How is Photos able to reduce video files sizes so much on export?

Discussion in 'Digital Photography' started by sbrown02, May 13, 2015.

  1. sbrown02 macrumors newbie

    Oct 25, 2013
    I realize this is the Digital Photography forum, but my question actually is a result of using Photos, so if this is posted in the wrong place let me know and I'll delete and move it.

    Recently I ditched the use of a camcorder for taking video and am now just using a Sony digital camera for all my still picture and video capture needs. When I plug my camera into my iMac and open Photos it imports both the still pictures and video files, so now I have video files in Photos.

    At first I thought this would be a problem as I assumed all video files should go into iMovie. But after using Photos to export video for other purposes I'm seeing that Photos is able to significantly reduce the file sizes of the exported content.

    So, to get a feel for what different video formats (MP4 and AVCHD) look like and their ensuing file sizes I decided to try a simple unscientific test. Here's what I did step by step.

    1) Using my Sony RX100M3 digital camera I took 10 seconds of exactly the same scene of video (video indoors of the kitchen table) using 4 different video modes. The 4 modes and their files sizes after being imported into Photos are:

    MP4 1440x1080 - 16.8MB
    AVCHD 24p 17M(FS) - 21.4MB
    AVCHD 24p 24M(FX) - 28.5MB
    AVCHD 60p 28M(PS) - 34.4MB

    Note: All AVCHD files in Photos are imported as .MOV and say they are 1920x1080 and H.264. What's odd is even the MP4 file that the camera says is 1440x1080 shows in Photos as 1920x1080, so I'm not sure how that works that the resolution changes from 1440 to 1920 going from the camera to photos. If anybody know here, please chime in and answer.

    2) Plugged the camera into the iMac and imported the 4 video clips into Photos

    3) Then exported the 4 video clips using the 1080p video quality option. Below are the new exported files sizes and the % reduction.

    MP4 1440x1080 - 16.8MB and same after - No Reduction
    AVCHD 24p 17M(FH) - Orig 21.4MB and New 13.6MB - 36% Reduction
    AVCHD 24p 24M(FX) - Orig 28.5MB and New 13.5MB - 53% Reduction
    AVCHD 60p 28M(PS) - Orig 34.4MB and New 22.8MB - 34% Reduction

    Anybody know in detail what Photos is doing to get these significant improvements (reductions) in video file size? In particular, the 24M(FX) reduced more than half. Another oddity is how the FX is reduced +50% while the FH and PS only in the mid 30% which essentially makes the FX file size the same as the FH.

    All that said, when playing/watching them I cannot tell a difference between any of these files (they all look great), yet the sizes are so much smaller. Note: This was done on my early 2011 MBP on Yosemite 10.10.3 and Photos 1.0. Doing this makes me think I should export all my video files out of photos at 1080p and them import them back in to save hard drive space. Thoughts anyone?

    Lastly, for anyone (like I was) that is curious I also exported these files using the 720p option just to see how much it would reduce the size and here are the results. And in my opinion 720p also looks great on an HDTV screen.

    MP4 1440x1080 - 16.8MB and New 10.6MB - 37% Reduction
    AVCHD 24p 17M(FH) - Orig 21.4MB and New 9.5MB - 56% Reduction
    AVCHD 24p 24M(FX) - Orig 28.5MB and New 9.5MB - 67% Reduction
    AVCHD 60p 28M(PS) - Orig 34.4MB and New 16.2MB - 53% Reduction

    Well, that's it. Thanks and I look forward to hearing from others what is going on behind the scenes here.

  2. alex.m90 macrumors member

    Nov 14, 2008
    Leicestershire UK / San Francisco USA
    I'd imagine its just compressing it, so the resulting footage is a slightly lower bitrate but not enough to notice any obvious differences.

    If you can't tell between the 2, and disk space is hard to come by, then I'd say go ahead and convert the lot!

    Mind you if you plan on editing in iMovie, higher bitrate footage will be easier to manipulate/colour correct due to the retained detail in the shadows and highlights etc.

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