In virtual reality, pupil swim is an image-distorting effect that occurs every so often when you move your eye around the lens of your VR headset.
It is the variation in lens distortion that users experience as their eyes rotate off the center axis and across the lens. Pupil swim often occurs with near-eye display even if you configure everything correctly, have your IPD on just right, and somehow measure your exact eye length.
Fixing pupil swim requires:
- Real-time eye tracking
- Eye position-dependent rendering
- Lens distortion correction
But even with the perfect distortion correction at the center viewing position, you will still experience some distortion in the form of seeming motion when you move your eyes around, especially towards the edges of your lenses, as the field of distortion at the edges is more significant than that at the center. This is true to varying degrees, depending on the design of your lenses and the implementation of distortion correction.
In other words, all VR lenses are subject to some degree of pupil swim, and there is no flawless geometric portrayal.
All you can hope for at best is a level of pupil swim that is tolerable enough for you to still enjoy the VR experience, and most lens manufacturers strive to achieve this and improve upon it with every new release.