focus detection may be executed via these focus detection pixels3and4through the split-pupil phase difference detection method.

Either of the following two methods may be adopted when setting the shape, the size and the positions of “photoelectric conversion areas”, a term designated to refer to areas where the right light flux and the left light flux having passed through the pair of areas, i.e., the right-side area EP1and the left-side area EP2at the exit pupil EP of the photographic optical system, are received via the micro-lenses3aand4aand the received light actually undergoes photoelectric conversion.

In the first method, photoelectric conversion elements3band4bare formed on a semiconductor substrate upon which the photoelectric conversion element array6is formed, by ensuring that the ranges over which photoelectric conversion may be executed on the light actually received at the photoelectric conversion elements3band4bon the semiconductor substrate upon which the photoelectric conversion element array6is formed, match the “photoelectric conversion areas”, as shown inFIG. 5.FIG. 5shows masks3cand4cand a wiring material disposed between the micro-lens array5and the photoelectric conversion element array6. Through this method, the “photoelectric conversion areas” are defined in correspondence to the photoelectric conversion-enabled ranges at the light-receiving surfaces of the photoelectric conversion elements3band4band accordingly, the masks3cand4chave openings greater than the photoelectric conversion areas. It is to be noted that the photoelectric conversion areas are set at the light-receiving surfaces of the photoelectric conversion elements3band4bin this case.

In the second method, the “photoelectric conversion areas” are defined in correspondence to the openings of the masks3cand4cdisposed between the micro-lens array5and the photoelectric conversion element array6. The photoelectric conversion areas are set on the plane upon which the masks3cand4care disposed in this case.

As explained earlier, there is still a challenge to be addressed in the image sensor1in that a pair of focus detection light fluxes (the right light flux and the left light flux) having passed through the different areas EP1and EP2of the exit pupil EP of the photographic optical system enter the photoelectric conversion elements3band4bwith less precision at focus detection pixels3and4disposed at positions on the light-receiving surface of the image sensor further away from the point at which the optical axis of the photographic optical system intersects the light-receiving surface, resulting in poorer focus detection accuracy.

The challenge is addressed in the image sensor1achieved in the embodiment by (1) optimizing the positions of the “photoelectric conversion areas” relative to the exit pupil distance PO and (2) optimizing the imaging performance of the micro-lenses3aand4aat the focus detection pixels3and4relative to the exit pupil distance PO.

FIG. 7is an optical path diagram showing a pair of light fluxes having passed through a pair of areas EP1and EP2, i.e., a right-side area and a left-side area, in the exit pupil, entering a pair of focus detection pixels3and4disposed near the central axis (the vertical line passing through the point at which the optical axis of the photographic optical system intersects the image sensor1) and focus detection pixels3and4disposed near the point assuming an image height h measured from the central axis. The following is a description given in reference toFIG. 7, of the optimization methods adopted to achieve (1) and (2) above in the embodiment. S inFIG. 7indicates a point in the exit pupil EP present over the exit pupil distance PO from the micro-lens surfaces of the image sensor1, at which the exit pupil intersects the central axis (the optical axis of the photographic optical system). In addition, the point at which the principal ray of light flux departing the point S at the exit pupil EP and entering a micro-lens assuming a position with the image height h intersects the light-receiving surface of the corresponding photoelectric conversion element is referred to as a “reference point”.

It is to be noted that if the present invention is to be adopted in conjunction with a plurality of different types of photographic optical systems, a value representing the statistical average of the exit pupil distances corresponding to the plurality of photographic optical systems, determined through a specific method, should be designated as the exit pupil distance PO. In addition, while the embodiment is described by assuming that the optical axis of the photographic optical system is aligned with the central axis passing through the center of the image sensor1, the central axis may be intentionally offset from the optical axis of the photographic optical system.

First, in reference toFIG. 7, the optimization method adopted to achieve (1) above is described. At the first focus detection pixel3assuming the position with the image height h measured from the point at which the central axis of the image sensor1(the optical axis of the photographic optical system) intersects the imaging plane, the position of the photoelectric conversion element3bcorresponding to the micro-lens3aat the first focus detection pixel3is offset to the “reference point” from the optical axis of the micro-lens3a. It is to be noted that the offset quantity Δh by which the photoelectric conversion element is offset is univocally determined optically in correspondence to the image height h, the exit pupil distance PO and the like. The first focus detection pixel3is disposed so as to allow the photoelectric conversion element3to range toward the central axis by aligning the right end of the photoelectric conversion element3bwith the reference point, so as to allow the right light flux having passed through the right-side area EP1of the exit pupil EP to be received at the photoelectric conversion element3bvia the micro-lens3a.

