Eye relief

Eye relief is the distance between the lens of a head-mounted display (HMD) and the user's cornea at which the user can obtain the full viewing angle. If the eye sits outside this distance, a reduced field of view is observed. In optical terms it is the distance from the last surface of the eyepiece to the plane of the exit pupil, the location at which the eye must be placed to see the complete image without vignetting or clipping. It is related to the eye box, the volume within which the eye can move while still seeing the full image.

Why it matters

Eye relief determines whether a viewer sees the entire image and whether corrective eyewear can be worn inside the headset. People who wear glasses have eyes that sit farther from the eyepiece than people who do not, so they require longer eye relief to keep the full field of view; the prescribed distance at which spectacle lenses sit from the eye is called the back vertex distance. Guidance for VR and AR optics commonly cites at least 15mm of clearance to accommodate ordinary glasses.

Eye relief involves a tradeoff with field of view: in general, greater eye relief reduces the usable field of view, and an exit pupil that is too small relative to eye position causes the image edges to vignette or clip. Bringing the lens closer to the eye delivers more of the rendered image to the viewer. The sensitivity can be high: Valve states that for designs targeting more than 90 degrees of field of view, even a single millimeter of excess eye relief reduces the field of view by about three degrees.

Adjustment in consumer headsets

Because eye relief affects both field of view and glasses compatibility, many headsets let the user change the lens-to-eye distance.

In the DK1 and DK2 Oculus Rift development kits, users can adjust eye relief using the lens depth adjustment screws located on the sides of the headband frame, which move the lenses closer to or farther from the eye. Road to VR reported that both the DK1 and the DK2 had this adjustment, while the consumer Rift did not outwardly share the feature.

The Valve Index provides a physical eye relief adjustment alongside its IPD adjustment. A knob moves the display assembly toward or away from the eyes, and Valve notes that the high sensitivity of field of view to eye relief can be observed directly by turning the knob through its range. The design lets the lenses sit close to the eyes, even with the gasket foam in place, in order to maximize field of view. The eye relief mechanism also helps the headset accommodate glasses.

Meta Quest headsets handle eye relief through the facial interface. On the Quest 3 and Quest 3S, eye-relief buttons next to the lenses release the facial interface so it can be slid forward or backward, changing the distance between the lenses and the face or any glasses worn; the interface has four marked positions. The Quest 2, which lacks this adjustment, instead ships with a separate glasses spacer that is inserted under the facial interface to create clearance and to keep glasses from contacting and scratching the lenses.