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Editing for Slow Motion

Jan 25, 2010 12:00 PM, By Jan Ozer

Editing for slow motion using Final Cut Pro

The approach for Final Cut Pro is similar to that for Premiere Pro. I'll edit this slow-motion footage using a 720p/24fps sequence setting, specifically that shown in Figure 6.

When I insert the 24p-record-rate/60fps footage from the JVC into the project, Final Cut Pro reads the frame rate as 23.98fps, knows that it's slow motion, and handles it as such. I don't have to use the Change Speed setting to create the slow motion.

On the other hand, Final Cut sees the footage from the Canon as 59.94fps, and will play it in realtime unless I do slow the speed with the Change Speed control, accessed by right- or CTRL-clicking the clip and choosing Change Speed.

Here, though, I have to be careful to disable Frame Blending, lest I create the faux-interlacing artifact shown on the left in Figure 8. To be clear, if you shot at regular speed (24fps in this case) and wanted to slow the video down, you'd probably want Frame Blending enabled. On the other hand, if you've shot at 60p and have the original frames to use—no interpolation required—make sure to disable Frame Blending.

Where's the beef?

For you Clara Peller fans out there (and who isn't, or wasn't?), I know what you're thinking: Where are the video files? Well, calm down, they're coming, after enough caveats to betray my legal training.

First, I decided to test slow motion using two different cameras, rather than the same camera with two different takes at two different settings. The first alternative gives you identical test footage to work with, so the jumps, spins, and jetés all occur simultaneously, with multiple, unavoidable differences due to camera positioning, framing, and different lens/CCD/CMOS/format setups. The second gives you similar but different test footage, with identical framing using the same camera. I chose the former, which enables comparative images like those below, but inevitably means that the two videos will look subtly different.

By the way, the shoot was for an audition clip for Tino Sauter, a rising star in my wife's ballet company whom she'll lose to a full-time school after this year. He's an amazing talent, and we'll all miss him. I shot in my wife's studio using only the overhead fluorescent lighting.

From a slow-motion perspective, the cameras did wonderfully, producing 60fps that slowed as expected in both editors using the techniques detailed above. That said, I wish I had done some things differently with both cameras at the shoot.

With the JVC HM700U, I had the 14X Canon lens option and shot with the iris wide open, gain set to zero, and shutter speed set by the camera, since part of the value-add of the variable-frame-rate technique is that the camera calculates the optimal shutter speed for realistic motion blur. The camera doesn't have autofocus, so I focused before each shot using both the focus-assist mode and zooming in close to the subject. I did not attempt to refocus during the shoot.

Looking at the JVC clip, the focus and exposure look very good (considering the ambient lighting), but the motion blur is evident—perhaps too much so. In a perfect world, I would have experimented in situ with manual shutter speeds to see if I could achieve a setting that delivered a slightly sharper frame.

With the Canon 7D, I used the EFS 10mm-22mm lens, with aperture at 4, which was as low as I could go, and shutter set at 125. I shot in Auto-ISO mode to let the camera determine the best ISO level. I used the Canon's autofocus assist before each shot, and looking at the results, wish I had used the zoom-focus controls as well.

In these images, the focus appears a bit soft, but the motion blur is very limited. The focus softness could have been due to the wide-angle lens, poor focusing on my part, or some combination of both. The video also looks a bit noisy.

In a perfect world, I would have shot with a longer lens with a lower ISO rating, which would have reduced noise and (assuming that it wasn't user error) produced a slightly crisper frame. However, in the relatively shallow studio, I was forced to use the wide-angle lens.

Long story short, what I want you to take from these videos is that both cameras can produce very sharp slow-motion videos if you shoot and edit as described above. You should draw no qualitative conclusions about either camera from these results. Given a different lens, more time, or a better shooter, both cameras probably could have done a lot better.

Finally, to produce the videos that you're seeing, I copied the 720p videos into a widescreen 24p DV sequence preset and adjusted the positioning so that Tino was more or less centered in the frame. Then I rendered out at 854x480 so as not to scale vertically at all. Essentially, this gives you a center cut of the original 720p video. The alternative would have been to scale the complete frame to my delivery target of 854x480, which would have kept Tino in the frame but made him much smaller.

Continue the discussion on “Crosstalk” the Millimeter Forum.
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