Strolling out of the theater after seeing the recent 3D movie, Gravity, my friend wondered why she felt slightly dizzy every time she watched a show in 3D. After all, isn’t 3D the way we normally see the world? What do our eyes do differently while watching a 3D film that they don’t do in real life, say when we‘re at a picnic?

Actually, the reverse question is a better one. What don’t our eyes do while watching a film that they do during a picnic? Answer: since the movie screen is a fixed distance away, our eyes don’t change focus even when objects in the 3D film appear to move in and out of the screen. For example, a real yellow jacket harassing you during the picnic and a 3D film of the same yellow jacket elicit different responses from our eye muscles. In the real-life case the lens muscle (ciliary muscle) of the eye is constantly changing the focal length of the lens as the insect buzzes closer and further, whereas in the 3D film the lens must stay focused on the screen--and not on the apparent position of the yellow jacket. The process of adjusting the focal length of the eye lens to bring objects at different distances into focus is known as “accommodation.” In real life the lens must accommodate for distance whereas in 3D movies the natural tendency of the eye to accommodate to the perceived distance would actually make the screen go out of focus.

By itself, this isn’t enough to start a quarrel between our eyes and 3D movies; the ciliary muscles can simply accommodate for the fixed screen location. Unfortunately for 3D movies, ciliary muscles aren’t the only muscles that participate in giving us a three dimensional view of the world. When we fix our gaze on an object in space, the muscles that move the eyeballs (extraocular muscles) rotate the eyes in opposite directions to make the lines of sight of the two eyes intersect at the location of the object (fixation). So when the yellow jacket flies closer to our face, our eyes cross and when it moves away our eyes un-cross. This instinctive response is known as “vergence.” The accommodation and vergence responses are well coordinated so that the point where the lines of sight of our eyes intersect is also the point where our eyes focus. In a 3D illusion this coordination between the two muscle types is disrupted, because the extraocular muscles want to verge on where the object appears to be, while the ciliary muscles want to stay focused on the screen. This conflict, many believe, is the main source of discomfort in 3D movies (see Figure 1).

Image Source:

I should add, though, that unlike my friend who gets dizzy, some folks’ problem with 3D movies is double vision.  Double vision happens because a frame of 3D film is actually two views of the same scene taken from different angles. In fact, this is exactly how having two eyes (binocular vision) gives us the ability to see the world in 3D. Since the eyes are separated by a distance (interpupillary distance), each eye gets its view of the scene from a different angle (see figure 2). The closer the object, the bigger the difference in the respective angles of view. The optics of 3D movie glasses make sure the left eye receives only the view angle meant for the left eye and the right eye receives only the view angle meant for the right eye. Our brain will then sort out a depth map of the scene based on the view angles of each eye provided the views are imaged on the correct location in each eye (retinal correspondence). If the two views don’t fall on the right locations in each eye, instead of a single view with 3D depth we experience two separate images. How do our eyes normally achieve the correct retinal correspondence? By making sure that their lines of sight intersect at the location of the object being viewed, in other words vergence. And as we saw above, in 3D movies the vergence response is sometimes disrupted.  Hence the double vision.

[Insert where appropriate pull quote: The optics of 3D movie glasses make sure the left eye receives only the view angle meant for the left eye and the right eye receives only the view angle meant for the right eye.]

The condition where the lines of sight of the eyes fail to intersect at the point of focus can occur outside the 3D theater. It is known as “binocular diplopia” and is corrected with eyeglasses that bend the incoming light just enough to achieve retinal correspondence between the eyes. Since prisms bend light, adding the right amount of prism to the eyeglasses accomplishes this fix. The often blank “prism” box in eyeglass prescriptions has to do with correcting for vergence dysfunction problems that cause diplopia (see figure 3).

3D display in relation to the functioning of the human eye is an active area of research beyond just the entertainment industry’s concern to eliminate the occasional headache and dizziness. The medical field is increasingly finding uses for 3D technology for performing microsurgery, creating a high-end demand for improved displays. Aptly, it turns out that the requirements of ophthalmic surgery have put ophthalmics at the forefront in adapting this technology.

Ari Siletz is president of CCDMETRIX. His company specializes in automated vision system inspection and metrology. With a background in both optical and software engineering, Ari has been developing instruments for the the ophthalmic and optical coating industries since the 1980s. Writing is one of Ari's serious hobbies. He is a published author whose short stories have appeared in numerous literary anthologies. He lives in Sebastopol, California.