Another overstatement that 3D won't work.

Roger Ebert, blog post : Why 3D doesn't work and never will. Case closed. with the help of Walter Murch, tries to use some science to explain that 3D won't ever work, but gets the science wrong.

Here is my reply (on his blog) in response to his post:

While there are issues with 3D presentation, the claim that the "convergence/focus" issue makes 3D unsolvable is false. There is an error made in the assumption that "the audience must focus their eyes at the plane of the screen"; while that is generally true for objects close to a viewer in space, it is not true for a movie screen "80 feet away."

In optics there is the concept of hyper-focal distances -- there is a focal distance in which a lens will resolve all objects at that distance and beyond such that they appear in focus. The human eye is just another lens. While calculating the hyper-focal distance of the human eye is tricky, and likely has a good degree of variance between subjects, the distance for theatrical viewing is well beyond the needed range for all but Superman. Various ways of computing the hyper-focal range of the eye suggests that objects from around 15 feet to infinity will appear in focus. That means a 3D presentation that has objects appearing no closer than 15 feet and beyond will appear in focus whether the audience is focusing at the screen plane or not -- the eye is free to converge and focus anywhere within the volume of space projected, just as it naturally would.

While there are many areas that can and should be improved, presentation brightness, left/right cross-talk, glasses comfort, and the artistic battle of shallow vs deep depth of field for 3D, eye focusing for theatrical presentations is not an issue.

I wrote more technical details about this when Daniel Engber's article in Slate made the same claim on my blog (here.)

David Newman

CTO, CineForm

Update 01/25/2011: I see that lots of people are reading this. Thank you for dropping by. Some twitter comments have asked "what about TV, that is less than 15 feet away?" Fortunately the same optical principles apply, just with a smaller 3D volume -- which I have covered prevously here. A theatrical 3D release which might have 3D preceived depth from half way to the screen to infinty, has its depth shrunk when placed on a 5-10X smaller screen, resulting in a 3D volume say from 6 to 30+ feet, which happens to be within the depth of field for the human eye for a screen placed 12 feet away. As the screen shrinks further, so does the 3D volume, allowing 3D to work within the eyes abilities at most scales.