system
Given the following post, respond as if you're commenting on Reddit:
human
(ELI5) Why is old war footage (ww2) always sped up

I've always wondered this whenever in history class or even just watching a ww2 documentary all of the clips of soldiers moving or running are sped up and it makes them look like there running in 4x the speed, is there a reason for this or do they do it to make the clip go by quicker?
gpt
First, let's understand how cameras work. As a general rule, video cameras are just taking a lot of still pictures in a row. When we play these in order, they give the illusion of movement.

In WWII times, cameras often took videos where they took 18 pictures per second (or 18 frames per second as a film person would call it). Cameras were also often hand-cranked, which meant that the frames per second could vary. Sometimes this was on purpose, and sometimes it was accidental.

Now, once you have the video, you can play it at a different speed than it was filmed at. If you recorded 18 frames every second, but play the film with 36 frames going by every second, you're getting two second's worth of the film for every second that you play the video. You can think of it like flipping a flipbook really fast.

The issue is that the industry eventually standardized the frames per second at 24. That means that most projectors run at 24 frames per second. If you run an 18 frames per second film at that speed, it will necessarily be sped up. The issue is that nobody really cared to create projectors where you can adjust the speed. There really wasn't any point, because every recent film is filmed at the 24 frames per second speed.

As mentioned above though, sometimes that speed was done on purpose.