1. A method for dynamic control of biometric image processing in association with an unconstrained subject, the method comprising: (a) acquiring, by a sensor, a first image of an iris of an unconstrained; (b) comparing, by a detection component, a quality metric of the first image against that of one or more images of the iris of the unconstrained subject that are stored in memory and acquired prior to the first image, the memory being progressively updated to store a best available set of images of the unconstrained subject for further processing including biometric matching, wherein the quality metric of the first image is determined based on at least one of sharpness, contrast, or area of the exposed iris on the acquired image; and (c) determining to update the memory by storing the first image in the memory in place of at least one of the one or more stored images responsive to determining that the quality metric of the first image is greater than that of the at least one of the one or more stored images, and to maintain the one or more images stored in the memory by discarding the first image responsive to determining that the quality metric of the first image is equal to or less than that of the at least one of the one or more stored images; (d) acquiring, by the sensor, a second image of the unconstrained object for determination of whether to update the memory; and (e) providing, within a predetermined length of time for iris image acquisition and update to the memory, the best available set of stored images of the iris of the unconstrained subject for further processing.