A head-mounted display comprising: at least one image sensor; and a computing device comprising: a logic subsystem; and a data-holding subsystem comprising instructions stored thereon that are executable by the logic subsystem to: identify, using image data from the at least one image sensor, a background object in a physical environment surrounding the head-mounted display as a real world target (318; 514); via the head-mounted display, display a virtual marker (306; 506) being visually alignable with the real world target (318; 514) by a user of the head-mounted display by moving the head-mounted display; receive an input from the user indicating that the user has visually aligned the virtual marker (306; 506) with the real world target (318; 514); in response to receiving the input from the user, acquire image data from the head-mounted display to determine a location of the real world target (318; 514); using the acquired image data, determine an estimated eye location relative to a location of the head-mounted display; and calibrate a predetermined eye location based upon the estimated eye location, the predetermined eye location indicating a location of an eye of a user relative to the head-mounted display, characterized in that the virtual marker (306; 506) is aligned with a plurality of real world targets (318; 514) in a particular order to enter a user's password. at least one image sensor; and a computing device comprising: a logic subsystem; and a data-holding subsystem comprising instructions stored thereon that are executable by the logic subsystem to: identify, using image data from the at least one image sensor, a background object in a physical environment surrounding the head-mounted display as a real world target (318; 514); via the head-mounted display, display a virtual marker (306; 506) being visually alignable with the real world target (318; 514) by a user of the head-mounted display by moving the head-mounted display; receive an input from the user indicating that the user has visually aligned the virtual marker (306; 506) with the real world target (318; 514); in response to receiving the input from the user, acquire image data from the head-mounted display to determine a location of the real world target (318; 514); using the acquired image data, determine an estimated eye location relative to a location of the head-mounted display; and calibrate a predetermined eye location based upon the estimated eye location, the predetermined eye location indicating a location of an eye of a user relative to the head-mounted display, a logic subsystem; and a data-holding subsystem comprising instructions stored thereon that are executable by the logic subsystem to: identify, using image data from the at least one image sensor, a background object in a physical environment surrounding the head-mounted display as a real world target (318; 514); via the head-mounted display, display a virtual marker (306; 506) being visually alignable with the real world target (318; 514) by a user of the head-mounted display by moving the head-mounted display; receive an input from the user indicating that the user has visually aligned the virtual marker (306; 506) with the real world target (318; 514); in response to receiving the input from the user, acquire image data from the head-mounted display to determine a location of the real world target (318; 514); using the acquired image data, determine an estimated eye location relative to a location of the head-mounted display; and calibrate a predetermined eye location based upon the estimated eye location, the predetermined eye location indicating a location of an eye of a user relative to the head-mounted display, identify, using image data from the at least one image sensor, a background object in a physical environment surrounding the head-mounted display as a real world target (318; 514); via the head-mounted display, display a virtual marker (306; 506) being visually alignable with the real world target (318; 514) by a user of the head-mounted display by moving the head-mounted display; receive an input from the user indicating that the user has visually aligned the virtual marker (306; 506) with the real world target (318; 514); in response to receiving the input from the user, acquire image data from the head-mounted display to determine a location of the real world target (318; 514); using the acquired image data, determine an estimated eye location relative to a location of the head-mounted display; and calibrate a predetermined eye location based upon the estimated eye location, the predetermined eye location indicating a location of an eye of a user relative to the head-mounted display, characterized in that the virtual marker (306; 506) is aligned with a plurality of real world targets (318; 514) in a particular order to enter a user's password.