As the popularity of 3-D movies grows, a new group of people has emerged: those who claim that they can't see the 3-D image at all, or say that they experience discomfort when watching a film in 3-D.
Their complaints are bringing to light the existence of many people who experience not just movies, but the whole world, in two dimensions. This inability to perceive depth is called stereoblindness.
Hank Green designed some glasses to turn the 3-D image back into 2-D
Columbia University ophthalmologist Dr Paul Krawitz writes on his blog, VisiVite, that strabismus - when the eyes don't align properly - and amblyopia, or a lazy eye, "are both eye conditions which affect someone's ability to enjoy 3-D movies and can cause the person to experience eyestrain, which can in turn lead to a headache".
The technology behind 3-D films works by taking advantage of the brain's mechanism for perceiving depth. A person with normal vision perceives depth because each eye sends a slightly different image to the brain.
The brain reconciles these images, fusing them into one. The discrepancies between the two images are perceived as depth.
A 3-D movie works by actually showing two separate movies on top of each other at the same time. A pair of glasses works as a filter, each lens blocking a different set of images.
When the two images are reconciled in the brain they result in the illusion of depth. But if viewers' brains are less adept at fusing the images, they must work harder, causing tiredness and even headaches.