A Broadband Diffractive-Optical Element (BDOE) as a lens whose f-number and numerical aperture are decoupled. The BDOE can include a substrate and an array of optical cells formed on the substrate to have a non-linear arrangement of cell heights to diffract light into a focal spot. The geometry of the focal spot can be designed to decouple the f-number from the numerical aperture for an imaging device that employs the broadband diffractive optical element as a lens.