by Patrick Appel

A reader writes:

Walter Murch on the 3D debate is mistaken. Beyond about 20 feet our eyes are essentially parallel and so there is very little change in convergence between 50 feet and infinity. Stereopsis in movies can work very well because of this. It might be a problem with home 3D, but in my dissertation work I used a distance of about 6 feet, and it worked fine.

Another reader goes into more detail:

Here's Murch's argument stated more concisely. For any light entering your eye (or your camera lens) to be a focused image then your eye lens has to be focused to the distance the light came from. Since all light comes from the scattering screen in a movie your eye must focus there. But steroscopic 3D relies on the eyes converging to different depths to see the apparent source of the light. Murch's claim is that we evolved in a world where the convergence point and focal point are always the same. Ergo we can't learn to like stereo 3D.

Sounds good but it's wrong, not just for one reason but two.

First, he's wrong about the focus. What he says is true only if your eyes had infinitely large and perfect lenses. For finite sized lenses the hyper focal effect dominates. What this means is that as long as the smeared out size of a defocused point source is smaller than the resolution limit of your vision (i.e. the blur circle of a perfectly focused spot) then the point is effectively in focus. In practice this happens for any object located far from you. For human eyes in a darkened room this might be something like 20 feet. If you are that far from the screen then, when your eyes focus on the convergence zone behind the screen, the screen itself is effectively still in focus. Thus 3D works for everyone who is not in the very front row.

Second, it's patently obvious that stereo-scopic vision does give a great 3D look. If you ever looked in one of those old view-master stereoscope you can see that not only was the object 3D but it was in focus as well. How does this Jibe with Murch then? Well Murch is mistaking two different things called a convergence zone. Your eye balls don't do the 3D processing, your brain does. It compares the two slightly different images to infer the 3D. It is your brain that is inventing the concept of a convergence zone not the eyeballs. Now for objects very very close to you, your brain will also notice that your eyeball muscle are pointing the eyes in a non-parallel way. And your brain may use this info as well. This is the physical eyeball sight line convergence that Murch is worried about, not the one that comes from the brain processing. And again for any object farther away than 10 times the distance between your eyes then your eyes are pointing nearly parallel and there is no sightline convergence info for the brain to consider. All that remains is the cognitive inference of depth and that works fine because that is EXACTLY how your brain was built to work.

Thus to the extent that Murch has a point it is these two: 1) don't sit in the front row. 2) film makers should not project the 3D objects out of the plane any closer than 10 feet from my nose for long periods of time. Other than that Murch is wrong.