More Thoughts On 3D Cinema Optics

May 7th, 2009 by Clint Posted in film | No Comments »

I don’t have the same pessimism for 3D in the long run as the article that David Newman was responding to with his “No Problem With 3-D”. I have the strong sense that humans are clever creatures, and we will find ways to fix most of the problems that bother us. However, I don’t think we have the technical aspects of 3D mastered. We don’t get nauseous from 3D effects while watching a play on stage, even when Gallagher smashes a watermelon in our face, so I think there is still something afoot.

I can’t yet dismiss the idea that seeking between objects with unmatched convergence and focal properties is a way to cause viewer discomfort, even though David is pretty convincing that we won’t suffer long-term damage. Since the convergence point of the stereo pair should track with the object distance (as David Stripinis has pointed out), doesn’t that limit us to a single plane of interest where the convergence point and focus coincide? That would mean that a close-up large object would have one area that hits the sweet spot, and maybe the further from that area that your eye travels, the more confused you will be.

To revisit a point I touched in “Where Have 3D Stereoscopic Movies Taken You?”, John Batter once discussed how it’s important to carefully pace the amount of depth effect. Poorly paced depth effects lead to increased fatigue, which could also be a nausea contributor.

I’m starting to think that hyperfocal areas don’t work the same with stereo pairs as they do with a single lens (like in David’s optics examples), because the perceived image includes ghosting from opposite eye images. For near objects, I suspect this ghosting dramatically decreases the effective focal depth of a pair of eyes. Maybe this is part of why deep focus establishing 3D shots are my favorite. The amount of blur in areas surrounding objects of interest provide a strong cue as to the expected seek distance to move focus to another object. The camera lenses bake in a single blur amount for the scene, but in real life, the amount of blur changes based on the location of the object of interest with relation to the other objects. I wonder if there is enough of a mismatch between the expected seek distance and actual seek distance to cause a feeling that the world is shifting around unnaturally.

Ordinarily, we shift our heads slightly to “fix” confusing or interfering occlusion and edge problems, but it’s impossible to have a head shift fix anything for projected film. Shifts in object occlusion in real life depend mostly on distance from viewer. If we happen to flinch out of the way of a near-field object flying at us, the projected image won’t shift at all, leading to a definite mismatch between expected and observed motion. This kind of mismatch is what causes seasickness according to my favorite explanation in Alton Brown’s Good Eat’s episode, Rise of the Rhizome (starting at 6:55).

This does hint at another reason for why I love deep-focus establishing shots. Maybe there is only a tiny amount of parallax shift expected for far off objects when I shift my position around, so the lack of shift on the screen isn’t unsettling.

One trick to detect stereoscopic problems would be to track the head movements of a test audience. The more movement for a scene, the more likely it is to be nausea inducing. This might also be related to the classic “audience engagement” metric, where the times when the audience shifts in unison are worthy of extra attention.

I think we should create a 3D film nausea index combining the amount of seeking required between cuts, the magnitude of mismatch between expected and actual seek distance from object to object in a shot, and the ratio of in-focus objects that don’t have matched focal and stereo convergence properties. Once we can quantify the factors that contribute to viewer discomfort, we can automatically warn the editor and maybe someday the director on set, when they’re pushing the limits.

I’m still eagerly awaiting the first 3D gag that breaks the widescreen matte for added effect, even if there is still some discomfort caused by the medium. While I hope 3D can help filmmakers connect with the audience better, all I really want is to be shown another great story.

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