Shooting for Slow Motion

slow-mo.jpgIf you want fast or slow motion in your video (like slowing this spinning ballerino), you have two choices: you can adjust the speed in your editor, which can produce interpolation artifacts, or you can adjust the frames per second captured by your camcorder, which should deliver higher quality. If you choose the latter approach, there are three components to consider: camcorder settings, editor settings, and the craft. In this article, I’ll discuss the theory behind using fast- and slow-motion on the camcorder side; in a separate article, I’ll detail the editor side, discussing both Adobe Premiere Pro and Apple Final Cut Pro.

I’ll detail camcorder settings for the JVC GY-HM700U camcorder, which I have in for testing. Panasonic guru Barry Braverman will discuss similar settings for the Panasonic AG-HVX200A camcorders on his blog and will also address the craft side of the equation. Specifically, beyond the obvious fast- and slow-motion effects, film producers have long used subtle speed changes to make a chase scene more exciting, or slow down the moves of novice actors. Braverman will share some of the hows and whys of these types of effects on his blog.

In the interest of full disclosure, while I normally try to write about techniques I’ve used extensively, this is more like on-the-job training, a learn-as–I-go experience. However, unlike you (har, har), I did read the manual, and was fortunate to have much more knowledgeable colleagues like Braverman take the time to tutor me on the topic. As Grisham would say, however, all mistakes are purely mine.

The other comment that I’ll make is that many of the decisions presented here have been argued heatedly over many pages of posts on a number of prominent video-related message boards. Typically when this occurs, it’s because either opinion has been substituted for objectivity, or because there is no single answer that’s right all the time. I’ll do what I can to totally duck these types of arguments, but when they exist, I’ll point you to the pages and let you draw your own conclusions.

Our target

Let’s set some project parameters to help shape the discussion. Assume I’m producing a project at 24fps for web output at full-resolution HD, which is 1920×1080@24fps. OK, throw film output in there as well; I always wanted to direct. These pinings aside, the point is that my goal is to produce the biggest, boldest, clearest, smoothest video at 1920×1080 resolution and 24 full frames per second (or 24p).

Most of the video footage in this production is normal speed, but I want to add some slow motion that takes the 100 percent speed down to say 40 percent. Let’s assume that I shot the bulk of my footage at 24fps in progressive mode (24p) and am editing on a 24p sequence in Premiere Pro or Final Cut Pro (in politically correct alphabetical order according to manufacturer). Just for fun, let’s assume it’s a documentary about ballet dancers for reasons that will become evident in the next edition.

Perhaps you’re thinking, “Why all the fuss? If I want slow motion, I should just tell Premiere Pro or Final Cut Pro to change the speed of the footage on the timeline.” That’s workable of course, but understand that if you’re working with footage shot at 24fps, both editors will simply spread the frames that you shot over a longer period of time and then interpolate to create new frames between them.

True, interpolation techniques have gotten wonderfully sophisticated and can produce very high-quality results. However, for the absolute clearest result, you’d want to shoot the action at a much higher frame rate (say 60fps) and then spread these frames out on the timeline. That way, you have real frames representing each of those 24 frames on your timeline, and reality is always more accurate than interpolation.

By way of background, one of the reasons that true film-based productions look so smooth is because film cameras can shoot at extremely high frame rates such as 150fps or higher. Historically, this was called over-cranking, since the poor Joes who actually had to crank the camera while shooting had to step it to record high-speed footage. Just to close the loop, when shooting at frame rates slower than 24fps, it’s call under-cranking.

Traditional tape-based camcorders didn’t have the flexibility to over- or under-crank, because they were tied to NTSC (or PAL) and the associated 29.97fps (or 25fps) timebase. With the advent of solid-state storage for camcorders, capture rates have swelled to upwards of 300fps on some consumer camcorders—though most of the professional models peak at 60 full frames per second in some modes, as opposed to 60 fields per second which they’re always done. This lets you slow down to 40 percent of speed (in a 24p project) without any interpolation.

This “in some modes” comment identifies the first decision that you have to make.

About Jan Ozer

I help companies train new technical hires in streaming media-related positions; I also help companies optimize their codec selections and encoding stacks, and evaluate new encoders and codecs.

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