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I observed when a helicopter rotor or fan is spinning the blades blur making it impossible to count the blades, but after a certain speed the blades seem to become visible making it possible to count the blades. Cameras can also do this better.

Do human eyes have a frame rate? What is that called and the most a person can see?

This video in the link shows how these blades seem not to move by timing with the cameras frames rate.

https://www.youtube.com/watch?v=R-IVw8OKjvQ

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  • $\begingroup$ The term you want is "flicker fusion rate". For humans, it's about 30 frames per second, though it varies somewhat between individuals: ps://en.wikipedia.org/wiki/Flicker_fusion_threshold $\endgroup$ – jamesqf Apr 27 '19 at 18:17
  • $\begingroup$ @jamesqf I revised it so your comment can be an answer. Thanks $\endgroup$ – Muze the good Troll. Apr 27 '19 at 19:11
  • $\begingroup$ @BryanKrause TEKKEN? $\endgroup$ – Muze the good Troll. Apr 27 '19 at 21:12