The Shape of Days

Everybody and his sister is emailing me about this interview with James Cameron in which the director says he’d rather shoot 2K at 48 frames per second than 4K at 24 frames per second.

This is crazy talk.

Look, I’m no expert, okay? But I’ve been shooting quite a bit lately. I’ve done my share of experimenting with funny frame rates. Most recently, yesterday I did some side-by-side comparisons of a 3D sequence rendered out at 23.976 with pulldown added to the same sequence rendered at 59.94i. See, I was asked to create a DVD downrez of one of my shows for playback on an old-fashioned interlaced projector, and I wanted to be sure I was giving the client the best possible product. So I rendered out a test sequence with pulldown and one at 59.94i to compare the results for myself.

Pardon me while I digress into a little bit of background here.

Movies are older than television. Compared to all the electronic gee-whizzery of television, movies are pure simplicity. You’ve got yourself some kind of lens, a strip of film and a metal half-circle that sits between them. The half-circle — the shutter — spins. Whenever the shutter lets light in, a frame of film gets exposed. Whenever the shutter is blocking the light from the lens, the camera pulls the film down so the next frame can be exposed. If you spin the shutter at 48 revolutions per second and pull the film down 24 times per second, you get a movie.

But for a lot of complicated reasons that I won’t go into here mostly because you can damn well google them yourself if you’re so inclined, television works differently. Specifically, television doesn’t — or rather, didn’t, back in the old days when it was first invented — draw a whole frame all at once. Instead it breaks the frame up into lines, and draws the frame from the top of the screen to the bottom, a line at a time.

Now, this was mostly okay, because old-school televisions worked by exciting a phosphor with an electron beam, and the phosphor continued to glow for a bit after the beam had moved on. But still, by the time the beam got to the bottom of the screen and started its trip back up, the top of the screen would have begun to grow dim.

So televisions that worked that way flickered noticeably. It was annoying.

Movie projectors had the same problem, at first. A movie projector is basically a camera in reverse; instead of letting light in to expose the film, it pushes light out through the film to project an image on a screen. Projectors have a shutter in them too; its job is to block the light coming through while the film is advanced one frame, just like in a camera.

But movie projectors that used a one-twenty-fourth shutter produced a noticeable and objectionable flicker. The light from the projector was being interrupted twenty-four times a second, so it worked like a very fast strobe.

The solution was simplicity itself: Just run the shutter twice (or even three times) as fast. The light from the projector still flickers, but it flickers forty-eight or seventy-two times a second, which your eye can’t really pick up.

Old-fashioned television worked on the same principle. The screen was constantly going from light to dark and back to light again, but by illuminating the even lines now and the odd lines now and then the even lines again, the flicker was reduced to the point where you could hardly notice it.

Of course, the catch is that every frame has to be divided up into fields — sets of odd or even lines — and those fields have to be drawn separately. And not just separately, but one sixtieth of a second apart. Well, almost. Technically the fields are drawn one hundred five-thousand-nine-hundred-and-ninety-fourths of a second apart. Which is precisely why everybody rounds up.

Anyway, back to the point: Television cameras record in essentially the same way televisions display. That is to say, they record first the even-numbered lines of the frame, then a sixtieth of a second later they record the odd-numbered lines. If you look at both sets of lines together, you’ll notice that they’re slightly out of sync. That’s because they were recorded at different times. But that’s okay, because they’re played at slightly different times, too.

Television, in other words, gives you half the vertical resolution twice as fast.

Movies work nothing like this.

And yet they show movies on television all the time. How can this be? Sorcery, I say.

Well, sort of. When movies are shown on television, each frame is converted to a pair of fields, and those fields are recombined in such a way that they turn into 29.97 field-pairs (sort of like frames, but slightly different) per second. This process — of converting 24-frames-per-second material into 29.97-frames-per-second material — is called adding pulldown.

But the sequence I was testing with yesterday didn’t originate in a camera. If it had, I would have had only one option: adding pulldown. Because I shoot everything at 24 frames per second, using a one-forty-eighth shutter, just like in a movie camera. But because I was working with synthetic footage, I could tell my computer either to render out 24 frames per second, or 59.94 fields per second. So I did. I had it generate one of each, then burned them both to DVD and watched them on the only old-fashioned television I could find in the office.

The result? The 59.94 version looked like shit.

Okay, that’s kind of an oversimplification. The truth is, it looked perfect. It looked exactly like it would have looked if I’d shot the scene in real life, using a 59.94-fields-per-second video camera.

But it didn’t look good.

See, material played back at 59.94 frames per second — which is what we’ll call this, since you’re basically getting 59.94 complete but half-resolution images per second — has an entirely different motion quality than material played back at 24 frames per second. And the miracle is, this remains true even if you insert pulldown and play back at 59.94. The fact that the material was recorded at 24 means your eye is interpreting what you see as 24 frames per second, even though those frames are being delivered in a slightly funny way. That’s just how the brain works.

Contrary to what Jim-call-me-James Cameron has to say, a movie shot at 48 frames per second doesn’t look clearer or sharper than a movie shot at 24 frames per second. It looks cheap, because we’re all used to seeing motion pictures played back at 24 and things like game shows and news broadcasts played back at 59.94. A self-fulfilling prophecy? Maybe. We’re used to seeing that, so that’s what we expect, and when we see something that conforms more closely to our expectations than it does to the truth, we can’t shake the feeling that something’s not right.

Maybe in a different world, James Cameron would be right. Maybe in a different world, a world where cheap TV shows are shot on film and only the biggest-budget motion pictures are shot on video at a higher frame rate, the situation would be reversed and everybody would be lusting after that 60 Hz look. But that’s not the world we live in. In this world, 24 frames per second looks cinematic because that’s what we all grew up watching in the cinema. And trying to change that now would be a huge uphill battle for no actual payoff, since in the end, they’re all just pictures flickering in the dark.

• Rudy says something beautifully
• Well I certainly wasn’t expecting this
• More H.264 demonstrations