1. Who Decided How Fast Silent Films Were Projected?
In the silent film era, there was no rule for "frames per second." Cameras were hand-cranked. The speed could change in the middle of a scene depending on how the operator turned the handle. The filming speed is said to have ranged roughly from 16 to 24 frames per second [1]. This is the speed during filming, which is a separate issue from the speed at which films were shown in cinemas.
Cinemas had similar situations. Projectionists could adjust the projection speed inside the theater by changing the voltage of the motor running the projector. It is said that their role was not just to choose a speed for the film, but to decide based on the audience's reaction [1]. Speeds often differed by genre; comedies were shown faster, while dramas were shown slower [2].
Changing the speed was not only for artistic reasons. An article introducing a paper by film historian Kevin Brownlow states that theaters sometimes showed films faster than the creators intended because showing them quickly allowed more film reels to be packed into a single show [4]. For example, records from a 1914 work by director D. W. Griffith show instructions to change the speed from 16 frames per second to over 20 frames per second for each reel.
Thus, an era continued where the same film was shown at different speeds in different cinemas. So, what were these speeds actually like?
2. Why Did Adding Sound to Films Require One Fixed Speed?
In September 1927, Earl Sponable, the technical head at Fox-Case, spoke at an SMPE (Society of Motion Picture Engineers) meeting about cinemas in New York. In the range Sponable investigated in New York, no projection slower than 105 feet per minute (about 28 frames per second) was found. On Sundays, extra showings were added, sometimes reaching 120 feet per minute (about 32 frames per second) [3].
This means that in large city cinemas at the end of the silent era, projection faster than the later standard was common. In this state, sound could not be added. This is because, with audio, speed variations are no longer permitted. It is explained that the ear is more sensitive to changes in frequency (pitch) than the eye [1]. For images, if the speed is a little faster, it just looks like a "hurried film." However, for sound, if the speed increases, the pitch of voices and instruments shifts directly.
If sound was to be added, the speed had to be fixed to one number. So, what number should that be? This is where "24" appears.
3. How Did Western Electric Choose 24 Frames Per Second?
The American electrical company Western Electric chose 90 feet per minute (24 frames per second) as the speed for talkies. A recollection by Stanley Watkins of that company, told in 1961 and quoted in Scott Eyman's book The Speed of Sound, states that this speed was chosen as an average of the projection speeds actually used in cinemas at the time [3].
There is an anecdote about how this choice was made. When Watkins asked Frank Rich, the chief projectionist at Warner Bros., he reportedly answered that large premiere cinemas used about 80 feet per minute (about 21.5 frames per second), while smaller cinemas wanting to increase showings used up to 100 feet per minute (about 27 frames per second). Watkins is said to have replied, "Then let's compromise at 90" [2]. This is a story based on later recollections, so read the details as an anecdote.
Western Electric also made Fox-Case use this 24 frames per second for the Movietone system, which used film-based sound. They aligned it with the 24 frames per second already set for the Vitaphone system, which used discs [5]. Even though there were two sound systems, the speed was aligned between them. However, it took time for this to become the standard. As studios updated their equipment between 1927 and 1930, 24 frames per second became the standard for 35mm sound films [1].
What is curious here is why the screen did not flicker with the small number of 24 frames.
4. Why Doesn't 24 Frames Per Second Flicker? Projectors Flash Each Frame Twice
24 frames per second is a small number. If shown as is, the screen would appear to flicker and flash. Therefore, common projectors used a shutter with two blades. The shutter allowed light to pass twice while projecting one frame. Even at 24 frames per second, the light flashed 48 times per second, making the flicker less noticeable. A method using three blades to show each frame three times, resulting in 72 flashes, was also used [1].
The number of frames did not increase. They were just showing the same picture twice. Still, the eye finds it harder to notice flicker when the light flashes more often, so it became much easier to watch.
Incidentally, the inventor Thomas Edison conceived a camera in 1891 that would take 46 photographs per second, stating he wanted to show the eye a "continuity of motion" [4]. 24 frames is a much smaller number than that, but it was compensated for by splitting the light into two flashes.
In other words, 24 is not a number based on "the limits of the human eye." It was a compromise that settled down due to the circumstances of the sound era and the ingenuity of the projector.
5. How Can You Investigate Flipping Speed and Movement with a Flipbook?
How does the impression of movement change with the speed of flipping? Draw about 10 flipbook frames of a stick figure walking in the corner of your notebook. Compare two ways of flipping the same 10 frames: slowly and quickly. You will see that the impression of movement changes. If you flip them quickly, the movement looks hurried. You can imagine a little bit how the same movie might have looked different in different cinemas in the past.
If you want to read more, look for books on film history or silent films at the library. It is interesting to compare how projection speeds are described in different sources.