1. An imaging device, comprising: a lens array including a plurality of compound lenses facing a subject, wherein a compound lens comprises a subject-side lens and an image-side lens on a same axis; an opening array disposed to face one surface of the lens array and having circular openings with substantially a same diameter, the openings of the opening array disposed to face the corresponding compound lenses such that effective diameters of the compound lenses become substantially a same; an image sensor obtaining a compound-eye image including single-eye images of the subject formed by the compound lenses; and a computing unit processing the compound-eye image obtained by the image sensor, wherein the compound lenses have different radii of curvature, substantially a same back focal length and substantially a same sag the computing unit extracts an in-focus image from the compound-eye image, the back focal length is the distance between an apex of an image-side surface of the image-side lens and an image plane, the computing unit includes an image capturing unit separating the compound-eye image into the single-eye images; and an in-focus image extracting unit extracting the in-focus image from the single-eye images, and the in-focus image extracting unit extracts one of the single-eye images that is most in focus as the in-focus image, calculates a difference between a highest luminance level and a lowest luminance level of pixels for each of the single-eye images, and extracts one of the single-eye images where the difference is the highest as the in-focus image.