When sound film was first developed in the latter part of the 1920s, the norm of 24fps (frames per second) became established for 35mm feature films. In film studios, where the cameras were by now driven by electric motors rather than cranked by hand, camera operators would ensure that the cameras really did run at this speed since this was essential to achieve synchronicity with the recorded speech of the actors.
However, most non-fiction films were still being shot at around 16-18fps. But even though many non-fiction film-makers were by now using clockwork-driven cameras rather than cameras cranked by hand, it was still not possible to guarantee that the norm of 16-18fps would always be exactly achieved. Some spring mechanisms were better than others; all spring mechanisms tended to slow down as the spring got towards the end of its unwinding.
It would not be until after the Second World War – by which time, many documentary film-makers were shooting on 16mm film – that 24fps would become the norm for non-fiction film-making as well.
In modern television and digital technology, the standard frame rate norm is a minimum of 24fps in the US and some other regions, while in Europe, it is 25fps. This poses a potential problem when transferring early film shot at 16-18fps to video.