Likewise, the position of the photoelectric conversion element4bcorresponding to the micro-lens4aof the second focus detection pixel4is offset to the reference point from the optical axis of the micro-lens4a. The second focus detection pixel4is disposed so that the photoelectric conversion element4branges further away from the central axis by aligning the left end of the photoelectric conversion element4bwith the reference point in order to allow the left light flux having passed through the left-side area EP2of the exit pupil EP to be received at the photoelectric conversion element4bvia the micro-lens4a. The second focus detection pixel4disposed on the central axis of the image sensor1assumes an image height h of 0 with the reference point thereof set on the central axis, and thus, the offset quantity by which the photoelectric conversion element4bmust be offset relative to the optical axis of the micro-lens4ais 0.

As described above, at the focus detection pixels3and4assuming the position with the image height h measured from the central axis of the image sensor1, the photoelectric conversion elements3band4bare offset to the respective “reference points” from the optical axes of the micro-lenses3aand4a. As a result, the images of the “photoelectric conversion areas” corresponding to all the first focus detection pixels3, projected via the individual micro-lenses3a, are superimposed, namely overlaid, in the right-side area EP1at the exit pupil EP and the images of the “photoelectric conversion areas” corresponding to all the second focus detection pixels4, projected via the individual micro-lenses4a, are superimposed, namely overlaid, in the left-side area EP2at the exit pupil EP.

AsFIG. 7clearly indicates, the right-side area EP1and the left-side area EP2at the exit pupil EP achieve right/left symmetry with respect to the intersecting point S at which the exit pupil EP and the central axis intersect and thus, even if vignetting attributable to the aperture of the photographic optical system or the like occurs at the right-side area EP1and the left-side area EP2, such vignetting shall affect the right-side area EP1and the left-side area EP2in a uniform manner, which makes it possible to minimize the decrease in focus detection accuracy.

While an explanation is given above in reference to the embodiment on an example in which the right end of each photoelectric conversion element3bis aligned with the corresponding reference point and the left end of each photoelectric conversion element4bis aligned with the corresponding reference point, as shown inFIG. 8A, the photoelectric conversion elements3band4bmay be set so as to achieve positional relationships with the reference points other than those described above.

For instance, the photoelectric conversion element3bat the first focus detection pixel3may be set further toward the central axis by positioning the right end of the photoelectric conversion element3bover a distance Δ1from the corresponding reference point and the photoelectric conversion element4bat the second focus detection pixel4may be set further away from the central axis by positioning the left end of the photoelectric conversion element4bover the distance Δ1from the corresponding reference point, as shown inFIG. 8B. In this case, focus detection may be executed with a greater opening angle, making it possible to improve the focus detection accuracy.

As an alternative, the reference point corresponding to the photoelectric conversion element3bof the first focus detection pixel3may be positioned over a distance Δ2from the right end of the photoelectric conversion element3band the reference point corresponding to the photoelectric conversion element4bof the second focus detection pixel4may be positioned over a distance Δ2from the left end of the photoelectric conversion element4b, as shown inFIG. 8C. In this case, the right-side area EP1and the left-side area EP2are allowed to partially overlap each other at the exit pupil EP.

While an explanation is given above on an example in which the photoelectric conversion elements3band4bof the focus detection pixels3and4assuming the position with the image height h measured from the central axis of the image sensor1are offset to the reference points from the optical axes of the micro-lenses3aand4ain order to define the “photoelectric conversion areas” in correspondence to the ranges over which the light actually received at the photoelectric conversion elements3band4bon the semiconductor substrate upon which the photoelectric conversion element array6is formed undergoes photoelectric conversion, as shown inFIG. 5, the “photoelectric conversion areas” may be defined in correspondence to the openings of the masks3cand4cdisposed between the micro-lens array5and the photoelectric conversion element array6, as shown inFIG. 6, by determining the positional relationships of the photoelectric conversion elements3band4bto the micro-lenses3aand4a, as described below.

