ENJOYING 3D WITHOUT GLASSES
David Seigle; In-Three, Inc.; April, 2008
Have you ever noticed pros watching 3D? They often slide their glasses up to view the movie. What do they find so interesting about blurry images?
The likely answer is that they’re viewing the scene’s parallax -- that is, the separation of objects -- in the images that are intended for their left and right eyes. This separation determines the perceived depth that viewers with glasses will experience. They may also be looking for discrepancies that can cause fatigue or even headaches; i.e., they’re checking how good the stereo effect is.
Parallax
Depth in or out of the screen is created when the left eye and right eye views cause viewers’ eyes to cross at a point in front of or behind the screen.
Adult eyes are separated by about 2.5 inches. Think about looking at a ball on a screen when the right eye’s view of the ball is 2.5 inches to the left of the left eye’s view of the ball (negative parallax). When exposed to viewers with a proper projection system each image will cause each eye to track the ball independently, crossing one half, or 50%, of the way to the screen. (If you question this, sketch the scene looking downward to see its geometry.)
On the other hand if the ball in the right eye’s view is 2.5 inches to the right of the ball in the left eye’s view (positive parallax), the axes of the eyes’ vision will be parallel. In this case the ball will appear to be at infinity -- just like when you look into the distance.
Normal eyes do not diverge. Therefore if you see positive parallax on the screen exceeding 2.5 inches, be aware that the scene is creating an abnormal and difficult viewing experience, one likely to cause discomfort.
On the other hand if the negative parallax exceeds 10 or more inches, the scene is presenting objects at or closer to you than 20% of the screen distance. That also may be difficult to view for any length of time.
At the end of this essay there’s a handy table to use when judging where a stereographer is placing objects in space:
Other Disparities
In addition to excessive parallax, pros may look for other disparities. For example imperfect lenses of dual cameras may cause an object to appear with vertical disparity causing one eye to track upward and the other downward. To detect vertical disparity, look for small bright objects. When displayed for each eye the object’s left/right images should be horizontally aligned. If they are displayed diagonally they will cause the viewers’ eyes to track vertically in opposite directions. This is a major cause of eye fatigue.
Also without glasses you can see differences in bright highlights. Sometimes these highlights are recorded differently in each camera (because of differences in the two lenses). The brain tries to make sense of the two different impressions of what it knows is the same image. Over time this too can cause fatigue.
Style
In addition to parallax and discrepancies, you can see the stereo style explicitly with your glasses removed. If there is no negative parallax in a scene, the cinematographer intends the scene to be played as if the screen is a window into the world.
If objects of primary dramatic interest have no separation, the director wants them to be played at 100%, i.e., at screen level.
If the parallax ranges from say -5 inches to +2 inches then the cinematographer has created the content for families. (Kids eyes are separated by about 2 inches; as a result, adult 3D content may be uncomfortable for them.)
And, of course, when the parallax ranges greater than negative10 inches the director wants that arrow to appear right between your eyes.
As you can see, there’s a lot to enjoy when watching 3D without glasses.
A Handy In-Three Guide: Distance to the Screen as a Function of Parallax
| Inches of Parallax | Distance from Viewer to Screen (%) |
| 2.5 | Infinity |
| 2 | 500 |
| 1.5 | 250 |
| 1 | 167 |
| .5 | 125 |
| 0 | 100 |
| -.5 | 83 |
| -1 | 71 |
| -1.5 | 63 |
| -2 | 56 |
| -2.5 | 50 |
| -3.5 | 42 |
| -5 | 33 |
| -7.5 | 25 |
| -8 | 24 |
| -9 | 22 |
| -10 | 20 |