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A focus detection device comprises a micro lens array disposed near an estimated focal plane of an image forming optical system and formed by arraying a plurality of micro lenses, a light receiving device provided in correspondence to each of the plurality of micro lenses, which includes three or mo

A focus detection device comprises a micro lens array disposed near an estimated focal plane of an image forming optical system and formed by arraying a plurality of micro lenses, a light receiving device provided in correspondence to each of the plurality of micro lenses, which includes three or more light receiving portions disposed along a direction in which the plurality of micro lenses are arrayed, and a focus detection circuit that detects a focal adjustment state with the image forming optical system based upon a signal output from the light receiving device.

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What is claimed is: 1. A focus detection device, comprising: a micro lens array disposed near an estimated focal plane of an image forming optical system and formed by arraying a plurality of micro lenses; a light receiving device provided in correspondence to each of the plurality of micro lenses,

What is claimed is: 1. A focus detection device, comprising: a micro lens array disposed near an estimated focal plane of an image forming optical system and formed by arraying a plurality of micro lenses; a light receiving device provided in correspondence to each of the plurality of micro lenses, which includes three or more light receiving portions disposed along a direction in which the plurality of micro lenses are arrayed; and a focus detection circuit that detects a focus adjustment state with the image forming optical system based upon a signal outputted from the light receiving device, wherein the focus detection circuit (i) calculates a single main area of a region which is formed by three or more light receiving portions onto which light flux originating from the image forming optical system is projected, (ii) determines a boundary line which divides the single main area into two sub areas, at least one of the sub areas including two or more light receiving portions adjacent to another light receiving portion of the same group, and (iii) splits all of the light receiving portions between a first group and a second group based on the boundary line, the first group being formed at one side of the boundary line, the second group being formed at another side of the boundary line. 2. The focus detection device according to claim 1, wherein: the sub areas assume shapes identical to one another. 3. The focus detection device according to claim 1, wherein: the focus detection circuit determines at least one of a position and a shape of the single main area based upon aperture information for the image forming optical system. 4. The focus detection device according to claim 1, wherein: the focus detection circuit calculates an image offset quantity, indicating an extent of offset manifesting in a pair of images formed with a pair of light fluxes passing through different pupil areas at the image forming optical system based upon the output signals from the first and second groups and detects the focus adjustment state of the image forming optical system based upon the image offset quantity having been calculated. 5. The focus detection device according to claim 1, wherein: the boundary line is determined so as to substantially equalize quantities of light received at the light receiving portions in one sub area to those at the light receiving portions in another sub area. 6. The focus detection device according to claim 4, wherein: the focus detection circuit determines a conversion coefficient to be used to convert the image offset quantity to a defocus quantity indicating an extent of defocusing at the image forming optical system in correspondence to the first and second groups. 7. The focus detection device according to claim 6, wherein: the focus detection circuit determines the conversion coefficient in correspondence to an opening angle formed by gravitational centers of the pair of light fluxes received at the first and second groups. 8. An optical system, comprising: an image forming optical system; a micro lens array disposed near an estimated focal plane of the image forming optical system and formed by arraying a plurality of micro lenses; a light receiving device provided in correspondence to each of the plurality of micro lenses, which includes three or more light receiving portions disposed along a direction in which the plurality of micro lenses are arrayed; and a focus detection circuit that detects a focus adjustment state with the image forming optical system based upon a signal outputted from the light receiving device, wherein the focus detection circuit (i) calculates a single main area of a region which is formed by three or more light receiving portions onto which light flux originating from the image forming optical system is projected, (ii) determines a boundary line which divides the single main area into two sub areas, at least one of the sub areas including two or more light receiving portions adjacent to another light receiving portion of the same group, and (iii) splits all of the light receiving portions between a first group and a second group based on the boundary line, the first group being formed at one side of the boundary line, the second group being formed at another side of the boundary line. 9. The optical system according to claim 8, further comprising: an image pickup device that includes two-dimensionally arrayed pixels each constituted with a second micro lens and a second light receiving portion and takes an image formed via the image forming optical system, wherein: the micro lens array and the light receiving device is constituted with some of the pixels at the image pickup device. 