1. A method for acquiring a series of images of a 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 the acquired image, a time period during which there is a high probability that biometric data from the unconstrained subject is available for acquisition; and (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.