1. An apparatus for iris image acquisition, the apparatus comprising: a sensor; a lens, in optical communication with the sensor, to produce an out-of-focus infra-red image of a subject's iris on the sensor and an in-focus non-infra-red image of a feature other than the subject's iris on the sensor; and a filter array, disposed between and in optical communication with the sensor and the lens, to filter light propagating to the sensor via the lens, the filter array comprising a plurality of filter elements at vertical and horizontal sampling intervals that form a plurality of infra-red cut regions that pass visible illumination for sampling the in-focus non-infra-red image with a first portion of the sensor, and a plurality of infra-red pass regions that pass infra-red illumination for sampling the out-of-focus infra-red image with a second portion of the sensor, wherein the plurality of infra-red pass regions are configured to sample the out-of-focus infra-red image at or below a corresponding Nyquist limit for the infra-red pass regions.