How many EXACT frames does it take for the eye not to notice the change from frame to frame? I’ve been told its about 20 but I’ve heard as low as 12. Any ideas?
It varies a bit from person to person, but 15 is a good average.
Your vision system will blur everything together at about 15 frames per second, but if there are harsh changes between each frame (like in a video game, where polygons are rendered exactly, as opposed to film where movement gets blurred onto the film negative) you can still tell that something isn’t quite perfect, even if the frames are too blurred together for you to exactly see what is going on. Once you get above about 50 frames per second you can’t tell the difference any more and everything looks smooth.
Silent movies used 20 FPS. When they added sound they needed to go to 24 FPS so that the lips would sync up with the voice; i.e., to have enough lip “samples” to make it look realistic.
You also need to have a refresh rate of ~ 50 Hz to avoid flicker - the exact figure depends upon viewing conditions. Movies do this by showing each frame twice and TV by interlacing, i.e. showing alternate scan lines on each pass. If you are doing this on a computer then the display hardware will do this for you.
I don’t think that’s true. Unless you’re talking about (some) animated movies.
Oh but it is true - Linky
The Wikipedia has some interesting things to say about this topic:
And then…
Well what do you know. I had no idea.
I thought interlacing was done in order to get a reasonable horizontal scan frequency while at the same time preserving vertical resolution.
Completely wrong. I’ll get to why in a moment.
Here, take a look at a fairly simple visual representation of HOW and WHY Persistence of Vision works. Click both images.
This link gives you an idea of how persistence of vision is related to both motion picture and high def video imaging.
First of all, to figure out why frame rates of 24 frames per second ( fps ) and 25 fps in Europe came about, it’s very helpful to consider how and why a slower frame rate of 16 fps nominally was arrived at. Fascinating to note that things were a bit wild and wooley in the pre-sound days. This is not to say that film speed picked up to 24 fps because of the movement of lips. Such is not the case. This is just proof of why 16 fps kind of became the settled-upon standard.
An excellent in-depth treatise on Persistence of Vision.
Excellent three-part deep history of sound on film
And finally, courtesy of a reasonably well-composed treatise in Wikipedia on Persistence of Vision, we see why film frame rates were settled at 24fps. ( 25 European, for reasons I will cover in a second ). The lower reasonable threshold is in the 16-18 fps range, but as has been pointed out in cites above, running film through a projector gate at 16 frames a second when it was nitrate based film was a recipe for disaster- the film didn’t pass the arc lamp fast enough and the film could ( and did ) catch fire. So, there’s one nice reason to speed things up.
The more important reason is to find the absolute minimum threshold at which our brains will drop away a feeling of flicker due to the shutter blades in a camera/projector and perceive true motion. When does this apparent effect fall down, even today? Any time you see a motion picture shot where the camera is panning a bit too quickly. As a professional cameraman, I learned many years ago just what the threshold was of panning sideways while shooting. You either pan slowly enough to avoid the strobing, or pan faster and have things blur. An excellent example of this mistake is in the theatres right now- go and see Inside Man. Great movie ! Near the very end, we are taken through some rooms in the lower level of a bank. We are looking around a room, and the camera shows us the room quickly, panning and tilting from left to right. The camera operator panned a bit too quickly, and there is a strong stroboscopic effect present. We’re trained to complete the missing bits, but in this case, not quite enough clear information is on each frame for our brains to fill in the blanks and so we see strobing or flickering.