FIG. 9is an optical path diagram showing a pair of light fluxes having passed through a pair of areas EP1and EP2, i.e., a right-side area and a left-side area, in the exit pupil EP, entering a pair of focus detection pixels3and4disposed near the central axis (the vertical line passing through the point at which the optical axis of the photographic optical system intersects the image sensor) and focus detection pixels3and4disposed near the point assuming an image height h measured from the central axis at the image sensor with photoelectric conversion areas defined by the openings in the masks3cand4c. It is to be noted that inFIG. 9, corresponding toFIG. 7, the same reference numerals and the like are assigned to elements equivalent to those shown inFIG. 7.

First, in reference toFIG. 9, the optimization method adopted to achieve (1) above is described. At the first focus detection pixel3assuming the position with the image height h measured from the point at which the central axis of the image sensor1(the optical axis of the photographic optical system) intersects the imaging plane, the position of the opening of the mask3ccorresponding to the micro-lens3aat the first focus detection pixel3is offset to the “reference point” from the optical axis of the micro-lens3a. It is to be noted that the offset quantity Δh by which the opening is offset is univocally determined optically in correspondence to the image height h, the exit pupil distance PO and the like. The first focus detection pixel3is disposed so as to allow the opening in the mask3cto range toward the central axis by aligning the right end of the opening in the mask3cwith the reference point, in order to allow the right light flux having passed through the right-side area EP1of the exit pupil EP to be received at the photoelectric conversion element3bvia the micro-lens3aand the opening in the mask3c. It is to be noted that the photoelectric conversion element3bat this first focus detection pixel3assumes a sufficient width as does the photoelectric conversion element3bdisposed near the central axis.

Likewise, the position of the opening in the mask4ccorresponding to the micro-lens4aof the second focus detection pixel4is offset to the reference point from the optical axis of the micro-lens4a. The second focus detection pixel4is disposed so that the opening in the mask4cranges further away from the central axis by aligning the left end of the opening in the mask4cwith the reference point in order to allow the left light flux having passed through the left-side area EP2of the exit pupil EP to be received at the photoelectric conversion element4bvia the micro-lens4aand the opening in the mask4c. It is to be noted that the photoelectric conversion element4bat the focus detection pixel4assumes a sufficient width as does the photoelectric conversion element4bdisposed near the central axis. The focus detection pixel4disposed on the central axis of the image sensor1assumes an image height h of 0 with the reference point thereof set on the central axis, and thus, the offset quantity by which the opening in the mask4cmust be offset is 0.

As described above, at the focus detection pixels3and4assuming the position with the image height h measured from the central axis of the image sensor1, the openings in the masks3cand4care offset to the respective “reference points” from the optical axes of the micro-lenses3aand4a. As a result, the images of the “photoelectric conversion areas” (openings in the masks3c) corresponding to all the first focus detection pixels3, projected via the individual micro-lenses3a, are superimposed, namely overlaid, in the right-side area EP1at the exit pupil EP and the images of the “photoelectric conversion areas” (openings in the masks4c) corresponding to all the second focus detection pixels4, projected via the individual micro-lenses4a, are aligned in the left-side area EP2at the exit pupil EP.

AsFIG. 9clearly indicates, the right-side area EP1and the left-side area EP2at the exit pupil EP achieve right/left symmetry with respect to the intersecting point S at which the exit pupil EP and the central axis intersect and thus, even if vignetting attributable to the aperture of the photographic optical system or the like occurs at the right-side area EP1and the left-side area EP2, such vignetting shall affect the right-side area EP1and the left-side area EP2in a uniform manner, which makes it possible to minimize the decrease in focus detection accuracy.

It is to be noted that while an explanation is given above in reference to the embodiment on an example in which the right end of the opening in the mask3cis aligned with the corresponding reference point and the left end of the opening in the mask4cis aligned with the corresponding reference point, the openings in the masks3cand4cmay be set so as to achieve positional relationships with the reference points other than those described above.

For instance, the opening of the mask3cfor the first focus detection pixel3may be set further toward the central axis by positioning the right end of the opening in the mask3cover a distance Δ1from the corresponding reference point and the opening of the mask4cfor the second focus detection pixel4may be set further away from the central axis by positioning the left end of the opening in the mask4cover the distance Δ1from the corresponding reference point, as shown inFIG. 8B. In this case, focus detection may be executed with a greater opening angle, making it possible to improve the focus detection accuracy.

