A method, comprising: receiving, on a device comprising a computer processor and a memory, a user authentication request for the device initiated by a user of the device; in response to receiving the user authentication request, capturing a first gate image of a face of the user using a camera located on the device; determining a first set of gate feature vectors for the first gate image; assessing the first set of gate feature vectors for the first gate image to determine whether a set of selected criteria for gate images is met in the first gate image; in response to the set of selected criteria being met in the first gate image, capturing at least one first unlock attempt image of the face using the camera; determining whether to authenticate the user as an authorized user of the device by comparing the face of the user in the at least one first unlock attempt image to one or more reference templates for the authorized user; in response to failing to authenticate the user of the device using the at least one first unlock attempt image, adjusting the set of selected criteria for gate images by placing a more restrictive tolerance on at least one criterion in the set of selected criteria for gate images; capturing a second gate image of the face of the user using the camera; determining a second set of gate feature vectors for the second gate image; assessing the second set of gate feature vectors for the second gate image to determine whether the adjusted set of selected criteria for gate images is met in the second gate image; in response to the adjusted set of selected criteria being met in the second gate image, capturing at least one second unlock attempt image of the face using the camera; and determining whether to authenticate the user as the authorized user of the device by comparing the face of the user in the at least second unlock attempt image to the one or more reference templates for the authorized user. receiving, on a device comprising a computer processor and a memory, a user authentication request for the device initiated by a user of the device; in response to receiving the user authentication request, capturing a first gate image of a face of the user using a camera located on the device; determining a first set of gate feature vectors for the first gate image; assessing the first set of gate feature vectors for the first gate image to determine whether a set of selected criteria for gate images is met in the first gate image; in response to the set of selected criteria being met in the first gate image, capturing at least one first unlock attempt image of the face using the camera; determining whether to authenticate the user as an authorized user of the device by comparing the face of the user in the at least one first unlock attempt image to one or more reference templates for the authorized user; in response to failing to authenticate the user of the device using the at least one first unlock attempt image, adjusting the set of selected criteria for gate images by placing a more restrictive tolerance on at least one criterion in the set of selected criteria for gate images; capturing a second gate image of the face of the user using the camera; determining a second set of gate feature vectors for the second gate image; assessing the second set of gate feature vectors for the second gate image to determine whether the adjusted set of selected criteria for gate images is met in the second gate image; in response to the adjusted set of selected criteria being met in the second gate image, capturing at least one second unlock attempt image of the face using the camera; and determining whether to authenticate the user as the authorized user of the device by comparing the face of the user in the at least second unlock attempt image to the one or more reference templates for the authorized user. <EOS>