Anyway, back to minimum threshold. The move from 18 fps to 24 fps had nothing to do with moving lips and everything to do with the lowest threshold of a clear non-strobing set of images. Proof? In the old days, one owned a record player. It had four settings. ( I owned my gramma’s record player/worldwide shortwave band radio. So, I know that regular consumer models appeared as such ). It had a setting for 16 rpm, 33 1/3 rpm, 45 rpm and 78 rpm. It is untrue to say that filmspeed moved to 24 fps to accomodate the movement of lips, just as it is untrue to say you cannot record the spoken voice below 33 1/3 rpm’s, because the sound of the spoken voice would blur or slur or be unintelligible. It is simply untrue. As long as you record and play back at the exact same speed, you can fit a heck of a lot of stuff on the side of a disk. Side note- my Gramma lost her sight to a large degree near the end of her life, and we got her a record player provided by the government, and books on disk to be played on the device. I don’t remember seeing a number but from watching it turn very slowly, it was clear to me that it ran MUCH slower than even the 16 rpm’s of the old set I’d had as a kid. The audio was fine. Was it the same high fidelity you’d get by recording a minute of talking at 78 rpm’s? Of course not. Was it completely clear, sharp and intelligible? Absolutely.
So, why 24fps? Because the flicker is gone, unless you screw up while shooting and defy the ability of the brain to compensate by panning too fast. At 24/25 fps, showing each frame twice of course, you are giving the brain enough data to create seamless moving images.
Ok, so that’s the lowest threshold. We accept the moving images, and see life-like motion. So what happens when you decide that 24 fps in a frame of film 35mm wide just isn’t enough? What if you want to saturate the eyes/brain with a lot MORE information per second?
A special-effects cameraman named Douglas Trumbull, whose work wowed us all in 2001:A Space Odyssey, came up with a clever system called the Showscan system. This used 65mm/70mm film. Motion picture cameras use 65mm negative, release prints are 70mm wide to accomodate the optical tracks. People are used to seeing ads for films shown in “70mm”, so frequently it is referred to as 65mm/70mm. In truth, nobody passes different width films through the gate of a movie camera. In using a larger negative area, and passing the film at 60 frames per second, the brain is delivered quite a bit more information.
I’ve only seen one Showscan film- Mr. Trumbull tried to make a go of it with this new technology making regular Hollywood movies. He came up with a very clever script to show off the stuff, and I got to see it when it came out, along with some other folks. The film bombed, unfortunately. It was called Brainstorm, and made excellent use of the Showscan system. It was also the film that Natalie Wood died while making.
Why didn’t early sound producers go for a higher frame rate than 24fps? Because film costs money and money drives Hollywood. They accepted the need for a certain amount of footage to be used per minute, but no more.
The way to keep motion picture cameras running at a truly constant speed, and running in synchronization with the sound recording device, was to use an oscillating quartz crystal that would be fed an electrical impulse from a power supply and would oscillate perfectly at a given frequency. In the USA, we use a 110 volt 60 Hz cycle of electricity. The math was made easy, and so running a camera tied by cable to a sound recording device whose crystal was oscillating at a 60 Hz cycle, meant that divisors of 60 were acceptable. 6x 4 is 24, 6x10 is 60. This is why 24, and not 23 or 25.
Why 25 in Europe? Because in Europe they run on a 240v 50 Hz cycle. The math was easier yet for the Brits, French and the rest of the folks there making movies. Make the quartz oscillate at a 50 Hz cycle, shoot at 25 fps. The hidden pulsetone that is recorded on the sound track of every film shot, is synchronzed with footage shot with a camera whose motor is slaved by a matched crystal motor.
Oh- the IMDB Trivia area is partially incorrect. The "Brainstorm"recorded sequences were in fact shot on Showscan. A perusal of the technical specs for that movie reveal a mixing of film stocks, aspect ratios and whatnot never seen quite that way in another film. ( I know, Oliver Stone’s J.F.K. mixed up stocks and color and film sizes. They still didn’t use Showscan ).
I hope this explains why we arrived at 24/25 fps as a filming and projecting rate, and a few other things as well.
Cartooniverse
I remember Showscan vividly. There was a Showscan theater near me some years ago in some kind of Pizza joint. I went to one movie with a friend, and though I don’t remember the movie, I do remember the short that ran first as a demo of the technology. It started out kind of staticy, as though there was something wrong, and then stopped to reveal some guy sitting behind the movie screen, backlit. He walks up to the screen and pushes on it so that it bulges out toward the audience. Absolutely 100% realistic and 3D appearing. So much so that my friend could not be convinced that it was a movie he was watching. He was absolutely certain there was actually a person behind the screen pushing it out towards us. The higher frame rate and higher resolution of the 70mm frame made for an amazingly realistic experience. I wish I could see it again.