As an alternative, the reference point corresponding to the first focus detection pixel3may be positioned over a distance Δ2from the right end of the opening in the mask3cand the reference point corresponding to the second focus detection pixel4may be positioned over the distance Δ2from the left end of the opening in the mask4c, as shown inFIG. 8C. In this case, the right-side area EP1and the left-side area EP2are allowed to partially overlap each other at the exit pupil EP.

The positional relationship between the micro-lens2aof an imaging pixel2and the “photoelectric conversion area” is now described.FIGS. 10A and 10Bshow the positional relationship of the photoelectric conversion elements3band4bto the respective micro-lenses3aand4aat the focus detection pixels3and4and the positional relationship of the photoelectric conversion elements2bto the corresponding micro-lenses2aat the imaging pixels2as observed in the embodiment, in illustrations facilitating comparison thereof.FIG. 10Ashows the image sensor1in a front view. The image sensor1includes rows of focus detection pixels3and4set at the center, a left-side position and a right-side position, as has been explained in reference toFIG. 1. It is to be noted that whileFIG. 10Ashows three rows of imaging pixels2, each made up with four imaging pixels2and three rows of focus detection pixels3and4, each made up with four focus detection pixels, to facilitate the explanation, focus detection pixels3and4may be disposed at positions and in quantities other than those shown in the figure.

As illustrated inFIG. 10B, the centers of the “photoelectric conversion areas”, i.e., the centers of the photoelectric conversion elements3band4b, are decentered further away from the optical axes of the micro-lenses3aand4afor focus detection pixels disposed further away from the center (the point at which the image sensor intersects the central axis of the photographic optical system in the embodiment), i.e., for focus detection pixels with a greater image height. In contrast, the photoelectric conversion elements2bof the imaging pixels2are always aligned with the optical axes of the corresponding micro-lenses2a, regardless of the image height measured from the center of the image sensor1, as shown inFIG. 10C.

It is more crucial for the focus detection pixels3and4to assure good imaging performance whereby the light fluxes arriving thereat from specific areas further frontward relative to the focus detection pixels3and4are allowed to enter the photoelectric conversion elements3band4bwith a high level of reliability, instead of giving priority to achieving superior condensing performance via the micro-lenses3aand4ato maximize the amounts of light arriving at the image-capturing surface from multiple directions and entering the photoelectric conversion elements3band4b. For this reason, the micro-lenses3aand4aare formed in a circular shape viewed from the front, the adjacent micro-lenses3aand4aare set so that they do not range continuously over their peripheries (in order to allow the micro-lenses to sustain the intended shape) and the centers of the photoelectric conversion elements3band4bare decentered relative to the optical axes of the micro-lenses3aand4ain correspondence to the image height, so as to ensure that the light fluxes arriving at the focus detection pixels3and4corresponding to varying image heights from the same areas further frontward relative to the pixels are allowed to enter the photoelectric conversion elements3band4bwith a high level of reliability.

In contrast, it is more crucial for the imaging pixels2to assure good condensing performance whereby the amount of light arriving at the image-capturing surface from multiple directions and entering the photoelectric conversion elements2bis maximized, instead of giving priority to providing superior imaging performance via the micro-lenses2ato allow the light fluxes arriving from the same areas further frontward relative to the pixels to enter the photoelectric conversion elements2bwith a high level of reliability. For this reason, the micro-lenses2aare formed in a rectangular shape with a significant width viewed from the front, adjacent micro-lenses2aare set densely in close proximity to each other so that they range continuously over their peripheries and the centers of the photoelectric conversion elements2bare always aligned with the optical axes of the corresponding micro-lenses2aregardless of the image height so as to assure the desired performance and facilitate the manufacturing process.

It is to be noted that while the “photoelectric conversion areas” are defined in relation to the photoelectric conversion elements2b,3band4bin the example presented inFIG. 10, similar positional relationships may be assumed when the “photoelectric conversion areas” are defined in relation to the masks (the masks3cand4cfor the focus detection pixels3and4and the masks used to regulate the areas where light actually received at the imaging pixels2undergoes photoelectric conversion (not shown)).

