1. A method for dynamic control of biometric image acquisition, the method comprising: (a) acquiring, by a sensor, an image of an iris of an unconstrained subject, without input from measurement of whether the image to be acquired is in focus; (b) determining, by a detection component, at least one of sharpness and contrast of the acquired image; and (c) determining, based on the acquired image, whether to trigger processing of an additional image of the unconstrained subject, and (d) if the additional image is to be processed, determining a time instance for triggering the processing of the additional image.