1. An eye-mountable device comprising: a transparent polymeric material that can be removably mounted over a corneal surface; a substrate at least partially embedded in the transparent polymeric material; and an image sensor disposed on the substrate, wherein the image sensor includes: (i) an opaque screen including a non-focusing aperture, and (ii) a photo-sensitive area occupied by a plurality of photo-sensitive elements, wherein the plurality of photo-sensitive elements includes a first photo-sensitive element located closer to the non-focusing aperture than a second photo-sensitive element, and wherein the first photo-sensitive element occupies less of the photo-sensitive area than the second photo-sensitive element, wherein the non-focusing aperture is smaller than the photo-sensitive area, and wherein each of the photo-sensitive elements receives unfocused light entering the non-focusing aperture from a respective direction.