Next, a method that may be adopted to optimize the image-forming performance of the micro-lenses3aand4aat the focus detection pixels3and4in reference to the exit pupil distance PO as described in (2) earlier is explained. The “photoelectric conversion areas” are defined either by the ranges where the light actually received at the photoelectric conversion elements3band4bon the semiconductor substrate upon which the photoelectric conversion element array6is formed undergoes photoelectric conversion or by the openings in the masks3cand4c. It is desirable to minimize the extent of blurring of the images projected onto the exit pupil EP via the micro-lenses3aand4aover these photoelectric conversion areas. The less the extent of blurring, the less the extent of vignetting occurring for the pair of focus detection light fluxes at the exit pupil EP, i.e., the right light flux passing through the right-side area EP1and the left light flux passing through the left-side area EP2and thus, better focus detection accuracy is assured. If a significant extent of blurring occurs, the signal intensity in the images of the photoelectric conversion areas having been projected is bound to decrease significantly over a greater range at the outer edges and vignetting occurs readily at the exit pupil EP, which shall result in degraded focus detection accuracy.

Accordingly, the highest level of definition is achieved over the areas separating the images of the “photoelectric conversion areas” projected at the exit pupil EP set over the exit pupil distance PO from the micro-lenses3aand4a, by (2-1) setting a specific curvature for the micro-lenses3aand4aat the focus detection pixels3and4, by (2-2) setting a specific distance between the micro-lenses3aand4aand the positions of the “photoelectric conversion areas” measured along the central axis (along the optical axis of the photographic optical system in the embodiment) or by implementing both (2-1) and (2-2) above.

It is to be noted that the distance between the micro-lenses3aand4aand the positions of the “photoelectric conversion areas” measured along the central axis is equivalent to the distance between the micro-lenses3aand4aand the light-receiving surfaces of the photoelectric conversion elements3band4bmeasured along the central axis when the “photoelectric conversion areas” are defined in relation to the ranges over which the light actually received at the photoelectric conversion elements3band4bon the semiconductor substrate undergoes the photoelectric conversion as has been described in reference toFIG. 7, whereas it is equivalent to the distance between the micro-lenses3aand4aand the surface upon which the masks3cand4care disposed measured along the central axis when the “photoelectric conversion areas” are defined by the openings in the masks3cand4c, as has been described in reference toFIG. 9.

It is also to be noted that through either of the measures described in (2-1) and (2-2) above, it is ensured that the surface upon which the “photoelectric conversion areas” are set and the plane of the exit pupil EP assume a conjugate relationship relative to the micro-lenses3aand4a.

At the image sensor1in the embodiment described above, (1) the positions of the “photoelectric conversion areas” are optimized relative to the exit pupil distance PO and (2) the imaging performance of the micro-lenses3aand4aof the focus detection pixels3and4is optimized relative to the exit pupil distance PO. As a result, the pair of focus detection light fluxes (the right light flux and the left light flux) having passed through the different areas EP1and EP2at the exit pupil EP of the photographic optical system are allowed to enter the photoelectric conversion elements3band4bwith precision even at focus detection pixels3and4disposed at positions further away from the point at which the light-receiving surface of the image sensor1intersects the optical axis of the photographic optical system, making it possible to assure the desired level of focus detection accuracy in conjunction with the split-pupil phase difference detection method.

It is to be noted that the desired level of focus detection accuracy may be sustained at the image sensor1in the embodiment simply by (1) optimizing the positions of the “photoelectric conversion areas” relative to the exit pupil distance PO.

Next, an embodiment in which the image sensor1described above is adopted in a single lens reflex digital camera (image-capturing device) equipped with an autofocus function is described. As shown inFIG. 11, A digital still camera201achieved in the embodiment includes an exchangeable lens202and a camera body203. The exchangeable lens202is mounted at the camera body203via a mount unit204. Any one of exchangeable lenses202equipped with various image-forming optical systems may be mounted at the camera body203via the mount unit204.

The image sensor1, having been described above, the body drive control device214, a liquid crystal display element drive circuit215, a liquid crystal display element216, an eyepiece lens217, a memory card219and the like are disposed at the camera body203. Imaging pixels are two-dimensionally arrayed at the image sensor1and focus detection pixels are also built into the image sensor over areas corresponding to focus detection positions.

