1. A method of processing images acquired using a single image sensor, comprising: acquiring, by an image sensor, a first image of an object 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 one or more noise sources; and processing the first image according to a selection from a first mode and a second mode, comprising: if the first mode is selected, acquiring the first image using a first portion of the image sensor, and processing the first image acquired via the first portion 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 if the second mode is selected, acquiring the first image using a second portion of the image sensor, and processing the first image acquired via the second portion 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.