This weekend I shot some precious 1080p footage on my Panny camcorder and duly imported it into iMovie only to discover that the external mic I had used with the camcorder was defective and had only recorded the left-hand channel. Luckily I had simultaneously recorded perfect audio on a Sony PCM-M10. So I added the new audio to my footage, synching it perfectly. So far so good, But exporting the finished movie - how do I do this without losing quality? I understand this may be an issue with the latest version of iMovie. Thanks in advance for any tips.
N

Depends on what you want to do with it after export. To retain the full quality, use the File> Share> File option, you can then choose what settings you want but bear in mind not all computers have the ProRes codec installed for playback. If the footage is that important to you make sure you retain the 'original' camera footage file or files (ie: before the import into iMovie) that can be imported into any current video edit program like FCPX.

Thanks for that. OK so if I use the ProRes codec, what format is iMovie spewing it out in? The difference in file size is enormous, I notice.

The ProRes codec will give you a .mov file, ideal to use as a 'Master' from which you can make further versions (or edits) at a later stage for YouTube, Vimeo etc using either iMovie or any of the other compression programs like MPEGStreamclip or Handbrake.

The high quality setting in iMovie will give you an mp4 (H264), which could be uploaded to YT as is but, is less suitable for further compression. Yes, ProRes will give you a much larger file but you said you wanted to retain as much quality as possible, and TBH, hard drive space is about as cheap as chips these days - when I started in NL video 4.3Gb SCSI drives were £2.5k each plus VAT!

Thanks Dave. Just 22m of 1080p footage ran to 22gb in ProRes format. Does that seem high to you?

As a very rough guide it's about 1Gb per minute for Prores 422 25fps 1920x1080p, so yes it's about right.

I usually figure on roughly 45 sec/GB for Prores HQ 4,2,2 (220mbit/sec) here in 29.97 fps land. It's a little lower than that, but that's what I use mentally when figuring out how much recording time I have left on the media.

https://support.apple.com/en-us/HT202410