The body drive control device214includes a microcomputer, a memory, a drive control circuit and the like. It executes drive control of the image sensor1, reads out image signals and focus detection signals, repeatedly executes focus detection calculation based upon focus detection signals and adjusts focus in the exchangeable lens202, processes and records the image signals and controls camera operations. In addition, the body drive control device214engages in communication with the lens drive control device206via an electrical contact point213to receive the lens information and transmit the camera information (indicating the defocus amount, the aperture value and the like).

The liquid crystal display element216functions as an electronic viewfinder (EVF). A live image provided by the image sensor1, brought up on display at the liquid crystal display element216by the liquid crystal display element drive circuit215, may be observed by the photographer via the eyepiece lens217. The memory card219is an image storage medium in which an image captured by the image sensor1is stored.

A subject image is formed on the light-receiving surface of the image sensor1with a light flux having passed through the exchangeable lens202. The subject image undergoes photoelectric conversion at the image sensor1and subsequently, image signals and focus detection signals are transmitted to the body drive control device214.

The body drive control device214calculates the defocus amount indicating the extent of defocus based upon the focus detection signals output from the focus detection pixels at the image sensor1and transmits this defocus amount to the lens drive control device206. In addition, the body drive control device214generates an image by processing the image signals provided from the image sensor1and stores the image into the memory card219. It also provides live image signals from the image sensor1to the liquid crystal display element drive circuit215so as to bring up a live image on display at the liquid crystal display element216. Moreover, the body drive control device214provides aperture control information to the lens drive control device206to enable control of the aperture211.

The lens drive control device206updates the lens information in correspondence to the current focusing state, zooming state and aperture setting state, the maximum aperture number and the like. More specifically, the lens drive control device206detects the positions of the zooming lens208and the focusing lens210and the aperture value set for the aperture211, and calculates correct lens information based upon the lens positions and the aperture value. Alternatively, it may select the lens information corresponding to the lens positions and the aperture value from a lookup table prepared in advance.

The lens drive control device206calculates a lens drive quantity indicating the extent to which the lens is to be driven based upon the defocus amount having been received and drives the focusing lens210to a focusing position based upon the lens drive quantity. The lens drive control device206also drives the aperture211in correspondence to the aperture value it has received.

FIG. 12shows the structure of a focus detection optical system used to detect the focusing condition through the split-pupil phase difference detection method by using micro-lenses. It is to be noted that the focus detection pixels are shown in an enlargement. EP in the figure indicates the exit pupil set over a distance PO along the frontward direction from the micro-lenses disposed at the predetermined imaging plane of the exchangeable lens202(seeFIG. 11). The distance PO is determined in correspondence to the curvature of the micro-lenses, the refractive index of the micro-lenses, the distance between the micro-lenses and the surface where the “photoelectric conversion areas” are set and the like, and is referred to as a focus detection distance in this description.

EP1indicates a range defined by the photoelectric conversion elements3bprojected via the micro-lenses3a, and this range is referred to as a right-side area in the description. While EP1is shown as an elliptical area inFIG. 12so as to simplify the illustration, the area actually assumes the shape of the photoelectric conversion elements36projected in an enlarged state. Likewise, EP2indicates a range defined by the photoelectric conversion elements4bprojected via the micro-lenses4a, and this range is referred to as a left-side area in the description. WhileFIG. 12shows EP2as an elliptical area so as to simplify the illustration, the area actually assumes the shape of the photoelectric conversion elements46projected in an enlarged state.

WhileFIG. 12schematically illustrates two first focus detection pixels3and two second focus detection pixels4disposed near the optical axis L of the photographic optical system, the photoelectric conversion elements of focus detection pixels distanced from the optical axis L also receive light fluxes arriving at their micro-lenses from the corresponding right-side area EP1and left-side area EP2. The focus detection pixels are arrayed in a direction matching the direction in which the pair of areas, the right-side area EP1and the left-side area EP2, are set side-by-side, i.e., the direction along which the photo electric conversion elements in each pair are set side-by-side.

