1. An iris-recognition (I-R) system for capturing images of one or both irises of a person over a large depth of field (DOF), comprising: an illumination system adapted to i) illuminate at an imaging wavelength λ IM a facial region of the person, wherein the facial region includes one or both eyes and at least a forehead portion of the person, and ii) to form a spot on the forehead portion; an optical system having an object plane within the large depth of field DOF, an image plane within a correspondingly large depth of focus DOF′ with an image sensor arranged thereat, and an amount of spherical aberration (SA) at an imaging wavelength λ IM such that the depth of field DOF increases by an amount between 50% and 500% as compared to the optical system being diffraction limited, the optical system configured to form on the image sensor an image of the facial region when the facial region is placed within the depth of field DOF, wherein said facial region image includes an image of the forehead spot; and a controller electrically connected to the image sensor and to the illumination system, the controller adapted to control and coordinate the operation of the illumination system and the image sensor, and adapted to perform image-processing on the facial-region image to form an enhanced image of one or both irises based on an enhanced modulation transfer function (MTF) formed using forehead distance information obtained from the forehead spot.