By the way, thanks for the informative post, Cartooniverse.
From here:
I also ran across this explanation in a book on video standards that I do not have handy at the moment.
I have to correct a few errors in Cartooniverse’s generally good post. First, all (conventional) motion pictures are shot and projected at 24 frames per second. There are no 25 fps films. I think you may be confusing the frame rate differences between US and European TV standards: 30 fps for US (NTSC) and 25 for Europe (PAL).
Second, although Douglas Trumbull hoped to release Brainscan in his 70mm/60 fps Showscan format, and and probably shot portions of it in the format, it was not released that way. Cite:
Trumbull did make several short (30 min.) Showscan films, including New Magic, the film flex727 saw. Pretty much the only place they were shown was the handful of purpose-built theaters the Showscan company managed to install in Showbiz Pizza Parlors. The format was also used in the 1980s and '90s in motion simulators and theme park attractions, like Tour of the Universe at the base of the CN Tower in Toronto. AFAIK, there are no Showscan theaters extant today, except possibly at the Futuroscope theme park in France.
flex is absolutely right that Showscan was a spectacular experience. In many ways it was better than IMAX. Its greatest benefit was that it virtually eliminated the strobing effects that Cartooniverse rightly complained about. Sadly, the company wasn’t able to overcome the chicken and egg problem that any new format faces–building enough theaters to generate production of enough films–and the format faded into obscurity. (The good news is that Trumbull’s patent on projection of 5-perf, 70mm film at frame rates between 49 and 72 fps has expired. So if you want to shoot and show your own Showscan film, go right ahead.)
BTW, I met Trumbull several times, and he told me that while developing Showscan he had experimented with frame rates up to and over 100 fps. He said there were definite improvements in image quality at speeds higher than 60 fps, and that he would have preferred 72. But it was more practical to use 60 fps, because it was easier to run projector motors at 60 Hz, and because 60 fps (and the Showscan aspect ratio) synched exactly to HDTV. In fact, quite a few productions intended for presentation in HDTV have been shot in Showscan.
Early part of the last century? Whoever wrote that doesn’t know his film history. Motion picture soundtracks were not recorded on magnetic tape until 1950, and magnetic soundtracks were not put on release prints until three years after that.
Kevin Brownlow, Silent FIlms: What Was the Right Speed?.
You’re wrong. In the United States, that is the framerate. I heartily encourage you to call up Sammy’s in London and tell them to change the crystals in all of their Panavision and Arriflex cameras because you said so. Sorry. You’re just wrong. I’ve HELD Euro Arri bodies in my hand and seen the marking for a 25fps crystal.
I had to poke around A LOT- but I admit you are correct here. What I saw was a 70mm print, not a Showscan print. ( Huge difference ). The movie moved along, showing a frame that was in fact largely black around the edges, with sound not fully coming from the six speakers. When the “Brainstorm” scenes ran, the image suddenly jumped from a masked-out frame to full 70mm image, with 6-speaker surround sound. Quite jarring. However, you are right, I was watching a 70mm print the whole time. 
Tough to do, since Cinema Products Corp which made the Showscan gear for Trumbull went bankrupt in 2000… sadly. Re: your chicken and egg comment- the same issues are very prevalent today. Wil Smith and others tried to spearhead a movement to have state of the art HD Video Projectors installed all over the country, linked through encoded satellite transmittion. In this way, a movie or pay per view event could happen just one time if need be, wherever the vendor wanted it to.
BTW, I met Trumbull several times, and he told me that while developing Showscan he had experimented with frame rates up to and over 100 fps. He said there were definite improvements in image quality at speeds higher than 60 fps, and that he would have preferred 72. But it was more practical to use 60 fps, because it was easier to run projector motors at 60 Hz, and because 60 fps (and the Showscan aspect ratio) synched exactly to HDTV. In fact, quite a few productions intended for presentation in HDTV have been shot in Showscan.
