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So you’ve shot some video at a high frame rate… what exactly should you do with it now?
There are so many elements in slow motion video that need to be considered in post-production. Luckily for you, we’ve considered most of them to bring you our guide to post-processing your slo-mo masterpiece.
Project Rate Versus File Frame Rate
An important checklist item when shooting in slow-motion – before you even edit – is to see if you’re working with the actual frame rate you’re shooting in or something called a project frame rate.
Basically, your camera is either going to export the video you are shooting in the actual frame rate (say, for instance, 60fps) or it’s going to convert things automatically to a project frame rate (typically 24fps, 30fps, or 25fps).
Higher-end professional cameras such as the RED Epic do the latter, but easier-to-acquire cameras – like the Panasonic GH4 – do as well, using a feature called Varispeed. Your iPhone or Galaxy down-converts automatically, too!
On the other hand, Canon DSLRs – like the 7D Mark II – will shoot 60fps and then export the video file as 60fps, playing back in real-time.
So You Need To Convert It
So what happens when you’ve shot footage that’s exactly 60fps? Well, you need to convert it if you want to watch it in slow-motion. 60fps is the most common high-frame-rate video export. It’s more commonly found at a quality of 720p among high-end consumer cameras, but it’s slowly moving its way up to 1080p.
It’s actually pretty easy to do a slow-down conversion from 60fps to something like 24fps. In your editor, just play it back at 40%. Heck, it’s even easier if you’re doing 30fps. That’s simply a 50% slow-down. Granted, there’s the possibility – and you probably aren’t ever going to encounter it – that you’ll try to slow down video at a different frame rate than 60fps. But even if you do, it’s just simple division…
Basically, you just equal a fraction featuring your desired project frame rate over your existing file frame rate, and then solve for x.
24/96 = x
In this case, x = 0.25 (or 25/100), so that means 25%. Playback is then 25%. Easy stuff – don’t overthink it.
Instead of having your 96fps video play back exactly at 25%, you could simply have the video play back at 50%. It will still be slow-motion, but it will be slightly less slower than doing a full 25% slow-down.
This means you’re treating the video as if it was shot at 48fps. It’s also important to try and do your division in increments that are divisible by your actual playback rate. For instance, Trying to play things back at 73% might look weird, but playing it back at 75% will look a little better. Granted, there are always anomalies and exceptions, so your results may vary.
Another thing to consider is possible flickering in lights. Many lights and signs – particularly LEDs – do not have a constant output – instead, they flicker (invisible to the naked eye). Many cameras actually capture the frames in between those flickers, so by playing back at an unorthodox frame rate, the flickering will be more pronounced.
Even playing back at a normal frame rate can make these flickers prevalent, so it may take some tinkering to get it just right.
If you’re working with any mid-grade video editing software, you’ll know about keyframes. Keyframes are basically certain frames in your editor that tell the software to do something – whatever that may be.
In our case, you can manipulate the speed at certain points based on your keyframes. For instance, at one keyframe you could have the video playback at 100% (normal speed), but at another, it will be 25%. The frames in between will then progressively move slower towards this rate, creating a pretty cool time manipulation effect. The movie 300 features some of the best examples of speed ramping to date.
Something Fun: Slow-Mo Music Videos
So we’ve got a lot of the technical stuff out of the way. Here’s something I like to do with music videos, but it usually works better with a very slow song. Basically, you speed the song up while shooting and then slow it down in post so the artist’s lips match with the words… but in slow motion.
Let’s say we’re shooting in 60fps and then down-converting to 24fps. Well, we know how to do that with video – what about sound?
In this case, we speed the song up by playing at 250% speed, and then we bring it back by making the video 40% slower… effectively making the music play back properly. Here’s a very simple example from Coldplay:
What other tips do you have for working with slow-mo video in post? What else would you like to know?