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>It's also worth noting that in 3D, each eye only gets a new frame at half of the rate (because each eye only sees half of the frames), so each eye is getting refreshed at the rate of a regular movie.

With active shutter 3d yes, but with polarized 3d no. I think most movie theaters are polarized 3d.

48fps was chosen because it was compatible with a certain type of 3D projector that most digital theaters had used. Thus theaters didn't need to invest in an entirely new projector just to show the HFR version. The IMAX-certified 3D projectors (note: not IMAX domes) that RealD was pushing around the time Avatar came out was capable of this.

Technically, I think those projectors can go up to 60fps in 3D, but I can't remember/easily find where I read it so I'm not that confident about that figure.

48fps was chosen because it was a direct multiple of 24. You can cut the framerate in half and project it in all the old 24fps projectors.

More importantly you have an entire video and audio editing/production pipeline built for the timings of 24 fps. If your going to increase the rate of all those systems, its far easier to double the rate than increase by some arbitrary fractional amount.

The rate that the eyes/brain cannot detect changes will depend on many things including size of objects and speed (and possibly smoothness) of motion.

Using polarised 3D both eyes should still get 48fps they just receive different(ly polarised) light. TVs using active shutter glasses do reduce the frame rate by half but will usually be reducing it from 96Hz or more

By the way - in the cinema at 24fps each frame is usually flashed twice which encourages the psycho-visual system to fill in the blanks and smooth out the motion so it appears sharper and less blurred than it would on a LCD/Plasma TV screen that usually holds the image for the duration of the frame. Some high end models can do black frame insertion if they are using the right type of LED backlighting and are set to the right mode.

R&D on Showscan showed 72-76 fps was the threshold.

From personal experience that is far too low, I'd like to see the source for this.

I've read it in Cinefex long time ago, might be there is something online too. Douglas Trumbull did some sort of a controller experiment in front of a crowd with 70mm projection.

Which shot images have you seen played faster than 72 fps? I've seen one test played back at 120 fps, but it was in a camera r&d lab in germany. 72 fps is really more than enough. Consider that 3D projection actually projects 3 frames per each eye, so 3D@72fps would actually mean projector is churning out 432 frames each second. Or you're talking about games, which is another thing entirely since input is a part of experience too. In that case, I agree.

They project each frame 3 times so that your eye can't pick up flicker from the light going on and off. I'd imagine that at the full 72fps it would not be necessary to flash each frame 3 times.

Finding a 'framerate' for the eye is difficult. Under certain conditions, 60hz (think florescent lighting) can be enough that your eyes won't notice a flicker. (your eyes hold a residual for some period of time, and you never see the darkenss). But if you are flashing a series of super-crisp photos, your eye is much more likely to pick up on stuttering because each photo has just as much light.

And then, if you have some action sequence in a movie where parts of the scene are really bright and fast-motion, and others are dim, the minimum amount of frames per second you need to create a 'perfect' experience for the eye is anybodies guess. It would probably also make a difference if you were a person used to looking for flaws in a picture, vs a person who is just trying to enjoy a movie and doesn't know anything technical about framerates, etc.

Regardless, all forms of currently available cinema have framerates with room for improvement.

While I am not positive, my assumption is that 48fps was chosen as it is exactly twice the normal frame rate, allowing them to easily toss away half the frames per second to create the 24fps versions of the film.