1. A method for acquiring a series of images of an unconstrained subject, the method comprising: (a) acquiring, by a sensor without input from measurement of whether an image of an unconstrained subject to be acquired is in focus, the image of the unconstrained subject; (b) determining, based on conditions about an iris of the unconstrained subject detected in the acquired image, a time period during which there is a high probability that iris biometric data from the unconstrained subject for biometric matching is available for acquisition, the conditions about the iris comprising an estimated area of the iris that is exposed and acquired in the acquired image, and sharpness of features determined in the acquired image; (c) initiating acquisition of a series of images by the sensor within the determined time period, each image acquisition corresponding to a focus value within a predefined focus range, without input from measurement of whether a corresponding image to be acquired is in focus; (d) replacing a first image stored in a storage device, with a second image from the series of acquired images, when a quality score based on measurements of the conditions about the iris in the second image is higher than that in the first image.