10. The optical system according to claim 8, further comprising: a storage device at which aperture information for the image forming optical system is stored, wherein: the focus detection circuit determines at least one of a position and a shape of the single main area in correspondence to the aperture information. 11. The optical system according to claim 9 wherein: the focus detection circuit calculates an image offset quantity indicating an extent of offset manifesting in a pair of images formed with a pair of light fluxes passing through different pupil areas at the image forming optical system, based upon the output signals from the first and second groups and detects the focus adjustment state of the image forming optical system based upon the image offset quantity having been calculated. 12. The optical system according to claim 9 wherein: the focus detection circuit determines the boundary line so as to substantially equalize quantities of light received at the light receiving portions in one sub area to those at the light receiving portions in another sub area. 13. The optical system according to claim 11, wherein: the focus detection circuit determines a conversion coefficient to be used to convert the image offset quantity to a defocus quantity indicating an extent of defocusing at the image forming optical system in correspondence to the first and second groups. 14. The optical system according to claim 13, wherein: the focus detection circuit determines the conversion coefficient in correspondence to an opening angle formed by gravitational centers of the pair of light fluxes received at the first and second groups. 15. The optical system according to claim 8, further comprising: an image pickup device that includes pixels each constituted with one of the plurality of micro lenses and the light receiving device and takes a subject image formed via the image forming optical system. 16. A focus detection method, comprising: forming a pixel row by arraying a plurality of pixels each including a light receiving device, the light receiving device including three or more light receiving portions disposed in correspondence to a micro lens and disposing the pixel row near an estimated image forming plane of an optical system; calculating a single main area of a region which is formed by three or more light receiving portions onto which light flux originating from the image forming optical system is projected; determining a boundary line which divides the single main area into two sub areas, at least one of the sub areas including two or more light receiving portions adjacent to another light receiving portion of the same group; splitting all of the light receiving portions between a first group and a second group based on the boundary line, the first group being formed at one side of the boundary line, the second group being formed at another side of the boundary line; calculating an image offset quantity indicating an extent of offset manifesting in a pair of images formed with a pair of light fluxes passing through different pupil areas at the optical system based upon output signals from the first and second groups; and detecting a focus adjustment state at the optical system based upon the image offset quantity having been calculated. 17. The focus detection method according to claim 16, wherein: the boundary line are determined so as to substantially equalize quantities of light received at the light receiving portions in one sub area to those at the light receiving portions in another sub area. 18. The focus detection method according to claim 16, wherein: the image offset quantity is converted to a defocus quantity indicating an extent of defocusing at the optical system by using a conversion coefficient when detecting the focus adjustment state at the optical system; and the conversion coefficient is determined in correspondence to the first and second groups. 19. An image device, comprising: an image pickup device that includes two-dimensionally arrayed imaging pixels each including a first micro lens and a first light receiving device, and focusing pixels each including a second micro lens and a second light receiving device that includes three of more light receiving portions disposed along a direction in which the second micro lenses are arrayed; and a focus detection circuit that detects a focus adjustment state with the image forming optical system based upon a signal outputted from the light receiving device, wherein the focus detection circuit (i) calculates a single main area of a region which is formed by three or more light receiving portions onto which light flux originating from the image forming optical system is projected, (ii) determines a boundary line which divides the single main area into two sub areas, at least one of the sub areas including two or more light receiving portions adjacent to another light receiving portion of the same group, and (iii) splits all of the light receiving portions between a first group and a second group based on the boundary line, the first group being formed at one side of the boundary line, the second group being formed at another side of the boundary line. 20. The focus detection device according to claim 1, wherein the focus detection circuit determines the boundary line based upon aperture information for the optical system.