[miki123211]

PCs (and screens in general, particularly old CRTs) are a b*tch and a half to film.

You typically film at E.G. 21 FPS, while your PC runs at 50 / 60 (which isn't an integer multiple of 21), so you get Vsyncs somewhere during each frame, making each filmed frame a superposition of two frames onscreen. You also have to be careful about how long the film is exposed for, as, on a CRT screen, the electron gun goes left-to-right, top-to-bottom, and the top-left corner of the frame is no longer visible when the gun reaches bottom-right. If your film isn't exposed for the entire duration of the frame, parts of it may never be visible.

This is why there was a cottage industry of PCs and PC monitors designed to run at Hollywood frame rates. Cathode Ray Dude has an excellent video on why this was a problem and how the problem was solved[1].

[1] https://www.youtube.com/watch?v=qicQUvSUbPM