Imagine yourself watching movie of an unbelievably slow fog. You don't see edges and sharp borders. Now play the movie with 10fps. It will look fluid. Why? Because the difference from one frame to the other is very low. The extreme would be a totally unmoving wall: Then 1 fps would equal 1000 fps. Now take your hand and move it slowly in front of your face. Then move it faster until it's blurry. How many frames per second do you see? It must be little, because you see only a blurred hand without being able to distinguish every change per millisecond, but it must be many, because you see a fluid motion without any interruption or jump. So this is the eye's trick in both examples: Blurring simulates fluidity, sharpness simulates stuttering. (It's similar to "rotation simulates gravity".)

Motion blur example1: Capture from a live performance of The Corrs "What can I do" at MTV Unpluged

Motion blur example2: Capture from "Basic Instinct", where you see a woman plunging an ice pick into a man's body while sitting on him. The fact is that the human eye perceives the typical cinema film motion as being fluid at about 18fps, because of its blurring. If you could see your moving hand very clear and crisp, then your eye needed to make more snapshots of it to make it look fluid. If you had a movie with 50 very sharp and crisp images per second, your eye would make out lots of details from time to time and you had the feeling, that the movie is stuttering.

Also 25fps but without motion blur: Footage from BBC's story about Ed Gein, the murderer, who's case inspired Hitchcock to make "Psycho" and Jonathan Demme to make "Silence of the Lambs". The music is from CNN's "Marketmakers" (0.52 MB). Just think of modern games: Have you ever played Quake with 18fps? There is no motion blur in those games, thus you need a lot of frames per second more. However, you see the spots and the dirt of single frames in a cinema film, don't you? And those movies are played at 24fps. So there is a difference between seeing motions fluid and seeing that there's something (dirt) at all. Read on.