1. A method of processing images acquired using a single image sensor, comprising: acquiring, by an image sensor, a first image of an object of a predetermined size and at a predetermined distance with respect to the image sensor, the first image including a time-invariant signal component corresponding to the object, and a random time-varying signal component introduced by at least one of shot noise, sensor noise or amplifier noise; and processing the first image according to a selection from a first mode and a second mode, comprising: if the first mode is selected, processing the first image by retaining signals from the time-invariant and time-varying signal components with spatial frequencies at and below a threshold spatial frequency predetermined for iris recognition, wherein the object comprises an iris, and performing iris matching that detects statistical dependence between the retained signals and signals from an enrolled image; and if the second mode is selected, processing the first image by reducing at least a portion of the signals from the time-invariant and time-varying signal components with spatial frequencies at or below the threshold spatial frequency.