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Shooting for Slow Motion

Jan 11, 2010 12:00 PM, By Jan Ozer

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 rate—you 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.

Continue the discussion on “Crosstalk” the Millimeter Forum.
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