why are a 4k and HD export the same file size?

[djmattyz]

im exporting a short film in 4k and HD

and it appears that the size is more or less exactly the same regardless of the resolution

on the other hand if i increase or decrease the bit depth, it has a dramatic effect on the file size

why is this?

i would have imagined that a 4k file would logically be bigger than a HD file

[My Trucking Skills]

what are the dimensions of the HD file?

[djmattyz]

1920 x 1080

vs

3840 x 2160

[R Neil Haugen]

Bit-rate is a per-second thing, not a per-frame-size thing. So how could two exports of the same sequence be different at the same bits per second?

This is why you need to change the bit-rate per second for larger frame-sizes ... as otherwise, it cuts down the details of the export to achieve the smaller bit-rate numbers. At least double the bit-rate of an HD export for a 4K export ... if not more. Test to check what you're comfortable with.

Bit depth is different, of course. That is how many bits per color channel. 8-bit is 256 bits/channel, 10-bit is 1,056 bits per channel of color information. Unless you have a goodly share of 10-bit material on the sequence, there's no reason to export a 10 bit file. Except of course if you're using the most excellent Cineform which only comes in 10- and 12-bit flavors, but never uses more 'space' than it needs in the file.

Neil

[djmattyz]

oh right, sorry i was talking about bit rate, not bit depth

but premiere defaults to a bit rate of 10 regardless of weather im exporting at 4k or HD

so you are saying that i should manually increase it for higher resolutions,  which i did by the way, (to 20)and it seems to look better

but how do i know what is the ideal bit rate for any given video if premiere doesnt do it automatically?

[R Neil Haugen]

There's no way PrPro can "know" what is the best bitrate for any particular media which is heavily a user-decided choice. Are you needing small file size? Is there a lot of motion needing a high bit-rate for most of the project? Is there very little motion so the project can go in a rather small bit-rate average but with a high setting for max bitrate?

It can't out-think you. As a pro app, that's your job. It's very easy to search online for best bitrates for different video export uses. You can get some good guidelines quick, then ... test. Always test to make sure something works the way you want it to.

Neil

[djmattyz]

i understand yes, its my job to know.   BUT premiere did guess a bit rate of 10 to begin with, so if ,as you say the bit rate MUST be increased as we increase the resolution,  in order to prevent a reduction in quality, then i would have imagined that premiere would increase it automatically as i increased the resolution to maintain the same quality.

i dont really need  small file size, i basically want a master copy of the film,  the best quality i can get. then i will do smaller copies for online use etc. where i can work at a reduced bit rate

20 seems to look pretty good at 4k

its made using sony A7sii camera.

[R Neil Haugen]

You might think ... but then, it's best not to assume, right?

I know a number of people that for a master might be up to say 25-30=avg, 40 or more max for a 4K output. All depends on how much detail you expect to preserve. If it's only gonna end up online anyway, well ... not nearly so crucial of course. Smaller sizes work entirely adequately for online use.

Neil

[djmattyz]

this has really explained a lot.  thanks

it makes me wonder now, if certain cameras 4k footage may be,  in a way, worse than the HD footage, unless the camera can also record the 4k at a higher bitrate

[R Neil Haugen]

Yea, one does wonder.

My GH3's All-Intra is nearly twice the bitrate of the 'standard' media, but included more noise. So most GH3 users took the best you could get without going All-I as well, it added a lot of time in post to de-noise as much as you needed to above ISO800. The camera's internal processing to H.264 seems to have eliminated much noise.

With improvements in noise processing in the computer, I'm now using the All-I and de-noising when/as needed. And enjoying the extra data.

Next cam is going to be the BMPCC4K ... and ... that's going to be a bit higher in bitrates ... !

Neil

[Mo Moolla]

Agreed

Render file Size is dependent on bitrate and output codec not so much resolution

So I could have a one min long 4K timeline with a file size of 5gigs from the RAW footage and export out to 50 or less MB in a Twitter HD format

I could also output in a Prores422HQ or H.264 with maxed out bitrate to over 5 gigs

I guess u understand where we are going wth this

Best of luck

Mo