Shooting for Slow Motion
If you want fast or slow motion in your video, 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 issue of HD Insider, I'll discuss the theory behind using fast- and slow-motion on the camcorder side; next time, 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.
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 1920x1080@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 1920x1080 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 camcordersthough 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.
Figure 1 shows the recording alternatives available with the JVC GY-HM700U, which is similar to other camcorders in its price range. Again, we're shooting for slow motion, so we want to capture the highest number of available frames per second. As you can see in the chart, while shooting in 1280x720 resolution, the camcorder can capture 60p, which means 60 full progressive frames per second. However, at 1920x1080, the highest frame rate is 60i, or 60 interlaced fields per second.
As you probably know, when shooting in interlaced mode, camcorders capture two separate fields for each frame, and each field contains half the vertical lines in the frame. This means that when capturing in 60i mode, each 1920x1080 field actually contains 1920x540 pixels, so you'll have to de-interlace (another interpolative technique) to produce the full frame.
To recount, we're producing here at 1920x1080@24fps, and our goal is to capture the best source video for producing slow-motion video. If we shoot in 1280x720@60p, we get full frames at 60fps but have to interpolate these up to 1920x1080 resolution. If we shoot in 1080i 60, we get 60 fields per second and have to interpolate each field vertically to full resolution.
Which is better? Let me say definitively that it depends. This is way out of my bailiwick, so in addition to scanning a bunch of posts, I spoke with George Palmer, who has credits on a number of movies including Miami Vice, Collateral, and Black Water Transit. I called Palmer because I read his post on the Cinematography Mailing List.
"I would only suggest that the natural line-to-line temporal offset of any interlaced format will materially deteriorate the slow-motion (and even the full-motion) performance in reproducing images with vertical-motion content," Palmer wrote in his post. "Progressive formats contain no such temporal defects, so they provide better slow- (and full-) motion temporal characteristics (e.g. slow-motion performance to 35mm or to any display methodology). Yes, I have seen the effect in side-by-side 1080i/720p and other simultaneous capture tests."
During our conversation, Palmer tempered this with one other variable: that you have to consider the format that you're using for your other footage, i.e. the footage that isn't slow motion. That is, if you're shooting your other footage in 1920x1080@24p, or 1920x1080@30p for a non-film production, though 1280x720@60p footage would provide smoother slow motion, you'll have to scale it to 1920x1080 to match the rest of your source footage. So it may be smoother, but it may look subtly different from your other footage.
Essentially, it's a trade-off. If you shoot 1920x1080@60i, you risk temporal artifacts. If you shoot 1280x720@60p and the rest of your footage at 1920x1080, you risk scaling artifacts. Basically, you have to test and see which produces the best result for your footage, and minimize the relevant consequences in post.
One other factor will become clearer below: At 1280x720p, you have many more frame rate options with most $6,000-to-$10,000 camcorders than you do at 1920x1080, especially if you want to under-crank and shoot at, say, 10fps to produce faster motion. For example, with the HM700U, at 1920x1080, you can shoot at 60i, 30p, 50i, 25p, and 24p. At 1280x720p, you can shoot at 10fps, 12fps, 15fps, 20fps, 24fps, 30fps, 40fps, 48fps, and 60fps. So if you want to under-crank and shoot fewer frames per second, or more subtle gradations of over-cranking, 1280x720p may be your only opiton.
If you're interested in more discussion about these issues, check out the following:
- 60i vs. 60p for slow motion at Rebel Café
- 1080i-60 or 720p-60 For 'Slow Motion' Out To 35mm at the Cinematography Mailing List
For now, let's move to camera setup.
If you're going the 1920x1080 @60i route, just dial those parameters in as normal. I'm doing this on the JVC GY-HM700 in Figure 2.
If you choose 720p resolution, you have two choices with the JVC and most Panasonic and Sony camcorders in its class. First, you can simply dial in the resolution and capture rateyou can see the latter, but not the former for the JVC camcorder in Figure 3.
Or, you can invoke a special mode called variable-frame-rate recording, which seems to correspond to the Slow & Quick (S&Q) motion function for the Sony PMW-EX1, and the PN mode for Panasonic camcorders. In this mode, you choose the record format, which corresponds to the frame rate of your project, and then choose the desired shooting frame rate. You can see the relevant options from the JVC manual in Figure 4. Note that you have many more frame rate options than you did at 1920x1080, including four under-crank options at 24p (10fps, 12fps, 15fps, and 20fps) as well as three additional over-crank options (30fps, 40fps, and 48fps).
Figure 5 shows the JVC configured to 60fps in variable-frame-rate recording mode. You can't see it, but the record format was set to 24p and the resolution to 1280x720p, the only resolution that enabled variable-frame-rate recording on the JVC (and Sony and Panasonic).
So, the first alternative was 1920x1080@60i, the second 1280x720@60p, and the third 1280x720@60p with a record format of 24p. What's the difference between bachelor number 2 and bachelor number 3? That's the interesting question and there are several answers.
When I capture the first 1280x720@60p (without the 24p record format), I'm telling the camcorder that I want to capture 60fps for display at 60fps. For this reason, if I play the clip back on the camcorder after shooting, it will display in realtime, not slow motion. That is, if I capture 10 seconds of video, it will play back in 10 seconds. Similarly, if I drop that clip on the timeline, it will consume 10 seconds of timeline real estate. Nonetheless, that same 10-second clip has 600 frames, so if I adjust the speed to 40 percent of normal in my editor, I'll get 25 seconds of slow motion video with no interpolation.
With the second 1280x720@60p clip, I'm telling the camcorder that I'm shooting 60fps to display at 24fps. If I shoot for 10 seconds, I capture the same 600 frames, but if I preview from the camcorder, it will play back in slow motion, sans the audio which isn't captured, since the camcorder assumes it would be distorted upon slow-motion playback anyway. Drop the 10-second clip captured in this mannter on the timeline, and the editor will read the metadata and spread the 600 frames over 25 seconds.
Also, when I choose the latter mode, I'm telling the HM700U to calculate the optimal shutter speed for the target playback rate, which is different at 10fps and 60fps. When I shoot in 60p mode without the timebase, I'm in charge of setting these parameters. Most importantly, when I choose variable-frame-rate recording, I have many more frame rate options than I do outside of this mode, including under-cranking and over-cranking.
OK, so that's the theory and camera side; join me in two weeks when I look at the editing side.