Wednesday, May 06, 2009

No Problem With 3-D

The title of the post is a come back to Daniel Engber's article in Slate "The Problem With 3-D", with the sub-title "It hurts your eyes. Always has, always will." As CineForm is entering the 3D post production world, I was curious whether his claims are valid. I'm personally one who doesn't find modern 3D film difficult to watch, and very much prefer to look at recent films like Coraline 3D over their 2D presentation, but I spend much of my day staring at images for quality analysis, so I'm not the best test subject.

Engber states the visual fatigue that "plague flight simulators, head-mounted virtual-reality displays, and many other applications of 3-D technology" is directly connected to 3D movie eye strain and that "no one yet knows exactly what causes this." Engber then goes to propose a reasonable sounding theory that our eyes want to refocus on objects that are not really closer or further away than the physical screen plane, and a likely cause of strain. This seems like a logical explanation for those experiencing eye fatigue, and he offers no other. The article then goes on to suggest "if 3-D becomes as widespread" the possible blindness (well "permanently cross-eyed") of your children -- Wow! Now I was initially going to accept the earlier claim of convergence without refocus being a potential cause of eye strain for some, but now that my kid's eyesight is involved I had to dig deeper.

I dug, and I now believe he is wrong, at least for most correctly presented theatrical presentations. I'm also proposing a theory without rigorous test data (just like Engber), but focusing on the optical characteristics of the human eye. I wondered whether hyperfocal distance, the particular range from x feet to infinity such that all appears in focus. While a typical lens has a single point of focus, there is range in which focus is still considered sharp. Whenever depth of field is discussed, that is talking about the same range of acceptable focus. So from Wikipedia "hyperfocal distance is a distance beyond which all objects can be brought into an "acceptable" focus." If the screen is beyond the hyperfocal distance of the human eye, all 3D images behind the screen plane still appear in focus, and a certain amount in front will still be in focus, with some simple rules. With all images in the theatrical 3D space appearing in focus, it doesn't matter if your eyes do change the focus range, so Engber's claim does not hold up, and your children are safe.

Basically the problem described in the article only happens in close screen viewing conditions or extreme "coming at ya!" 3D that has been losing favour as 3D projection becomes more common. In a typical movie theater the viewing distances are such that the eye can do its natural convergence and refocusing without losing focus on the presented 3D world.

Now to calculate the acceptable distances for 3D, we needed to calculate the human eye's hyperfocal distance. With some online research I was able to determine the eye is approximately a 22mm lens system (seems about right,) with a maximum f-stop of 2.4 (darkened theater would do the trick.) There is a great article on The Photographic Eye from which I gather numbers I used (they agreed with many sources.) Now we can plug these numbers into a lens calculator, and get a number for 35mm cameras -- 22.3 feet hyperfocal, focus range 11.1 feet to infinity. So if eyes were 35mm cameras, as long as the 3D object remains more than 11 feet away from us we can comfortably and safely view it and everything behind it into the 3D world. But of course our eyes are not 35mm cameras, and are more complex to model, but the heart of all this is the Circle of Confusion (CoC - the amount of allowable blur.) So instead of guessing the camera system that models the human eye, let's calculate what as acceptable blur for the typical theater viewing environment.

For our theater model, we have nice 40 foot horizontal movie screen at a viewing range of one and half screen distances, i.e. 60 feet away, using a common 2K projector (99% of all digital projection is either 2K (2048) or 1920 line HD.) So the amount of allowable blur is related to the pixel size, as we don't see a lot of chunky pixels, the resolution is high enough that it fuses into a continuous image for the audience. So let's estimate that a half pixel blur is OK and is still perceived as sharp. For the approximate 2000 pixels around 40' screen, 0.5 pixels will be 0.5/2000*40 = 0.01 feet or a blur of around 1/10th of an inch. The viewing angle for our blur at 60' is calculated as 0.01 degrees. As Circle of Confusion or CoC is calculated at 25cm, the 0.01 degrees results in a CoC of 0.04mm. Now using the CoC number in our lens calculator we get these results: When viewing the screen at 60' away, all objects from 13.1' to infinity will appear in focus. If an object jumps 75% off the screen and is perceived as 15' away, and you focus on it at 15', it and the screen plane are still in focus, so no source of eye strain. We now have the safe/enjoyable range to present a 3D image. You might be thinking the amount of allowable blur at 0.5 pixels was overly generous, and it was, in 3Ds favor. Wikipedia and other sites place the average acceptable CoC at 0.2mm, yet the numbers above are five times sharper than that (so there is plenty of headroom for the average viewer.)

This potentially points to the home screening environment having issues with the screen being so much closer. Yet using the same human eye lens modelling, a 3D depth range can be created such that the eye can focus at will without causing blur issues at the screen plane, or introducing eye strain. Plus as the average home environment is not as dark as the theater experience, we can use a different lens f-stop in our calculations. If our eye is more typically at f/4 for home viewing (totally guessing -- love help here) for a screen at 12' distance, 3D images can be placed at 6' (half out of the screen) to infinity (still using the same very sharp 0.04mm CoC.) So there is a reformatting required between theatrical and home release, but this was already an accepted factor to adjust for the smaller home screen.

This is not to say that there aren't other factors that contribute to eye strain in today's 3D technologies, such as imperfect filtering causing left/right cross-talk and poor quality glasses introducing other optical distortions. Yet the biggest factor to eye strain is more likely due to inexperience of 3D film making, such that there are good and bad presentations, which have nothing to do with the technology. The film making and the tech can only improve, and there is no inherent cause of eyestrain in today's 3D presentation technology.


Here is a fun aside. For those wanting to experience the 2D and 3D version of a film without going to see the film twice (or closing one eye for prolonged periods of time), create yourself a pair of 2D glasses. These will also come in handy if you happen to see one of those bad 3D movies. Get two pairs of standard RealD glasses and pop out the left eye lens from one and the right eye lens from the other, and swap them. With a pair of scissors to cut the lens down a bit, you can restore the lenses to the frame with the opposite position (be careful not to flip the front and back side of the lens), so that you have one set of glasses that is a left only and another that is right only. At any time during a 3D film, put on your 2D glasses, to experience that retro 2D world.