In natural vision, the eyes converge on a target and the lens accommodates (changes focal power) to the same distance — vergence and accommodation are neurally coupled. A fixed-focus VR headset breaks that link: light from the display always leaves the lens as if coming from one fixed optical distance (the lens's focal plane), no matter how close or far the rendered stereo object appears. Your eyes still converge on the virtual depth cue, but your lens must stay focused on the physical panel — creating the vergence–accommodation conflict (VAC). This 2D version draws the same optics top-down: the eye baseline along the bottom, depth increasing upward, so the two sightlines, the object and the lens's focal plane are all exactly the geometric picture the formulas below compute.
Vergence distance d_v = object depth (m)
Vergence angle θ = 2 · atan( IPD / (2 · d_v) )
Accommodation dist. d_a = headset focal distance (fixed),
or d_a = d_v if the display is varifocal
VAC (diopters) = | 1/d_a − 1/d_v |
- Object depth slider — moves the virtual target closer or farther; the two sightlines re-converge to match.
- IPD slider — real eye spacing varies person to person (~54–72 mm); it changes the vergence angle for the same depth.
- Headset focal distance — the one distance a fixed-focus HMD's optics are actually sharp at (commonly ~1.3–2 m); the dashed line marks it.
- Varifocal toggle — simulates an experimental varifocal/light-field display that shifts its focal plane to match vergence, driving VAC to zero.
- Research (Hoffman et al. 2008; Shibata et al. 2011) places comfortable viewing within roughly ±0.5 D of the lens's natural focus, with sustained conflict beyond ~1 D linked to eye strain, headaches and slower refocusing — the blur halo around the object grows with the diopter mismatch to visualize that retinal defocus.