In certain embodiments, a head-mounted display may include one or more displays for displaying images. A left and right movable disk may be placed respectively between the left and right eyes of a user and the one or more displays. The user may wear the HMD, and may adjust the spacing between the left and right disks while viewing the computer-displayed image or images through holes in the plastic discs. For example, the HMD may display a stereo colored bulls eye pattern, of which the user can only see a portion because their vision is restricted to on-axis viewing by the discs. The computer may instruct the user to move the HMD disks until the user can only see the color in the center of the bulls eye. In certain embodiments, by performing the foregoing adjustment, the eye may be oriented on the proper optical axis to look through an optimal location of a lens.