1. A portable electronic device, comprising: an imaging sensor that, selectively during operation of the portable electronic device, acquires one or more images; a processor electrically coupled to the imaging sensor; and memory, electrically coupled to the processor, which stores a program module that, selectively during operation of the portable electronic device, is executed by the processor, the program module including instructions for: providing an instruction to a user to move the portable electronic device relative to a face of the user; acquiring, using the imaging sensor, a series of images while the portable electronic device is at different positions relative to the face of the user, wherein the series of images include at least two images relative to a symmetry axis of the face of the user; determining a change in a sign of a nose angle in at least the two images; and authenticating the user based on at least one of the series of images using a facial-recognition technique.