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Version: 17 Jan 2024

Vergence-Accomodation Conflict

Visual discomfort is a longstanding challenge of engineering for extended reality (XR) headsets. These devices must display virtual information to our eyes using a system that is different from how people naturally see the world around us. This unnatural viewing behavior, also known as the Vergence-Accommodation Conflict (VAC), can induce visual discomfort and motion sickness.

In the physical world, there are two ways our eyes adjust to view objects at different distances:

  • The eyes rotate inward (converge) for closer objects, and rotate outward (diverge) for more distant objects (so-called vergence eye movements).

  • The eyes adjust to focus on objects at different distances by changing the shape of the crystalline lens in the eye (so-called accommodation).

A diagram showing two human eyes adjusted to rotate and focus on a far object, and another diagram showing two human eyes rotated and focused on a near object.

When looking at objects in the physical world, these two adjustments most often occur simultaneously and adjust to the same distance.

In contrast, in current XR head-mounted display technologies, the appearance of virtual objects at different depths is created by changing their relative positions on the two displays. However, there is only one fixed focal plane (at the focal length of the display system), where the virtual contents are rendered.

This means that the eyes rotate (diverge or converge) to fixate on an object in the same way as when looking at physical objects. But the eye must focus on the fixed plane of the display to allow for clear vision of the virtual object. When the distance between where the eyes are focused and the point they fixate is different, the so-called Vergence-Accommodation Conflict (VAC) occurs.

A diagram showing two human eyes rotated to focus on an object at a far distance and focused on the closer focal plane of the headset.

Because this mismatch is an unnatural state for the human eyes and brain, it can lead to varying degrees of discomfort, including eye strain, blurred vision, headache, eye fatigue, double vision, dizziness, and nausea. However, not all people are affected to the same degree, and discomfort also depends on the type of content that is displayed. Please see more about designing comfortable experiences in the Comfort Content and Placement Guidelines article.

The Magic Leap 2 device was designed to protect users from this type of discomfort by placing the device's focal plane at a distance that would safely allow the biggest range of content distances possible – from as close as 0.37m (the default near boundary of the Display Zone) to optical infinity (10m). Previous research confirmed that the VAC experienced within the default Display Zone leads to only minimal discomfort symptoms in the majority of users, creating a setting that works for most use cases and users.