1. A standoff image acquisition system comprising: an image sensor; an optics assembly coupled to the image sensor; and a processor coupled to the image sensor and the optics assembly; wherein: the optics assembly is configured to convey images of a subject suitable for biometric imaging to the image sensor with a varying focus distance while the standoff image acquisition system is disposed at a standoff distance from the subject; and the processor is programmed to control the image sensor to acquire a sequence of images from the optics assembly of the subject and control the optics assembly to vary the focus distance monotonically during acquisition of the sequence of images.