Do contemporary cinemas really project at 25fps?

This is one of those interesting questions which gets more complex (and harder to answer) the more you learn about it. Unfortunately I cannot answer definitively how theater projectors work. I'll explain why I don't think that question can be answered. And I'm going to reference 100fps.com a bit.

First, I don't think the question can be answered now, because projector hardware is kinda stuck. Look at how few screens in your area converted to 3D in the past few years. Yes, most theaters likely have 1 or 2 screens capable of 3D, but the expense of converting them all just isn't supportable. Because of costs like this, most theaters are using projection methods which are likely very out of date; so even if newer/better projection methods exist, it's likely you're still seeing older methods. But this question likely couldn't be answered in the past, because of different video standards. American cinema companies developed one set of film standards, while european companies developed another. This carries into broadcast encoding technologies for TV, and pretty much every corner of video.

One great bit of discussion can be found here. It looks at the question of how the human eye perceives video images, and some of the trade-offs necessary to make movies look fluid. Note in particular this observation:

The fact is that the human eye perceives the typical cinema film motion as being fluid at about 18fps, because of its blurring.

This makes the main page at 100fps.com more relevant to the question of how cinema can get away with such low frame rates, as the main page is focused on interlacing (and image de-interlacing). Cinema blurs the images in each frame (by combining time slices) it gives the illusion of smoothness to motion.

As near as I can tell, most theaters in the US run at 24 frames per second, with 2 or 3 exposures per frame. The higher exposure rate prevents the eye from seeing the black shutter. The interlaced frames make the image look smooth. By comparison, most theaters in Europe show 25 frames per second, still with 2 or 3 exposures per frame. This low frame rate (by modern video game standards) works because the images are not crisply rendered computer images. So motion blur works to make it look better.

Some further reading: NTSC on Wikipedia, PAL on Wikipedia, Projectors:Shutter on Wikipedia, and the Flicker Fusion Threshold.

TV presentation of movies in Europe will always run at 25fps to synchronise with the TV framerate (anything else would require expensive conversion or incur terrible flicker as the slightly different frame rates interfere with each other). The difference is barely noticeable (but the movie will play with a 4% shorter run time and, for a non digital soundtrack, a 4% higher pitch for sound). In the USA where TV is 30fps more complex solutions are required.

There is no good reason for a projector in a movie theatre to do this: movies are shot at 24 Hz and shown at 24 Hz, so why would they? However digital projectors have no physical limitation on the framerate they use and it would be easy to project at different rates if there was a good reason to do so. But you wouldn't see big benefits unless the rates were much faster and many of these benefits would only appear if there was more data to project (it isn't entirely obvious that projecting the same frame twice leads to observable gains, for example, and TV sets that do this from 25 or 30fps signals also cause new and sometimes annoying artefacts on the perceived image).

Some modern movie directors (I believe Peter Jackson and James Cameron are pioneering the idea) are using digital shooting at 48 fps which will look a lot better than current movies if projected at 48 fps. And, note, their choice of 48 fps not 50 fps is dictated by compatibility with the current standard of 24 fps. Presumably, none of this will matter when everything is digital, but compatibility with the existing standard will make showing old movies originally shot at 24 fps less of a problem.

We don't have 3:2 pulldown - the film (audio and all) is simply sped up by (25/24) or about 4 percent. Same thing that is done for Television then 2:2 pulldown is applied to convert it to interlaced TV format. At least that is how things are done in Australia.

The reason for the frame rates is a historical one, Basically it is easier to rely on the AC supply to provide a timing source than create your own. The US uses 60Hz AC supply, so it is easier for them to use multiples of 6 for their timing (hence 24, 48, 60 fps). Countries which followed the European electrical standards use 50Hz AC supply, so it is easier to use (25, 50, 100 fps) Because of this, film and television standards developed in both areas around what was easiest to display (and thus made TVs and Projectors cheapest to build).

There is also a second reason why 25fps is preferred - the light globe in the projector may have 100Hz flicker and running at 24fps would produce a noticeable variation in image brightness. 25 fps allows synchronising with the AC supply.

This would only apply to older film projectors though, as new stuff can use high-frequency drivers for the light source, as well as stepper motors and microprocessors for control, thus allowing for any frame rate you want. Digital projectors are only restricted by the response time of their base media (DLP or LCD panel) and data processing capability.