Like the sibling comment, I wasn't sure what you mean when you write "50 FPS (max 25Hz)". "Hz" is literally "per second"; "50 frames per second" is "50 frame-H

Like the sibling comment, I wasn't sure what you mean when you write "50 FPS (max 25Hz)". "Hz" is literally "per second"; "50 frames per second" is "50 frame-Hz". Because we're talking about frame-oriented video, the "frame" is implied; FPS and Hz are equivalent units. Based on this "divide by 2" confusion and your mention of lowpass filtering, I'm thinking that the confusion is that you're thinking of this as audio or other wave-based signals. With audio the "volume" (amplitude) is a relatively minor detail and the key thing-of-interest is the frequency at which that amplitude is oscillating; not so with video. The signal doesn't follow a wave-pattern and we don't (normally) care about the frequency one bit. To apply video-numbers to audio; to represent a 60Hz tone, you'd need to represent that the amplitude goes down and back up 60 times per second; a 60Hz tone for 1 second is 120 "discrete" events, 60 "down" and 60 "up". If the samples are only at 60Hz then it would seem that the amplitude is staying constant, you wouldn't be able to observe that it's oscillating so quickly. And that oscillation is how we perceive audio. But it's not how we perceive video, light amplitudes don't normally oscillate like that, they stay steady and when they do change we notice that as a discrete event, not as part of a frequency. In fact, many displays (especially CRTs) do flicker, and are specifically counting on us not being able to notice this oscillation. If the video signal emits a new frame at 50Hz but the screen updates at 60Hz, naively that means that 80% of the video's frames will be on-screen for .0166s, and 20% of them will be on-screen for .033s. While humans might not notice a .0166s difference on its own, we're pretty good at noticing changes in rate and will notice the jitter. You could argue that similar to a lowpass filter for audio, that the display should interpolate between its samples to hide this jitter. It could; some TVs do this, but computer monitors don't because this necessarily introduces a delay of at least one frame (but realistically the software to do this will introduce more delay than that), and for interactive use we sure do care about lag/latency; even more than we care about jitter. TVs that do this will often have a "video game mode" that disables this and other features that introduce latency.