Why Do Movies Play at 24 Frames per Second?

How do still frames become moving pictures?

A film is a sequence of still images, and when they are shown quickly enough they cross what BBC researchers call the threshold for "apparent motion": the point above which the eye sees moving objects instead of a series of still photographs.^[1]

A 1931 handbook from the American Society of Cinematographers describes how a projector does this: each frame is pulled into place at the projector's opening and held still, and a rotating shutter blocks the light while the film moves and lets it through while the film is stationary.^[6] The handbook says experiments had shown that forty-eight flashes of light per second were needed for the screen to look steadily lit.^[6] So each frame was flashed more than once: three times at the silent-film speed of 16 frames per second, and twice once sound films settled on 24.^[6]

BBC researchers note a side effect of repeated flashing: moving objects can look doubled or jumpy, an effect called "film judder."^[1]

Did silent films run at 24 frames per second?

Mostly not. The Library of Congress says most silent films were neither shot nor projected at 24 frames per second, which it calls "sound" speed.^[3] For a 1909 D.W. Griffith film, it chose 16 frames per second mainly because that looked best, noting that no records from the time show the exact rate.^[3] A BBC research paper says silent-era frame rates were not standardized, and projectionists were advised to vary the speed to suit the subject on screen.^[1]

Sources describe the silent era differently. A 1962 article in the trade journal International Projectionist called 16 frames per second the international standard until sound arrived, and a 1933 British handbook said projecting at 16 per second had been common.^[5][4] But in a discussion recorded in the 1929 Transactions of the Society of Motion Picture Engineers (SMPE), one member said 16 "was not very rigidly in practice," and another said most studios cranked their cameras at 85 to 90 feet per minute while believing they were filming at 16.^[2]

Why did sound films need a fixed speed?

In the same 1929 discussion, one member said silent films often needed a little speeding up in projection, but with talking pictures any attempt to hurry the dialogue was at once noticeable.^[2] Another said that in sound film, 24 pictures a second was "rigidly definite."^[2]

Sound also needed the film to move faster: for a soundtrack carried on the film, the 1933 handbook explained, quality depends on how much film passes the reader's light beam each second, not on how many pictures it holds.^[4] It said 24 pictures per second was needed to reproduce high notes properly, which raised the film's speed from 1 foot to 18 inches per second.^[4] The 1962 article agreed that the jump from 16 to 24 frames was made not so much to reduce flicker or smooth out motion as to speed up the film for sound recording.^[5]

Saving film had long mattered as well: according to the 1962 article, the Lumière brothers had cut Edison's rate of 48 or 50 frames per second to 16 because the faster rate was "wasteful of precious film."^[5] By the 1933 handbook's figures, going from 16 to 24 frames per second used one and a half times as much film every second (our calculation).^[4]

When did 24 fps become the standard?

The SMPE's 1929 Transactions record initial approval of 24 pictures per second as the standard filming and projection speed for 35 mm sound pictures, to be presented again for final adoption after six months.^[2] Other sources date the change differently: a BBC News report said 24 had been the standard since 1927, the BBC research paper places it in the 1920s, and the 1962 article puts the arrival of sound in 1928-1929.^[8][1][5]

Why is TV 25, 30 or 29.97 frames per second?

According to the BBC paper, TV systems used a field rate equal to the mains electricity frequency: 50 Hz in Europe and 60 Hz in the United States.^[1] This was done partly to avoid "beating" against the flicker of studio lights powered by alternating current.^[1] Each frame was sent as two interlaced fields, so 50 fields a second make 25 frames and 60 make 30 (our calculation).^[1][7]

In 50 Hz countries, the BBC paper says, the difference between 24 and 25 frames per second is generally imperceptible, so each film frame can fill two video fields.^[1] In 60 Hz countries, every other film frame has to fill three fields, a process called "3:2 pull-down" that adds more judder.^[1]

U.S. broadcast rules tie the line-scanning rate to the frequency of the color signal and set the field rate at nominally 60 Hz, with an actual value of 59.94 Hz.^[7] With two fields per frame, that gives the odd-looking figure of about 29.97 frames per second (our calculation).^[7]

Why do movies still use 24 fps?

Faster rates have been tried: the 1962 article said the first few Todd-AO 70 mm productions used 30 frames per second, but later films went back to 24 to make standard 35 mm copies easier to prepare.^[5]

In 2012, Peter Jackson's "The Hobbit: An Unexpected Journey" opened in a version shot at 48 frames per second.^[8] Critics were split: the Independent called it "kitsch and alienating," and the Telegraph said it gave the film "a sickly sheen of fakeness."^[8] Jackson said 24 frames now looked "primitive" to him and that audiences, not critics, would decide.^[8] The BBC reported that only a small proportion of cinemas would show the 48 fps version; most would show the film at 24.^[8]

Some engineers argue for change: a 2008 BBC research paper said film and TV frame rates had been fixed for most of a century and that many believed they were near an upper limit; its authors argued that higher rates bring a "step change in picture quality."^[1]