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.
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.
Dr Pian Yip Pui-wai, an ophthalmology specialist at the Ken Lee Medical Centre in Central, says amblyopia is the most common cause of stereoblindness here but estimates that sufferers number no more than five per cent of the city's population.
A local sufferer of amblyopia (who preferred not to be named) says he tried attending a 3-D movie screening "once" and "got a very uncomfortable feeling".
It was so uncomfortable he hasn't dared go back. He is not alone in his experience and says that "a few of my friends have similar problems".
The viewer says that it does not particularly bother him that he can't enjoy 3-D movies, because "there are plenty 2-D choices on the market". But, as the popularity of 3-D viewing explodes, there is a fear that a significant segment of the global population will be left behind viewing-wise.
Dr Kenneth Ciuffreda, a professor at New York's State College of Optometry, has estimated that as much as 20 per cent of the population there cannot fully see in 3-D. There have been complaints about cinemas only showing the 3-D versions of certain films, something which leaves fans with depth-perception issues out in the cold.
How about couples where one partner is stereoblind? Are they doomed to enjoy films apart?
American entrepreneur Hank Green faced this problem, so he designed a pair of 2-D glasses that could change the film back into 2-D for his wife, who experienced discomfort while watching 3-D films.
Yip says there is an easier solution: just wear the 3-D glasses provided, and cover one eye. You won't be able to see in 3-D, but any discomfort or blurriness should be gone.
For now, those left out of the 3-D boom should rest easy, as it is unlikely that 3-D movies will do away with 2-D ones entirely.
After all, even superstar actor Johnny Depp has never seen any of his films in 3-D blockbusters, as he is stereoblind.