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So you're filming in slow motion?

First, let’s dive into what creates slow motion.

Slow motion is created by playing a video back at a slower frame rate than normal. For example, if your footage is recorded at 24fps, it can be converted into slow motion by increasing the amount of time this clip stretches over. The higher the frame rate when filming, the more opportunity there is for your slow motion to playback smooth. If you stretched 1 second of footage at 120fps by 50%, you would have 60fps each second. Compare this to stretching 1 second of footage at 30fps by 50%, and you would have 15fps of footage per second, which would be incredibly choppy.

...So how do we create that buttery smooth slow motion footage?

Footage must be filmed in 60fps or higher, as a general rule of thumb. To be converted into slow motion, footage needs to be recorded in a higher frame rate than the sequence is playing back in to ensure there are no gaps in footage once it’s slowed. Typical frame rate for European videos is 24fps, or 30fps for American video.

Your shutter speed should be at approximately twice the speed of your frame rate. For example, if you’re shooting at 60fps, you should shoot at at least 1/120 shutter speed.

If you go higher than this, it will look super jittery. If you go under, you will get a whole load of motion blur.

To make footage slow motion, reduce its speed on the sequence. To do this on Premiere, right click on the clip, click ‘speed/duration’ and reduce the speed by a percentage rate. Depending on your sequence frame rate and your footage frame rate, the rate to how much you can slow your footage will vary.

A simple formula to remember is: Sequence frame rate (usually between 24-30fps) / Footage frame rate = slowest frame rate possible (without jittery footage).

E.g. 24 fps / 120 = 0.2 = 20% speed.

Of course, sometimes frames are stretched beyond their capabilities. That’s where frame interpolation comes in. Imagine you filmed in 120FPS, your sequence was 24FPS, but you needed to stretch a clip so much to fill a certain duration that it ended up being 17% speed, instead of the lowest 20% speed as we just worked out.

This is where you can play with Premiere’s time interpolation feature. To find this, right click on the clip, ‘modify’, ‘time interpolation’, and you will see 3 options, frame sampling, frame blending and optical flow.

Frame sampling duplicates frames in between each missing frame. It looks a little jolty as there are not enough new frames to fill the sequence, as you can see below.

Frame blending is essentially Premiere adding cross fades between each frame to fill the gaps, as you can see below. This typically gives a motion blur effect.

Optical flow is Premiere’s AI feature which estimates the frames that would come next in a smooth flow. This, in my opinion, is the best option for time interpolation. This gives you the option to comfortably decrease speed beneath what you should be able to (to an extent!). See below.

FYI: In order to see the time interpolation effects in real time, you must render the frames before they can be played back.

Here I have the same clip, filmed at 50fps, in a 24fps sequence. The slowest I should be able to slow this down to to ensure a smooth slow motion is 50%, however I have slowed down each clip to 40% of its original speed, meaning that there are not enough different frames to smoothly play back slow motion, and tested each time interpolation feature.

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