December 27, 2016 at 3:34AM


Downscaling 4K to 1080p

I've been shooting a lot of 4K for a doc I'm working on, so I can crop and frame to create multiple cam angles with one camera. The problem is, it seems like when I export my 4k into 1080p footage, to use later in editing my final FCPX project, the 1080p file is BIGGER than the original 4K file?! Why is that? The project is set up to be Pro Res 422, and my export is Pro Res 422.

I know this all has to do something with codecs, but I'm not very familiar with all this technical stuff, so I'm hoping some one out there can help me understand.

My noobie assumption is if I export 4K directly to 1080 without cropping, the file size will be HALF as big. If I do that, then I can be comfortable RE-IMPORTING my new 1080 footage, and it will appear the same as all the other 1080 footage I shot as 1080 (not formerly 4K).

But that does not seem to be the case. I'm shooting on a Sony a7sII, and it looks like the codec its shooting in is AVC (which I believe is the same thing as H.264).

My only choice when setting up a Project File, is an Apple Pro Res format or Uncompressed 10 bit. I've been choosing Apple Pro Res 422.

Upon exporting, I've been selecting Source - Pro Res 422...and the result has been 1080p files that are %50 bigger than the original 4K files. So my small 76MB 4K file is now a 115MB 1080p file. You can see how this fills up my hard drive quickly when Im converting hour long interviews.

What is the correct workflow? I can't find anything this specific on youtube.

Should I just keep the 4K files in my final project timeline, skip downscaling all together? I don't think my Early 2013 15" MacBook Pro can handle all that 4K footage, especially since everything is stored on an external thunderbolt drive.

Or, am I doing this wrong? Should I be using a different export codec? A different project codec?



The AVC codec is highlight compressed, likely using 4:2:0 Chroma sub-sampling, which is 1/2 the data rate of 4:2:2 Chroma sub-sampling. But more importantly, AVC uses a much more compressed video codec than ProRes 422 (which is made for editing high-quality videos across multiple generations). ProRes LT is a 422 format that is light enough that it can be used over 1Gbps networks. ProRes Proxy is a highly compressed format that is suitable for viewing, but which has too much generational loss to be good for general editing. There are numerous other ProRes formats that are less compressed than 422.

FCPX is supposedly smart about generating proxies to let you work in 4K without killing yourself or your iMac. Use ProRes 422 as your project domain, allow FCPX to create and use Proxies, and have the ton of disk space that 4K editing actually requires. After some time it will all seem normal. Including the frequent new 8TB drives you have to order from every time you start a new project ;-)

December 27, 2016 at 4:15AM


Thanks Michael, much appreciated!


March 2, 2017 at 3:27AM

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