1. An image-capturing apparatus comprising: an imaging optical system; a microlens array having a plurality of microlenses two-dimensionally arrayed with a predetermined pitch in a vicinity of a focal plane of the imaging optical system; a photoreceptor array having a plurality of photoreceptors for each of the microlenses, each of the photoreceptors receiving bundles of rays passing through one of different regions at an exit-pupil of the imaging optical system; and an arithmetic unit that generates a synthetic image in an image plane at a desired position optionally set in an optical path of the imaging optical system, based on photoreception signals output from the photoreceptor array, wherein the arithmetic unit generates the synthetic image in the image plane at the desired position optionally set in the optical path of the imaging optical system, by obtaining a light amount of bundles of rays incident on a region a size of which is optionally set at the desired position, based on the desired position, the size of the region, a solid angle of bundles of rays incident on the region corresponding to a desired aperture, and photoreception signals output from the plurality of photoreceptors for each of the microlenses.