Did I Judder

As part of my responsibilities at AbelCine, I handle a number of technical support questions from our professional clients in the field. Recently, I had two different clients inquire about a "judder" they witnessed in their new cameras. They described a stuttering effect from frame to frame that was visible when shooting 24p. Both believed this jumpy footage could be a sign of a problem with their cameras, so I asked them to bring in examples. The two scenes were shot with very different cameras—one clip was of an actor against a greenscreen; the other was of students passing in front of a blown-out window. Both cameras were set to a standard 180º (1⁄48) shutter. When I viewed their footage on a computer screen, however, I could see their issue.

What was causing this 24p video to look jumpy? Surely, 24p, the standard of movie theaters across the country and preferred format for all things "cinematic," couldn’t produce such jumpy video? But there was nothing technically wrong with these clips—counting frames revealed a full 24 frames per second. So why did these two experienced cinematographers, and many more before, think their video was jumpy? Well, the simple answer is that it was jumpy, but exactly why is a little trickier to figure out.

A BRIEF HISTORY OF 24 FPS

Back at the turn of the century, most film was captured at around 16 fps, giving it that jumpy look that we all associate with old films. It wasn’t until the introduction of sound in 1926 that 24 fps was standardized, and then it wasn’t because of anything to do with motion capture. Slower film rates weren’t able to properly record high sound frequencies. With 24 fps, not only was motion more accurately reproduced, but sound also worked. Today, because of these historical economic and technical decisions, we all think of 24p as being "cinematic."

HOW WE VIEW 24 FPS

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. To see the difference, we recorded native 24p out of a Sony F3 and simultaneously recorded a 60i conversion—the difference is quite noticeable. An example of this can be found at www.abelcine.com/r/24p60i. 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.

But what about theaters? 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.

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