What causes 24p video to look jumpy?

Everyone wants to shoot 24p but it only looks “natural” when viewed at a higher framerate:

A large amount of content is produced in 24p. In theaters, 24p is the standard, but narrative television is also often produced in 24p. Yet, we don’t experience any obvious jumpiness when watching 24p on television or in theaters. Why not? One of the reasons is that we don’t often actually see 24p in either of those environments. In the U.S., we broadcast all video at 60 Hz. NTSC video is broadcast at 60i (59.94 interlaced fields per second), 1080 video is also broadcast at 60i, and 720 at 60p (59.94 progressive frames per second). To show 24p in the 60 Hz world, we need to convert it using a 2:3 pulldown method. This process not only conforms the video to the standard, but also has a smoothing effect. DVDs are mastered this way, as well, giving the same smooth result. Home televisions also often double their display rates (a 120 Hz TV is easy to find at your local electronics store), and this smoothens video on home screens even further.

24p film doesn’t have pulldown, so why isn’t it jumpy? Well, there’s something special happening that helps reduce the effect. Thomas Edison determined that for comfortable viewing in theaters, 46 fps was the minimum display rate of projectors. Projectors used a multiple-bladed shutter to show 24 fps film at 48 times a second, doubling the display of each frame. Many modern projectors actually will show each frame three times, giving 72 frames per second on screen. This has the same smoothing effect to our eyes that we see on TV sets. In fact, some of the only places we don’t see this smoothing effect is on production monitors and computer screens, so we can understand why cinematographers and editors may get a little uneasy about the 24p jumpiness.

Read the rest at HDVideoPro

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