Virtual reality technology has improved dramatically in recent years, with the latest round of headsets offering significantly better performance than previous prototypes.
But there's one thing that VR still isn't very good at, and that's simulating objects very close to your face. In reality, our eye muscles send depth information to our brain as they focus on objects in three dimensions. In virtual reality, your focus always stays in the same place, so objects that get very close to you don't feel realistic.
Now, however, electrical engineers Robert Konrad and Gordon Wetzstein from Stanford University and perceptual scientist Emily Cooper from Dartmouth University, have teamed up to try to solve the problem.
They've borrowed a trick from ophthalmologists - lenses filled with liquid that can have their focus tuned in real-time. In 2008, inventor Joshua Silver used the technology to create a pair of ultra-cheap glasses could correct the vision of the world's poorest people.
Konrad, Wetzstein and Cooper installed a set of the liquid-filled lenses in an Oculus Rift VR headset and tweaked it so that the system could control their focus. When an object is far away, the lenses are adjusted so you need to focus far away. When it's nearer, the lenses change so that your eyes need to focus close up to see it clearly.
But the system also lets the researchers experiment with another concept from ophthalmology which dates back to the Victorian era. Monovision is a technique allows each eye of an observer to focus to a different distance - useful in patients with age-induced presbyopia, or farsightedness.
In Victorian times it was accomplished with the help of a monocle. Today, it's more commonly done in contact lenses or laser surgery. But in virtual reality it can prove handy too - especially if an object appears close to your right eye but not your left, for example.
In user satisfaction tests, the monovision system didn't score as highly as the adaptive focus system, but both scored far higher than a normal pair of VR glasses.
"In addition to showing how adaptive focus can be implemented and can improve virtual reality optics, our studies reveal that monovision can also improve user performance in terms of reaction times and accuracy, particularly when simulating objects that are relatively close to the user," said Konrad.
Cooper added: "Practical optical solutions for virtual reality are crucial to moving this technology to increasingly more comfortable and immersive experiences. Our work shows that monovision has the potential to be one such solution."
The team's findings will be presented in May at the ACM Conference on Human Factors in Computing Systems.