Frame rates of early movies

There's an interesting blog at Vanilla Video discussing this and state it is due to the hand-cranked nature of the projectors and cameras:

The earliest cameras and projectors needed to be hand-cranked to advance the film through the gate. This lead to varying frame rates. Early silent films had frame rates from 14 – 26 frames per second, which was enough to provide a sense of motion, but the motion was often jerky or uneven. You can imagine how film cranked by hand when photographed, and then cranked by hand again when projected, would make it nearly impossible to portray true-to-life motion.

It then discussed the introduction of a mechanical crank:

Late in this period, motion picture cameras and projectors developed mechanized cranks, which allowed for constant speeds of recording and projection. Even so, individual scenes were oftentimes filmed and projected at varying frame rates due to filmmakers favoring different speeds for different scenes (usually between 18 and 23 fps). Often film reels were delivered with instructions as to how fast or slow each scene should be shown. Additionally, exhibitors and projectionists favored certain frame rates as well, creating further inconsistency.

The blog finally discusses why films settled on 24fps:

Eventually, sound was synced to film by actually printing an optical track on the filmstrip alongside the image. This practice linked frame rate to the limitations of audio technology of the time. Given that film is an expensive medium, it was in Hollywood’s best interest to consume as little film as possible during a production. Although silent films ran at an average of 16 fps, it wasn’t possible to produce a quality soundtrack at that frame rate. Eventually, the studios decided on 24 fps because it was the slowest frame rate possible for producing intelligible sound; which means the decision was not an aesthetic decision, but a technical and economical decision.

Source GIZMODO :

Cinematic frame rates have been getting undercut by the economic interests of the moving-making industry. The earliest silent movies were shot at around 16 to 20 FPS—since that was the bare minimum that actually generated the continuous motion effect—but were also limited by the arm strength of the cameraman, who had to manually crank a reel of film through the camera. Movie houses at the time would often play them back at a slightly faster rate than that at which they were filmed but this caused the on-screen motion to appear jerky.

When Talkies hit in 1926, projectionists could no longer vary the frame rate on the fly like they used to, because it would throw off the pitch of the sound playback, so the film industry had to pick a stable frame rate at which to project.

The industry settled on 24 FPS, mostly because that was the slowest (and therefore least expensive to produce) frame rate that could still support audio when played from a 35 mm reel.

Actually, a few early films ran at higher rates that 24 fps. The early color process Kinemacolor ran around 48 fps so that it could display red and green frames quickly for the illusion of color.

The earliest films were shot at about 16 to 18 fps, because that is all that was required to reproduce motion without much flicker. Comedy producer Mack Sennett used a slow fps to save film (and costs) and because his comedies would be fast-paced when projected slightly faster. Silent comedies were actually meant to be projected slightly faster than real life, so that the comedians' movements would be more smooth.

Very early projectors were hand-cranked. By the 1910s they had speed governors on them, but since different speeds were used by different cameramen, sometimes on the same film, the projectionists would have to adjust the projection speed. And theaters would usually show films slightly faster so that they could get more shows in a day (but this did not happen everywhere).

While there were sound film experiments all thoughout the silent era, the talkies didn't officially arrive until Warner Brothers introduced Vitaphone sound films in 1926-1928. By this time, the average camera speed was indeed around 24 fps, but many films were taken faster, even at 26 fps. Western Electric sound engineers surveyed cameramen and setted on 24 fps because it was as close to an industry standard as they could get back in these analog days.

When watching a video of a silent film, especially an older video, don't forget that some restorers did not speed-correct the films for modern television. Charlie Chaplin stretch-printed his early films by duplicating some of the frames so that they could project at 24 fps in sound projectors and on TV. While this corrected the speed, it does make movements slightly jerky. Many producers of TV documentaries in the 1950s through 1970s ran early films at 24 fps even though they had been taken at 16-18 fps, making the action unintentionally comical.