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07 Oct

Slow Motion Explained, Quickly.

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Slow motion's a simple idea, but the concepts behind it aren't always as simple as they seem. Phil Rhodes takes a quick look at the science of slowing things down.

"Slow Motion"  sounds pretty simple, doesn't it?  But the terminology is counter-intuitive – I mean, “high speed” camerawork makes things appear to move slowly? It's not the most obvious choice of words. Then again, we also have techniques to make things appear to move quickly, and we call that “under-cranking”, which is less than the most obvious way to describe what's going on, especially as cameras haven't routinely been hand-cranked for decades.

That said, it isn't a difficult technique to understand. To view any event in slow motion we can simply play back the frames of film or video footage at a slower speed than they were recorded, and that's possible no matter what speed they were shot at. Most people will be aware, though, that recording a scene at 24 frames per second and playing it back at six might give you a slow motion effect in which everything takes four times longer to occur, but it won't give you an illusion of fluid motion.

Speeding up to slow down

Even if we take 24fps as our benchmark for smooth motion it's clear that we can't produce convincing, fluid slow motion simply by slowing down playback; instead, we need to shoot more frames to begin with, which is where the phrase high speed comes from. A camera such as the Sony FS700 will shoot at 240fps which, when played back at 24fps, results in things moving at one tenth the usual speed.

So far, so straightforward, but it's this requirement to record scenes at very high framerates that makes this an interesting issue. It wasn't long ago that merely shooting HD resolutions at normal frame rates was a technological challenge, so it should be clear that doing the same thing at high or very high frame rates can still present problems that are only solvable with technology that's exotic and rare enough to be expensive. Back when this sort of work could only be done on film, at least at resolutions suitable for theatrical release, costs were ferocious, with 35mm film stock thundering through the gate at a rate sufficient to make the production accountant whimper like a frightened puppy.

Varicam

The very first device directed at filmmakers which was capable of shooting video at rates significantly in excess of those used for broadcast TV was probably the Panasonic Varicam. While limited to 720p resolution, it offered rates up to 60fps, suggesting it was aimed at sports coverage, which is often broadcast at 720p60. This was useful for music videos but not much to write home about if you happen to be making a sequel to The Matrix. Panasonic's later HVX-200 camera was widely viewed as the affordable, low-end equivalent to the Varicam, and was widely purchased for its 60fps capabilities even though it was otherwise not famous for producing excellent images.

Now, the FS700 and the Canon C500 are among the first non-film cameras to offer what we might call proper slow motion in a package that costs less than a very nice car, and we can probably expect to see a rash of slow motion on Vimeo over the next year or two, much as very shallow depth of field was perhaps a bit more popular than it should have been after the Canon 5D mark 2 became popular.


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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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