VR eye strain

Eye strain is a consequence of several factors to which folks have wildly different sensitivities.

Binocular overlap is that field of view seen by both eyes.
  3D perception works only within this region.
  Some folks report discomfort when this FoV fraction is relatively small (e.g < 75%)

View out of focus (or near focus limit for [e.g. older] folks with limited accommodation)

Wrong IPD, impacting ability to comfortably fuse binocular vision.

Eye strain provocations increase with pupils outside lens' eyebox (eye relief and exit pupil)

Getting users' pupils in VR lenses' eye boxes may require

modifying or replacing provided head strap and/or face mask.

Many prospective users, expecting plug-and-play, are unprepared to improvise...

Low IPD can be problematic for headset optics with small exit pupil

and lenses that collide with nose at small IPD settings.

Users' IPD have no direct impact on binocular overlap,

so long as IPD setting can center headset exit pupils on your pupils.

Optics with larger exit pupils allow folks to "cheat" binocular overlap vs total FoV

by tweaking location of their pupils relative to lenses' exit pupils
by increasing or decreasing IPD settings:
  eyes nearer inner edges of exit pupils increase FoV while decreasing 3D FoV

If your eyes *feel weird* after removing VR headset

focal distance, eye relief and/or IPD are/is probably off..

Stress becomes strain only when uncomfortable.

In practice, some folks can tolerate significant IPD maladjustment
with some headsets, to increase either measured FoV or binocular overlap

Brighter and sharper headsets, typically those with aspheric optics,
stimulate strong focus and binocular fusion responses in users' visual systems,

increasing likelihood of discomfort for users' pupils not centered in lens eyeboxes,
as well as sensing artifacts not detected in dimmer and less sharp scenes.