1. A method of iris image acquisition, the method comprising: acquiring, by a sensor, a plurality of images of a subject, the plurality of images including a first image of an iris of the subject; determining, by at least one processor, a quality metric for the acquired first image, the quality metric determined using an estimated area of the iris visible in the acquired first image and a level of focus measured from the acquired first image; and responsive to the determined quality metric, acquiring, by the sensor, another image if the determined quality metric is lower than a predetermined threshold, and storing, in a memory, the acquired first image of the iris of the subject if the quality metric is higher than the predetermined threshold, wherein storing the acquired first image in the memory comprises including the first image within a predetermined maximum number of images allowed to be stored in the memory, each of the allowed images having a quality metric higher than the predetermined threshold.