An iPhone slo-mo video that plays at normal speed on a Mac

An iPhone slo-mo video plays at normal speed on a Mac because the slow motion was never part of the file. The iPhone records an ordinary clip at a very high frame rate, and the Photos app slows a section of it during playback. Copy the bare file to a Mac and you get the recording without that instruction. The fix is to export the clip from Photos with the effect rendered in, or to slow the original down yourself.

Why the slow motion disappears

Slo-mo mode on an iPhone records 120 or 240 frames every second instead of the usual 30. Nothing in the picture is slowed at that point. Played at its true rate, a 240 frames per second clip looks like any other video, only smoother.

The slow motion is an edit that Photos lays on top. Open a slo-mo clip on the iPhone, tap Edit, and a row of tick marks appears under the frame strip. Closely spaced marks play at normal speed, widely spaced marks play slowly, and two taller bars set where the slow section starts and ends. Photos remembers those two positions as an edit. It does not rewrite the video.

That is why the route to the Mac decides what you see:

• Image Capture, or copying from the phone over a cable, hands you the original recording. It plays at normal speed in QuickTime Player. A file with the same name ending in .AAE often arrives beside it; that is the edit, in a form only Photos reads.
• Photos on the Mac, through iCloud Photos or an import from the phone, keeps the recording and the edit together, so the clip plays slowly inside Photos.
• AirDrop from the iPhone normally renders the effect first, which is why the phone pauses to prepare the clip. If All Photos Data is switched on under Options in the share sheet, it sends the original and the edit instead.

Check what you have

Open the file in QuickTime Player and choose Window, Show Movie Inspector. The inspector lists the frame rate.

• Around 120 or 240. This is the original high frame rate recording. Everything needed for slow motion is there; it only has to be slowed.
• A normal rate such as 30, and the clip already plays slowly. The effect has been rendered in. This file looks the same in every player.
• A normal rate, and it plays at normal speed throughout. This is an ordinary video, or a copy that was converted along the way and lost its extra frames. Slowing it will look jerky because the frames are no longer there. Go back to the phone for the original.

Get a copy with the slow motion built in

From Photos on the Mac. This is the cleanest route when the clip is in your library.

1. Select the slo-mo clip in Photos.
2. Choose File, Export, Export 1 Video.
3. Pick a Movie Quality and click Export.

The exported file plays in slow motion in any player, on any device, because the slow section is now really in the video. Avoid File, Export, Export Unmodified Original for this job: that command gives you the high frame rate recording again, at normal speed.

To change where the slow section falls before exporting, double-click the clip, click Edit, and drag the vertical bars under the frame viewer.

From the iPhone. Open the clip in Photos, tap Share, check under Options that All Photos Data is off, and AirDrop it to the Mac. The file that lands in Downloads has the effect rendered in.

From a bare original already on the Mac. Drag the file into Photos. Photos usually files a high frame rate iPhone recording under Slo-mo and offers the slow-motion bars in Edit. If it does, the export steps above apply. If it does not, use iMovie.

Slow it down yourself in iMovie

iMovie comes free with a Mac and slows any clip, whether or not Photos treats it as slo-mo.

1. Open iMovie, create a new movie, and drag the clip into the timeline.
2. Select the clip and click the Speed button above the viewer (the speedometer icon).
3. Open the Speed pop-up menu, choose Slow, and pick a percentage. Choose Custom to type your own.
4. Choose File, Share, File and save the result.

The arithmetic for a smooth result is simple. At 12.5 percent, a 240 frames per second recording shows 30 of its own frames each second, which is one eighth speed. At 25 percent, a 120 frames per second recording does the same at one quarter speed. Slow either one further and there are no more real frames to show, so the software has to repeat or blend them and the motion starts to stutter.

Just watching it slowly

On recent versions of macOS, QuickTime Player has a Playback Speed command in the View menu with a half speed option. It changes what you see, not the file, and half speed is far gentler than the eighth speed a 240 frames per second clip was shot for. It is fine for a quick look and no use for sending the clip to someone.

Sending the result

A rendered slo-mo clip is a normal MOV, and it can be large for its length. The high frame rate original is larger still, since it holds four or eight times as many frames per second as a regular video.

Manatee is not a video editor, so it cannot re-time a clip or apply the slow section for you. Do that in Photos or iMovie first. What it handles is the delivery afterward. Shrink takes a size in MB, such as the limit of an email service or a messaging app, and finds the quality that fits. Convert turns the MOV into an MP4 for people who are not on Apple devices. Each result is a new copy written beside the original, which is never modified, and everything runs on the Mac with nothing uploaded. Every feature is free for 24 hours, then it is $9 once.

One caution: do not convert the high frame rate original and expect slow motion to appear. A conversion keeps the timing the file already has, in Manatee or anywhere else.

For the wider picture, why some videos play on iPhone but not on Mac covers the other surprises in iPhone footage, and MOV vs MP4 helps you pick what to send.

Questions

Can I get normal speed back from a clip that was exported in slow motion? Not cleanly, because the slow section is now real footage. Go back to the clip in Photos instead. Export Unmodified Original gives you the full recording at normal speed, and in Edit you can drag the two bars together so that no section is slowed.

What is the .AAE file next to my video? It is a small text file holding edits made in Photos on the iPhone, including where the slow section sits. Other apps ignore it, and deleting it does not harm the recording. iPhone photos that arrive as a HEIC and a MOV pair explains the other files an import leaves behind.

The sound in the slow section is deep and stretched. Is the file damaged? No. The audio is slowed along with the picture, and slowed sound drops in pitch. The normal-speed sections on either side sound as they were recorded.