We propose a square aperture as a simple and practical alternative to existing coded aperture. A spatial derivative converts sensor measurements taken with a square aperture mask into measurements taken with a pinhole or slit aperture mask. Thus square aperture shares the properties of both large and small apertures, yielding excellent light efficiency while an artificial small aperture results in an infinite depth of field. We developed a prototype lens to confirm the feasibility of our blur size estimation and image deblurring approach.