I love 3D displays, but 3D movies are usually disappointing.

The key concept in a 3D movie is simple. You take two films simultaneously, from two eye-distance-apart cameras, and then project them in such a way that one eye sees one film and the other eye sees the other. Done right, this is a beautiful thing indeed. I have been fascinating by variants of this style of stereoscopic image for almost twenty years.

The problem with this technology is its sensitivity to many variables. In a theater you are almost guaranteed to have a less-than-perfect experience. In particular, two undesirable phenomena often appear: uniform and non-uniform depth scaling.

Non-uniform scaling happens when the infinity point is at the wrong width. When you look at clouds or stars or other very distant objects both of your eyes point in almost precisely the same direction. Thus, the left-eye’s image of infinitely-far objects needs to be to the left of the right-eye’s image of the same object by exactly the gap between your eyes: for me that’s 65 millimeters. If the gap is wider than that then distant objects are vastly stretched out in depth; if narrower then the sky looks like a painted backdrop and nearby objects are vastly stretched out in depth.

The other kind of scaling has several equivalent causes. If you move twice as far from the screen, or if you double the gap between your eyes or shrink the screen to half its normal size while keeping the infinity point correct then all depths appears to double: everything stretches out in the depth direction. You can mix and match these freely: sitting twice as close to a half-sized screen creates the correct depth cues.

If a theater is careful about calibrating a 3D movie they can make it look right to people with average-width eyes for a few rows of seats. You have to be lucky to guess which rows those will be. But most 3D movies I’ve seen don’t even get the non-uniform scaling right, making the images only 40 or 50 cm apart instead of 65: the sky looks to be not much past the screen, nearby objects are stretched out and distant ones are squashed.

Some people are troubled by another characteristic of 3D films: the lens focussing depth cue doesn’t match the stereoscopic depth cue for nearby objects. I personally don’t find this very annoying, but it is a problem for which I know of no solution.

I still love 3D movies, even though I rarely find one without non-uniform scaling problems and often have difficulty finding the right distance from the screen for those that are uniform. Ideally, a 3D theater would have only a few rows of seats at an optimal distance from the screen and would calibrate each film for those seats. Alas, I know of no theater that even attempts to achieve this ideal.