In the early days of silent cinema, there was a norm that film should pass through the gate of the camera at a frame rate of 16fps (frames per second). This was only slightly over the threshold rate of 10-12fps above which the human eye can no longer discriminate between the various independent images in a sequence and sees them instead as a representation of continuous movement.
As there were 16 frames in a foot of 35mm film, which was the standard film gauge in early cinema, this meant that the film was being passed through the gate of the camera at the rate of one foot per second.
Prior to the 1920s, most cameras used in non-fiction film-making were cranked by hand. They generally worked on the principle that one crank of the mechanism would send one foot of film through the gate. There was therefore supposedly a neat correspondence between cranking and frame rate: one crank per second produced the standard speed of 16fps. Camera operators developed various verbal formulas, which they repeated as they cranked, that were intended to help them crank at the standard speed.
However, in practice, there was considerable variation in the speed at which operators actually cranked. A problem noted by the ethnographic film pioneer, Baldwin Spencer, among others, was that there was a temptation to vary the speed of cranking in accordance with the speed of movement of the subjects [cited in Dunlop 1979, p.112].
But if camera operators cranked too fast, when the film was projected at the normative speed of 16fps, this would have the effect of slowing down the movement on the screen. Alternatively, if they cranked at less than 16fps, this would have the effect of speeding the action up when the film was screened.
A further issue was that while 16fps was generally regarded as a norm, some professional production companies simply did not acknowledge it, with some shooting systematically at higher speeds, and others at lower speeds.
Today, when footage originally shot at 16fps is transferred to a digital format, it frequently happens that the transfer is made at the now-standard 25fps, with the result that the actions of the subjects appear too rapid.