There are three main factors that contribute to the negative effects the two Visual Sins can have on the audience:
1.) Where is the audience looking? The Visual Sins can’t cause problems if the audience doesn’t look at them. Every shot has a subject and a lot of non-subjects. The audience spends most of its time, or all of its time, looking at the subject. The subject is the actor’s face, the speeding car, the alien creature, the adorable dog etc. If the Visual Sins have impacted the subject, the audience sees the problem and gets brain strain.
But most of a scene is not the subject. Peripheral objects, backgrounds, unimportant characters, crowds etc. are all non-subjects that the audience acknowledges but tends to ignore in favor of the subject. Non-subjects can tolerate most of the Visual Sins because the audience is looking elsewhere.
2.) What’s the screen size? The problems caused by the Visual Sins can occur on any size screen, but the problems become more severe as the screen gets larger.
3.) How long is the screen time? Time is critical. The longer the audience looks at the Visual Sins the greater the risk of brain strain. All of the Sins have degrees of strength and may cause instantaneous discomfort or take more time to have a negative effect on the audience. Brief 3D movies like those shown in theme park thrill-rides can get away with using the Visual Sins in ways that would be unsustainable in a feature-length movie. An audience can even tolerate the Visual Sins in a long movie if the Sins’ appearance is brief.
Fortunately, the Visual Sins can be avoided or controlled to create a comfortable 3D viewing situation. The following discussion assumes the 3D is being presented on a 40-foot theatre screen.
Sin #1: Divergence
A stereoscopic 3D movie may require the audience’s eyes to diverge. This can be a serious viewing problem and can cause brain strain.
Divergence occurs when the viewer’s eyes turn outward in opposite directions to look at the subject in a scene. In real-life, our eyes don’t diverge. Ever. Look at yourself in a mirror and try to simultaneously force your left eye to look at your left ear and your right eye to look at your right ear. It’s impossible to do. Both eyes want to look at the same ear at the same time.
In the real world, both eyes converge on the same object at the same time.
But when watching 3D, our eyes can be forced to diverge or angle outwards in opposite directions to look at an image pair. Divergence can be a problem when it involves the subject of the shot because that’s where the audience is looking.
Consider how our eyes see a stereoscopic image pair for a subject that appears behind the screen. The left eye sees the screen left image and the right eye sees the screen right image. Human eyes have a 2.5-inch IO. If an image pair’s actual measured parallax on the screen surface is 2.5 inches or less, the audience’s eyes will not diverge.
On a 40-foot theater screen with 2K resolution, a 10-pixel parallax will measure 2.5 inches or about 0.5 percent of the screen width. The 2.5-inch parallax separation forces the audience’s eyes to look in parallel but that will not cause eyestrain. In real life, we do the same thing when we look at any object more than about 40 feet away.
As the measured parallax widens past 2.5 inches, divergence will occur. The tolerance for subject divergence varies, but most people can watch subject divergence up to about 7.5 inches of measured screen parallax without feeling eyestrain. A 7.5-inch parallax is +30 pixels or about 1.5 percent of the screen’s width.
A parallax separation greater than 7.5 inches is called hyper-divergence. It can be used briefly for extreme subject punctuations but sustained hyper-divergence for a subject can cause eyestrain and headaches. Hyper-divergence can be used successfully for peripheral non-subjects without causing eyestrain because the audience isn’t looking at them directly; it’s watching the subject. Non-subject divergence can add depth that would be difficult to assign to the subject.
Watching hyper-divergence can be aesthetically distracting, and visually tiring. It’s like trying to hold a heavy weight. Initially, the weight feels tolerable but as time passes your muscles fatigue, the weight feels heavier, and eventually you collapse. The same pattern occurs with hyper-divergence and it becomes visually stressful.
Hyper-divergence is less likely to occur on television screens. A 60-inch (measured diagonally) consumer HD 2K television has an actual measured screen width of approximately 52 inches. A parallax of +92 pixels (4.75 percent of the screen width) measures about 2.5 inches. Any background object with a +92 pixel parallax places that object at infinity, and will not cause divergence.
A pixel parallax up to +280 or 14.25 percent is theoretically tolerable but is unusable in practice because other problems occur like ghosting. In practice, a television background object’s parallax of up to +92 pixels is tolerable, won’t cause eyestrain, and is extremely useful directorially. Placing objects farther away than +100 (5.25 percent of the screen width) isn’t necessary.
Divergence’s eyestrain is actually due to a combination of screen-measured parallax and the viewer’s distance from the screen. See Appendix C for a full explanation.
Hyper-divergence can cause another problem for the audience. If an object’s image pair is too far apart, the audience won’t be able to fuse them into a single 3D image. Even when wearing 3D glasses, the image pair appears as two identical objects rather than a single, fused stereoscopic image. The non-fused image pair visually disconnects the stereoscopic depth and the 3D illusion collapses.
Sin #2: Ghosting
Ghosting (sometimes called cross-talk) appears because most 3D viewing systems cannot completely separate the left and right eye images of the stereoscopic pair. Each eye gets some “contamination” and sees a faint “ghost” of the image meant for the other eye. Ghosting is most visible in high contrast image pairs with a large parallax.
Put on your 3D glasses and look at these photos. Moon #1’s stereoscopic pair shows severe ghosting because it has high contrast and a large parallax. Even with your 3D glasses on, you can still see two moons instead of one. The ghosting is less noticeable in Moon #2 because there is less parallax. Moon #3’s ghosting has been eliminated by completely removing the parallax but it’s lost its depth.
Lowering the tonal contrast between Moon #4 and the background reduces the ghosting. Moon #5 uses a glow to decrease the contrast and minimize the ghosting.
Ghosting can be reduced by art direction and lighting. Avoiding high tonal contrast in sets, locations, set decoration, and costumes can reduce the problem. A fill light can reduce the tonal contrast and add light to deep shadows to avoid the ghosting.
Single person 3D viewing systems, like those pictured here, eliminate ghosting because their mechanics completely isolate the image for each eye.
Excerpt from 3D Storytelling: How Stereoscopic 3D Works and How to Use It by Bruce Block and Philip Captain 3D McNally © 2013 Taylor and Francis Group. All Rights Reserved.