A head-mounted display assembly, comprising: a first eyecup and a second eyecup configured for respectively positioning a first lens and a second lens in front of intended locations of a user's eyes when the head-mounted display assembly is donned by the user, wherein the first eyecup and the second eyecup are movable relative to each other to adjust for the user's interpupillary distance; and a detent mechanism configured to hold the first eyecup and the second eyecup in two, three, four or five distinct distances from each other corresponding to two, three, four or five IPD settings of the first eyecup and second eyecup; wherein the detent mechanism comprises: a first rack extending inwardly from the first eyecup; a second rack extending inwardly from the second eyecup; a pinion engaged with the first rack and the second rack; a detent base coupled to the pinion, the detent base including a plurality of grooves in a face thereof; and at least one detent extension biased toward the detent base to be positioned within a groove of the plurality of grooves to hold the pinion at an IPD setting of the two, three, four or five IPD settings; characterized in that the pinion is mounted on the detent base; and in that the grooves are provided in a back face of the detent base opposite the pinion. a first eyecup and a second eyecup configured for respectively positioning a first lens and a second lens in front of intended locations of a user's eyes when the head-mounted display assembly is donned by the user, wherein the first eyecup and the second eyecup are movable relative to each other to adjust for the user's interpupillary distance; and a detent mechanism configured to hold the first eyecup and the second eyecup in two, three, four or five distinct distances from each other corresponding to two, three, four or five IPD settings of the first eyecup and second eyecup; wherein the detent mechanism comprises: a first rack extending inwardly from the first eyecup; a second rack extending inwardly from the second eyecup; a pinion engaged with the first rack and the second rack; a detent base coupled to the pinion, the detent base including a plurality of grooves in a face thereof; and at least one detent extension biased toward the detent base to be positioned within a groove of the plurality of grooves to hold the pinion at an IPD setting of the two, three, four or five IPD settings; characterized in that the pinion is mounted on the detent base; and in that the grooves are provided in a back face of the detent base opposite the pinion. a first rack extending inwardly from the first eyecup; a second rack extending inwardly from the second eyecup; a pinion engaged with the first rack and the second rack; a detent base coupled to the pinion, the detent base including a plurality of grooves in a face thereof; and at least one detent extension biased toward the detent base to be positioned within a groove of the plurality of grooves to hold the pinion at an IPD setting of the two, three, four or five IPD settings; characterized in that the pinion is mounted on the detent base; and in that the grooves are provided in a back face of the detent base opposite the pinion.