1. A system for acquiring a series of images of an unconstrained subject, the system comprising: a sensor configured to acquire one or more images of an unconstrained subject; a controller configured to: detect an eye that is open or partially open in the acquired image; monitor quality criteria comprising an estimated extent of an iris of the unconstrained subject that is exposed according to the openness of the eye and acquired in the one or more images; determine, according to the monitored quality criteria comprising the estimated extent of the iris that is exposed and acquired, a time period during which there is a high probability that iris biometric data valid for biometric matching can be acquired from the unconstrained subject; increase, responsive to the determination, a rate of acquisition of images of the unconstrained subject within the determined time period, without input from measurement of whether a corresponding image to be acquired is in focus; and replace a first image stored in a storage device with a second image acquired within the determined time period, when a quality score of the second image is higher than that of the first image.