The micro-lenses3aand4aare disposed near the predetermined imaging plane of the exchangeable lens202(seeFIG. 11). Images of the photoelectric conversion elements3band4bdisposed behind the micro-lenses3aand4aare projected via the micro-lenses3aand4aonto the exit pupil EP set apart from the micro-lenses3aand4aby the exit pupil distance PO, and the projected shapes of the “photoelectric conversion areas” define the right-side area EP1and the left-side area EP2. Namely, the positional relationship between the micro-lens and the surface at which the photoelectric conversion element is disposed at each focus detection pixel is determined so that the projected shapes of the photoelectric conversion areas at the individual focus detection pixels are aligned on the exit pupil EP located over the distance PO.

The photoelectric conversion elements3beach output a signal corresponding to the intensity of an image formed on the micro-lens3awith a light flux23having passed through the right-side area EP1and having advanced toward the micro-lens3a. Likewise, the photoelectric conversion elements4beach output a signal corresponding to the intensity of an image formed on the micro-lens4awith a light flux24having passed through the left-side area EP2and having advanced toward the micro-lens4a.

By linearly disposing the two types of focus detection pixels each structured as described above in large numbers and integrating the outputs from the photoelectric conversion elements at the individual focus detection pixels into output groups each corresponding to either the right-side area EP1or the left-side area EP2, information related to the intensity distribution of the pair of images formed on the focus detection pixel row with the individual focus detection light fluxes passing through the right-side area EP1and the left-side area EP2is obtained. Image shift detection arithmetic processing (correlation arithmetic processing, phase difference detection processing) of the known art is subsequently executed by using the information thus obtained so as to detect the extent of image shift manifested by the pair of images through the split-pupil phase difference detection method. Then, by executing a conversion operation on the image shift amount in correspondence to the distance between the gravitational centers of the pair of areas EP1and EP2, the defocus amount is calculated.

FIG. 13presents a flowchart of the image-capturing operation executed in the single lens reflex digital camera in the embodiment. The body drive control device214starts the image-capturing operation in step S110and subsequent steps as the power to the camera is turned on in step S100. In step S110, the data from the imaging pixels are read out through a discriminative read and the data from the imaging pixels are displayed at the electronic viewfinder. In the following step S120, a pair of sets of image data corresponding to the pairs of images, are read out from the focus detection pixels3and4.

In step S130, the image shift detection arithmetic processing in the known art is executed based upon the pair of sets of image data having been read, to calculate an image shift amount and ultimately calculate the defocus amount through conversion. In step S140, a decision is made as to whether or not the current condition is close to being in focus, i.e., whether or not the absolute value of the defocus amount having been calculated is equal to or less than a predetermined value. If it is decided that the current condition is not close to being in focus, the operation proceeds to step S150to transmit the calculated defocus amount to the lens drive control device206which then drives the focusing lens210at the exchangeable lens202to the focusing position. Then, the operation returns to step S110to repeatedly execute the operation described above.

It is to be noted that the operation also branches to this step if focus detection is not possible to transmit a scan drive instruction to the lens drive control device206. In response, the lens drive control device drives the focusing lens210at the exchangeable lens202to scan between the infinity position and the close-up position. Subsequently, the operation returns to step S110to repeatedly execute the operation described above.

If it is decided in step S140that the current condition is close to being in focus, the operation proceeds to step S160to make a decision as to whether or not a shutter release has occurred in response to an operation of the shutter release button (not shown). If it is decided that a shutter release has not yet occurred, the operation returns to step S110to repeatedly execute the operation described above. If it is decided that a shutter release has occurred, the operation proceeds to step S170to transmit an aperture adjust instruction to the lens drive control device206and thus set the aperture value at the exchangeable lens202to the control F number (an F number selected by the photographer or an automatically set F number). As the aperture control ends, the image sensor1is engaged in an image-capturing operation and image data originating from the imaging pixels2and all the focus detection pixels3and4at the image sensor1are read out.

In step S180, image data at positions assumed by the individual pixels in the focus detection pixel rows are generated through pixel interpolation based upon the data at imaging pixels2present around the focus detection pixels3and4. In the following step S190, image data constituted with the data at the imaging pixels and the interpolated data are recorded into the memory card219, and then the operation returns to step S110to repeatedly execute the operation described above.

It is to be noted that while an explanation is given in reference to the embodiments on an example in which the two types of focus detection pixels are disposed alternately to each other over a central area, a left area and a right area at the image sensor1, focus detection pixel rows may be set at positions, in quantities and along directions other than those described in reference to the embodiments. For instance, focus detection pixels may be disposed so that a vertical focus detection pixel row and a horizontal focus detection pixel row intersect each other in a cross shape.

