How to add slow motion and speed ramps to a video (so the slow point lands)

Slow motion is the most overused effect in amateur video and the most precise one in good video. The difference is where the slow point lands. A clip that is simply half speed is a slideshow; a clip that runs at full speed into the hit, hangs for half a second on the apex, and snaps back is a speed ramp, and it is how a jump, a goal or a punchline gets more weight than a cut can give it.

This is how speed ramps, freezes and rewinds are made in Narrative: what a ramp is, the numbers that make one read, what happens to the sound, how to ask, and the snowboard edit, which slows into a frozen rider on the word “COLD”.

A ramp is a curve, not a speed

On the timeline a clip has either a constant speed or a ramp, never both. A constant speed is what the hand tools set: 0.25× to 4× from the presets, or any value from 0.05× to 20× in the speed popover, with the pitch of the clip’s sound kept (the hand-editing post covers those). A ramp is a curve: a handful of points that say which second of the file is on screen at which second of the edit. Between points the footage runs at whatever speed gets it from one to the next; the curve can ease in and out of each stretch for slow-ins and slow-outs, or run straight for whips and freezes.

Two consequences fall out of that. A flat stretch of the curve, where the file second does not change, is a freeze. A falling stretch is a rewind. And because the clip’s edit length is simply where the curve ends, everything after the clip moves to fit; nothing has to be re-timed by hand.

The numbers

• The slow point goes on the hit. Contact, apex, the look to camera: the frame where it happens is the slowest point of the curve, and it sits on the beat if there is music. Everything else is built outward from it.
• Ramp in over at least six frames. A fifth of a second from full speed down to slow. Shorter reads as a stutter.
• Hold for 8 to 20 frames. Past about 0.7 s a crawl loses people unless the frame is beautiful.
• Slow drama is 0.15× to 0.3×. Below 0.15× the picture barely moves and a freeze says it better. Fast approaches and exits are 2× to 4×; a whip is 8× to 16× over a fifth to two-fifths of a second.
• Freeze for a third to four-fifths of a second, on impact, a face, a title landing. Never on a blur.
• Rewind only on a clear reversal: a fall, a shot, a jump. A fifth to three-fifths of a second back, then forward again. Never rewind someone talking.
• Cut out while slow when you want an accent. A cut out of slow motion hits harder than a ramp back up.

These are the rules the editor works to. Ask for “slow motion on the jump” and this is what you get; ask for numbers and it uses yours.

What happens to the sound

A ramped clip’s own sound is silent. Nothing can play a curve’s audio live, and there is nothing for the export to stretch. So the editor does one of two things. Most ramps live on a beat, and the music carries them; the clip’s natural sound would fight the slow motion anyway. When the sound matters (a crowd rising into slow motion, an engine), the editor bakes the clip’s audio over the same curve into a file and plays that alongside, at normal speed, anchored to the clip. If you need the clip’s own sound live at a constant speed, that is not a ramp; it is a speed, and the hand tools set it with the pitch kept.

Dialogue is never ramped. Anything whose sound has to stay intelligible keeps its speed.

Where it plays smoothly

On the iPhone app, playback and export interpolate frames inside slow stretches, so a 0.2× hang moves rather than steps. On the web and in a server export, frames are held: at 0.2× every picture shows five times. Whips and freezes look identical everywhere; only the slow stretches differ, and a 60 fps source gives twice the real pictures in them, so shoot at 60 if you know you will slow it down. The editor will not fake in-between frames, which is why the slow stretch on the web looks like slow motion has always looked in a cut.

How to ask

• “Slow motion on the jump, the apex on the beat.”
• “Speed ramp into the goal: fast approach, hang on the strike, snap out.”
• “Freeze on his face when he lands, half a second, then go.”
• “Rewind the fall and play it again.”
• “Whip into the next shot.”

At 0:14 the dunk: run in at double speed, hang at the rim for half a second on the downbeat, cut out of the slow motion to the crowd.

Then revise by naming the moment: “hang longer”, “the slow point is a few frames early”, “no freeze, just the ramp.” A ramp is a declared thing on the clip, so a later request or your own trim keeps it; it is not buried in code.

See one

Open the snowboard edit and scrub between 0:09 and 0:13. Opening it needs no account, your first change starts a three-day free trial, and the web editor runs on a computer; on a phone, the iPhone app is the way in.