Export iMovie for Mac File as MP4
(from Apple Support documentation)
Select a movie, trailer, or clip in the browser.
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Click the Share button in the toolbar (upper right corner), and then click File.
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In the dialog that appears, do any of the following:
Set the title of the shared movie: Click the name at the top, and type a new name.
Set the description of the shared movie: Click in the Description field, and type new text.
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Set the output format of the shared movie: Click the Format pop-up menu and choose an option.
You can choose to export video and audio, or just audio.
Set tags for the shared movie: Click in the Tags field, and type tag names separated by commas.
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Set the resolution of the shared movie: Click the Resolution pop-up menu and choose an option.
Unavailable resolutions appear dimmed.
Note: You can export a 4K clip or movie in its native resolution.
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Set the output quality of the shared movie: Click the Quality pop-up menu and choose an option.
To manually set the output quality of the shared movie, click the Quality pop-up menu, choose Custom, and drag the slider to set the target bit rate.
As you drag the slider, the estimated file size that appears below the preview thumbnail is updated.
Choose a compression setting: Click the Compress pop-up menu and choose an option.
Add your movie to the Theater: Select the Add to Theater checkbox.
Click Next.
In the dialog that appears, type a name in the Save As field, or leave the default name, which is the name of the movie, trailer, or clip.
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Navigate to the location where you want to save the file, and click Save.
A progress indicator appears on the right side of the toolbar. Click the progress indicator to see details. The indicator disappears when the operation has finished.
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After the share process is complete, the movie opens in QuickTime Player. A “Share Successful” notification also appears.