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And, your indirect quote from Trumbull supports my comments regarding crystalled frame rates. You actually just provided information that argues with your first comment to me re: Euro cameras and framerates. Hmm. :dubious:
Hope to meet Trumbull at the N.A.B. next week…
I’m also looking around for a damned production still from either The Shining or…hmm… 2001? Kubrick, standing right next to one of the Arriflex cameras that he owned and had maintained. Piece of camera tape, white, clear as a bell, marked " 25 fps ".
It is just the way it is.
To get back to the original question: Rod and cone cells refresh at slightly different rates: rod cells can detect movement up to about 30 frames per second, which is the reason that you can see a tv flickering out of the corner of your eye (where there are proportionally a greater number of rod cells). Some people can detect “movement” at the 50 frames per second level.
Ya really got under my skin with this. It is annoying to be attacked over such a plain and simple fact- one I have known and worked by for over 25 years- by someone who is not a professional cameraman. So, a cite.
Professional Cameraman’s Handbook ( revised edition ), by Verne and Sylvia Carlson. Amphoto, an imprint of Watson-Guptill Publications. Copyright 1970, and again 1981.
For the Arriflex lovers, I give you :
And, for the Panaflex and Panaflex-X fans in the audience,
Again. In the United States of America, to make best use of the 60Hz cycle apparent in electrical current, crystal sync is set at 24fps/60Hz. In Europe and a lot of the rest of the filmmaking world, a crystal sync of 25 fps/50Hz is used. I cannot make it much clearer than this to you. Your post is wrong. It has zero to do with video or television standards.
Oh, one other thing. That bit in the Arriflex quote about a camera being set to 24fps/50Hz? That’s not a typo. When shooting overseas to match footage shot in the USA< one wishes to shoot at the same crysalled rate. However, one has no control over power supplies. So, one wants the option of shooting at American standard ( 24fps) , but running at a 50Hz crystal sync.
Clear?
It is true that in more modern cameras, one can eliminate the “roll bar” apparent when filming a t.v. set or computer monitor by adjusting the frame rate in fractions. 29.975 fps gives one a film shot with no roll bar- and through a clever device on many cameras, one can slowly roll to that framerate as you turn on the camera, therefore clearing the roll bar out of view, instead of locking it in mid-screen as was the case years ago.
Cartooniverse, I, too, have been involved in the film business (as projectionist, IMAX theater manager, and journalist) for more than 25 years. First off, I didn’t intend to insult or “attack” you, and I apologize if my post was a bit curt. Second, I’m not refuting your cites or what you say you have seen.
But the fact that cameras are capable of shooting at 25 fps does not mean that theatrical films are shot or released at that rate. Cameras have that capability so that they can shoot directly for for the PAL TV format. But I still insist that all theatrical films are shot and projected at 24 fps, regardless of where they are produced. Cites:
A US-based DP asks if European films are shot at 25 fps. John Pytlak of Kodak replies: “The worldwide standard for 35mm theatrical exhibition is 24fps, per standard SMPTE 40.”
A German site: “Motion picture films are shot in 24 fps (frames per second - full-images - progressive) usually on 35 mm film material. The film has a frame rate of 24 frames per second.”
Also, if you search IMDb’s technical section for “25 fps” you get no hits. If there were two frame-rate standards, I expect the IMDb would indicate this. For instance, Showscan films are listed as “70mm, 60 fps.”
Finally, consider what a 24/25 disparity would mean for any Hollywood release shown in Europe or a European film shown in the US: either it would run 4% too fast or too slow, with obvious effects on appearance and the sound track, or one frame would have to be repeated or removed from each second of the film. This would require expensive optical processing and create visible artifacts. (The 24-25 fps conversion for PAL television is done electronically–much cheaper–and in way that lessens the artifacts.)
I’m sorry, but you’re mistaken on this point.