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Why Doesn't 3D Work for Some People?

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Stereoscopic 3D just doesn't work for some people. And I don't mean on a philosophical level, as in, "I just don't think it adds a whole lot to the film." I mean on a physical level, as in, "What's all the fuss? All I see are blurry lines."

As many of you may already be aware, the modern stereoscopic illusion is created by feeding a slightly different image to each eye—the larger the difference between the two images, the more pronounced the 3D effect will appear.

Offsetting the right and the left images directly simulates a real world characteristic of the human visual system known as binocular disparity, which is a product of the inches-wide gap between your right an left eye.

Because our eyes are a few inches apart, even when they're focused on the same point in space our brain receives slightly different information from each retina. This is one of the many things that aids human depth-perception, and it's the principle that forms the basis of the stereoscopic illusion we see in theaters.

1. So what causes the effect to fail?


Any physical condition that disrupts your binocular disparity is going to lessen the effectiveness of stereoscopic 3D in theaters, or cause you to be unable to witness it at all.

Disorders like amblyopia, where one eye transmits significantly less visual information than the other to the brain brain, as well as unilateral optic nerve hypoplasia (underdevelopment of the optic nerve), and strabismus (a condition where the eyes are not properly aligned) can all be causes.

Amblyopia is particularly common because the condition can be quite subtle and unnoticeable in normal human vision, often going undetected until late in life.

2. My vision is decent, why can't I see 3D?

Perhaps the most surprising thing for people who have trouble seeing the 3D illusion in theaters is that more often than not their day-to-day vision is perfectly capable. The most common question is, "If my depth-perception works in the real world, why doesn't it work at the cinema?"

That answer is that in the real world, our ability to perceive depth comes from many, many factors that go beyond binocular disparity. There are numerous powerful monocular depth cues (meaning you only need one eye to pick them up)—motion parallax, relative scale, aerial and linear perspective, and texture gradients all contribute extensively to our ability to perceive depth.

So, you could easily have a condition like Amblyopia disrupting your binocular disparity, but have your depth-perception remain pretty much intact in the real world, simply because your visual system is still receiving quite a bit of information that pertains to depth and distance.

Close one eye and look around you. Your visual field might feel a bit compressed, and it might feel like you're looking at the world through a telephoto lens, but you probably aren't going to bump into any walls, because our brain is quite capable of compensating for the lack of binocular vision.

However, stereoscopic 3D in theaters is an illusion that's wholly reliant on binocular disparity—take it away and the effect fails.

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