Vergence–accommodation conflict
In nature, the two eyes both converge (rotate) on the same point and focus on it. The focusing of the eye's lens is called accommodation. On a stereoscopic display the two separate: the eyes converge on the perceived point (vergence), but focus (accommodation) has to stay on the screen. This separation is called the vergence–accommodation conflict.
Diopters
Focus and vergence distances are compared in diopters: the reciprocal of the distance in meters (1/m). The size of the conflict:
where: Zp the (perceived) distance the eyes are directed at, V the screen distance the eye focuses on; both in meters
Substituting Zp = V·e / (e − P), the conflict comes out directly proportional to parallax: |P| / (V · e) (V and e in meters). It is zero at the screen plane; behind the screen, for a point going to infinity, it reaches 1/V; in front of the screen it has no limit.
The eye's depth of field
The eye also sees a small range around its focus distance sharply. This range is taken as about ±0.2 diopters (Devernay and Beardsley); Lambooij et al. cite ±1/3 diopter. With focus on the screen, the distances at which vergence can stay free of conflict:
where: t the eye's depth of field (diopters), V viewing distance (meters)
| Screen | Viewing distance | Range with ±0.2 D |
|---|---|---|
| Cinema | 16 m | 3.8 m – ∞ |
| TV | 2.5 m | 1.7 – 5.0 m |
| Tablet | 0.4 m | 0.37 – 0.43 m |
In terms of parallax this limit becomes P = ± e · t · V (V in meters). It is independent of screen size and grows with viewing distance. This is why a large screen viewed from a distance leaves a wide range for depth, and a small screen viewed up close a narrow one.
Why it matters
Large, prolonged conflicts are associated with visual fatigue and discomfort (Lambooij et al. 2009; Shibata et al. 2011). The conflict is considered one of the main causes of fatigue in stereoscopic content (Terzić and Hansard 2016).
- Main subject near the screen plane: the conflict is smallest where the viewer looks longest. See Zero parallax plane.
- Transitions at cuts: if the depth of the main subject is kept close across consecutive shots, the eyes do not have to converge anew at every cut.