In addition, while the two types of focus detection pixels are disposed alternately to each other in the description of the embodiment provided above, the two types of focus detection pixels may be disposed with a pattern other than that assumed in the embodiments. For instance, they may be disposed with the following pattern; first focus detection pixel-first focus detection pixel-second focus detection pixel-second focus detection pixel-first focus detection pixel-first focus detection pixel-second focus detection pixel-second focus detection pixel . . . . In addition, focus detection pixels may be disposed over two successive rows as well.

While the first focus detection pixels3each include a focus detection pixel element3bassuming a rectangular shape corresponding to the left half of the focus detection pixel, viewed from the front, and the second focus detection pixels4each include a photoelectric conversion element4bassuming a rectangular shape corresponding to the right half of the focus detection pixel, viewed from the front, in the description provided above, the shapes of the photoelectric conversion elements at the focus detection pixels viewed from the front are not limited to these examples and they may instead assume, for instance, semicircular shapes corresponding to the left half and the right half of the focus detection pixels, instead.

It is to be noted that while the embodiments and variations are described above in reference to a pair of focus detection light fluxes, the right light flux and the left light flux, having passed through two different areas, i.e., the right-side area and the left-side area, of the exit pupil of the photographic optical system, these light fluxes simply represent an example and it shall be obvious that the present invention may be adopted in conjunction with light fluxes passing through areas set at other positions on the exit pupil plane, e.g., top/bottom positions or diagonal positions, along any direction assumed around the optical axis of the photographic optical system. In addition, the pair of light fluxes passing through the exit pupil of the photographic optical system, i.e., the right light flux and the left light flux, as described in the embodiments and variations are equivalent to a pair of light fluxes, i.e., a right light flux and a left light flux, passing through the photographic optical system.

The following advantages are achieved through the embodiments and variations. At an image sensor that includes focus detection pixels each constituted with a micro-lens and a photoelectric conversion element, disposed so as to occupy part of the two-dimensional array of imaging pixels each constituted with a micro-lens and a photoelectric conversion element, a light flux having passed through a photographic optical system is received at each photoelectric conversion element via the corresponding micro-lens. The focus detection pixels include a plurality of first focus detection pixels each of which receives mainly one of light fluxes in a light flux pair transmitted through the photographic optical system and a plurality of second focus detection pixels, each of which receives mainly the other light flux in the light flux pair transmitted through the photographic optical system. The positional relationship between the micro-lens at each focus detection pixel and the corresponding photoelectric conversion area where the light flux transmitted through the micro-lens of the focus detection pixel undergoes photoelectric conversion at the corresponding photoelectric conversion element, is determined in correspondence to the image height at the image sensor light-receiving surface, so as to ensure that a plurality of images of the photoelectric conversion areas of all the first focus detection pixels, projected via the respective micro-lenses onto the exit pupil plane of the photoelectric optical system, set over a specific distance from the micro-lenses are aligned with one another and that a plurality of images of the photoelectric conversion areas of all the second focus detection pixels projected via the respective micro-lenses onto the pupil plane are aligned with one another. As a result, even at focus detection pixels disposed at a position further away from the point at which the image sensor light-receiving surface intersects the optical axis of the photographic optical system, the pair of focus detection light fluxes (the right light flux and the left light flux) having passed through the different areas of the exit pupil of the photographic optical system are allowed to enter the photoelectric conversion elements of the focus detection pixels accurately and, as a result, the focus detection accuracy achieved through the split-pupil phase difference detection method is sustained at a high level.

In addition, a substantially conjugate relationship is achieved for the photoelectric conversion areas and the exit pupil plane by setting a specific curvature for the individual micro-lenses, by setting a specific distance between the micro-lenses and the photoelectric conversion area setting plane, measured along the optical axis of the photoelectric optical system, or by implementing both measures. Consequently, an image of the photoelectric conversion area projected onto the exit pupil plane via the micro-lens at a given first focus detection pixel and an image of the photoelectric conversion area projected via the micro-lens at the adjacent second focus detection pixel achieve a high level of definition and since vignetting does not, therefore, occur readily, the focus detection accuracy achieved through the split-pupil phase difference method sustains a high level.

The above described embodiments are examples and various modifications can be made without departing from the scope of the invention.