Compression problem, frame size inversely related to file size?

[Loch Phillipps]

I have compressed a long string of 4k video hoping to create a file close to 1GB in H.264. When I compress it at 960x480, at medium resolution, I get a 2.67GB file. Strangely, when I compress the same sequence to a 640x360 fram size at medium resolution, I get a 5.86GB file. This makes no sense to me.

It could be that my resolution is changing as I muck with the frame size setting and add a time code window, but I don't see why it would be twice a big a file. Am I doing something wrong? Should I compress to a format other than H.264? Is there a way to keep the resolution the same, because every time I change it, the frame size reverts to the original size, which is 4k?

Thanks!

[Michael Grenadier]

what do you mean by "medium resolution?"  Resolution usually refers to pixel dimensions (at least I think it does).  do you mean bit-rate or quality?"  Try working with one of the h264 presets (such as those for youtube) to start with and then adjust the parameters if necessary...

[Loch Phillipps]

Yes, my bad. I meant quality.

[R Neil Haugen · Accepted solution]

File size is mostly determined by bitrate times the length of the video. The framesize only sets how sharp things are in the image. The bitrate sets the file size as far as bytes.

Neil

[Loch Phillipps]

I'm surprised by this! So the frame size doesn't have much to do with it? That is very helpful. So I can simply reduce my bitrate/quality and leave the framesize. I'll give that a try.

[Michael Grenadier]

Neil, are you saying that a 4k file with the same compression settings will be the same size as a 720 file?

[Ann Bens]

Yes: you can check this yourself in the export settings. Make everything the same except the height and width.

[R Neil Haugen]

Yes, mgrenadier and everyone else. Like Ann has constantly tried to remind people, the BITRATE is exactly that, how many bits are put in the file per second of playing time. That is put another way, how much data per second is put in the file.

All the file is, is a header with info on what's included (a few measly kb) and ... the data of the file itself.

More bits per second, means more data, means bigger file.

Bitrate times length. That's what matters for file size on disc.

So what does changing frame-size do then? Changes resolution. In a 4k (actually, UHD) export at say 25Mpbs, your data is "spread out" over 3840x2160 pixels, where a 'full HD' of 1920x1080 puts the same amount of data over 1920x1080 pixels. It has more data per pixel.

Neil

[Michael Grenadier]

ah, but I mainly just use the vimeo or youtube presets for web delivery which will have different bitrates for different pixel dimensions...  So that's why I never had this "aha" moment.  If I'm outputting for full rez archival purposes, I use the appropriate flavor of prores and again that doesn't enter in to it...  but thanks for setting the record straight...

[Michael Grenadier]

and this is one of the main reasons why I find monitoring this forum
so valuable. Although I know alot and pretty much everything I need
to know to work, I do have some areas I do not know as thoroughly as I
might. Right now, I'm going thru
"Premiere Pro 2020 Essential Training" on Linkedin Learning wherever I
have a few spare minutes. Gotta say I know at least 90% of it, and
have some serious issues with some of the workflow choices (and have
found some mistakes), but everyonceinawhile I discover something very
useful. So thanks Neil, Ann and all the other heavy hitters here.

[R Neil Haugen]

There are so many things to a complex app like an NLE. Any of us will always find something "basic" we didn't know, no matter how many years on in the daily use we are.

And you're right, that's one of the huge benefits of a user forum like this.

Neil