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I am just importing some AVCHD (.MTS) files into iMovie and even if Import them as 960x540 (original is 1920x1080) it uses more space than the original.

If I import it at full size, the .mov file is 5 times bigger than the original!

That means 4h of video which are originally 25 GB suddenly become 400GB on my disk!

What am I doing going wrong here? Why can't iMove work with the original file and only convert it when I export the file?

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Video is practically never stored uncompressed because the data rates are insane if they were. Each image in a video is 1920 by 1080 pixels and takes 3 bytes per pixel (one each for red, green and blue). That's 6.2 megabytes per frame. There are 24 frames in a second, that's roughly 150 megabytes per second for 24p video. That's 9 gigabytes per minute.

To reduce this size, codec's (encoders/decoders) are used that can compress the data either losslessly(similar to a zip file, they uncompress to produce identical input to the raw, uncompressed data) but typically can take as little as 1/3 of the space, or lossy, where the video is altered to make it easier to compress without significantly distorting the video. Most HD video is shot with a minimally lossy codec due to the size (H.264). AVCHD also records using H.264.

If you are outputting to uncompressed or lossless compression for the MOV, then the file size would increase significantly from the compressed AVCHD file, even if the number of pixels being stored has been reduced some. If you change to a similar H.264 data rate for the mov file, then you should not see an increase in file size.

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iMovie does it because it is (or was? it is 2018 now) a crap video editor. It cannot handle proper AVC HD with interframe encoding, so it converts it into 960x540 with intraframe encoding, much easier to handle but lower resolution and larger files. See https://en.wikipedia.org/wiki/IFrame_(video_format)

Just throw iMovie away and use a proper editor that does not require you to convert your source files into specific format for editing. On Windows, Vegas proved to be a very fast editor that accepts tons of formats and does not require pre-rendering, that is, converting to some internal format. Not sure what you can use on Mac.

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