Stereoscopic viewing systems may be adjusted for a user's inter-pupillary distance (IPD). An inter-pupillary distance (IPD) value is determined from a signal from a position sensitive device coupled to two moveable sighting fixtures mounted to a head mounted display (HMD) having two fixed optics configured for a specific inter-pupillary distance. The position sensitive device is configured to produce a signal that corresponds to a distance between the two sighting fixtures. The images are warped with a processor to optimize display of the images with the two fixed optics for the determined IPD value.