A lot and not much. ;^) How’s that for an answer? The human eye is a very complex vision system that is integrated with an even more sophisticated processing system (the brain). There is a lot going on when we see stuff. One piece of the puzzle is how we perceive motion. Some suggest that humans have a persistance of vision (how many images we see each second) of about 70 or 80. For a long time, televisions updated 1/2 of the picture at 60 images per second and it was thought that this was fast enough to eliminate flicker. Motion Pictures have 24 images per second but each image is shown 3 times so the eye is presented with a “refreshed” image 72 times per second. When coupled with camera shutter speeds of 1/60th of a second or slower the motion blur of the source image with a 60 or 72Hz refresh rate was deemed sufficient to provide fluid motion. Nice of “them” to deem that huh? Hey, why would we ever need a shutter speed faster than a 60th of a second?
Oops, this entertainment thing called sports made people complain that the baseball was blurry. Couldn’t that be fixed? Sure, use a faster shutter speed and the ball comes into sharp focus. But wait a minute. When you watch it the ball stutters across the screen. Well, that’s because we process motion seemlessly with our eyes and brain. We don’t see things in 1/30th of a second 1/2 resolution flashes or 1/24th of a second images flashed three times. *
So, there was now a need to increase the number of images per second. And the scientists are trying out all kinds of stuff. One process interpolates inbetween images with a computer to to “simulate” a more seamless presentation at faster refresh rates. Some of the 120Hz televisions work that way. And they sort of work. Unfortunatlely, when we deviate from reality everything becomes subjective as to what is good enough. And we are so far removed from reality with digital video lossy compression and colorspace lossy compression and interpolated spatial and now temporal image information that you can argue yourself into a zillion directions.
Now we add 3D to the mix. This process cuts the refresh rate in half. So that 120 Hz that was so critical for sports is now back to 60Hz for each eye. And the baseball is blurry again or it stutters if a fast shutter speed is used. And what are the manufacturers touting as super cool 3D? You guessed it: Sports!
What we have now are inconsistent continuously variable levels of quality coming at us over fibre optic or satellite or cable. And now we are going to add 3D? The problem is that there is no way that the 3D will match the fluidity and pleasantness of 2D on the same television. That is unless they cripple the 2D performance of the TV so that it doesn’t exceed the 3D performance. Sure, like that’s going to happen.
Consumers are going to get very confused and it is one step forward for 3D and two steps backwards. And remember I said at the start of this that the perception of motion is only one piece of the puzzle? There are many other issues.
However, I am not pessimistic. I think the future is pretty good for 3D because a lot of people are working very hard to improve the technology. Many things will look absolutely fantastic on 3D televisions and at motion pictures. But many things will also look like total crap. We will just have to wade through the mess for a few years until things mature and incrementally improve.
As to 120Hz, in general it is much better than say 60Hz and 240Hz has the potential to be better than 120Hz. But there is a lot of funky marketing stuff going on and as I said before, it is only a piece of the total puzzle.
*It is not known what the limit is to the processing of images by human beings because it depends on things like after image retention and the variability of the transmission of signals from the cones in the retina to the brain. As it turns out they all don’t work at the same exact speed. Our vision system and indeed the whole human body is an analog processing system that works differently as compared to an all digital processing system where there is always either an on or an off condition. Case in point? A military test showed that a pilot was able to identify accurately the type of aircraft that was flashed on a screen for 1/220th of a second (source www.100fps.com).