MOV and MP4 are nearly the same container
MOV (QuickTime) and MP4 share the same box structure, and an iPhone's MOV holds H.264 or HEVC video with AAC sound — codecs MP4 accepts as they are. Converting can therefore be a rewrap: the compressed picture and sound are copied into a new MP4 box without decoding, which takes seconds even for a 2 GB file and changes nothing you can see or hear.
Because nothing is re-compressed, the file size stays about the same. To make it smaller, use Compress MP4 afterwards or the Video Compressor.
Fast copy or maximum compatibility
- Auto: copies H.264 and HEVC video and AAC sound; converts uncompressed PCM sound from cameras to AAC.
- Fast: copies whatever MP4 can hold.
- Maximum compatibility: re-encodes to H.264 with AAC sound, which plays on any device, at the cost of time and a small quality loss.
HEVC in MP4 plays on iPhones, Macs, Android phones and current browsers, but Windows' built-in app may ask for the HEVC extension. If the file is for an older PC, a TV or an upload form that rejects HEVC, pick Maximum compatibility.
iPhone videos in browsers that can't read HEVC
Firefox can't decode HEVC, so it can't re-encode iPhone videos — but the fast copy needs no decoding at all, so MOV to MP4 works in Firefox too. Only Maximum compatibility needs Chrome, Edge or Safari for HEVC sources.
What isn't carried over
Spatial audio (APAC) is not supported. When a normal audio track with a recognized codec is also present, that track is used instead.