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I exported two videos from Adobe Premiere. Same length, same quality settings, but one was 4K (the original footage was 4k) and one was 1080p. I was trying to put these on Facebook so I was trying to crunch file size.

But with the exact same settings they were both 24.4MB. How can this be? The 4K one should be much much larger. Quality stayed the same so when the resolution changed file size should have changed, but it didn't.

Why?

Side note: This was done with variable bitrate, might that have been the reason?

  • If you say you used the same settings, does that include the average and maximum bitrate settings? – MoritzLost Jul 26 '15 at 10:17
  • Yes it does. Which is why this is so confusing for me. – meed96 Jul 26 '15 at 12:31
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Well you've kinda answered your own question: Both videos have the same size because you used the same settings, specifically the settings for target bitrate and maximum bitrate (assuming you're using the H264 codec, other codecs might only have an average bitrate setting available). If you use identical bitrate settings, videos of the same length will always have the same size, regardless of it's resolution and framerate. This is because the bitrate is exactly that: The (average) amount of bits per second. If you have a 10 second video with a bitrate of 10 Mbit/s, it's size will be about 100 Mbit, simple math.

To elaborate on how this is possible with videos with different resolutions: Most commonly used video codecs (as well as image formats) use highly efficient compressing algorithms that can use different levels of compression, where a higher compression level yields smaller video files at the cost of some video quality loss. For example, a JPG image will combine nearly monochrome chunks of the image to one block of information. The higher the compression, the higher the tolerance for similar pixels, resulting in compression artifacts:

jpg compression

(Source)

So what does that mean for your video? To accommodate for the higher resolution of your 4k video, you will need to use a higher bitrate in order to yield better quality, as there is much more information in a 4k video. If you use the same bitrate, the 4k video might actually look worse because there are more compression artifacts.

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    For some reason I thought that bitrate was linked to number of pixels, not overall data flow. It seems so obvious now. Thanks a ton for explaining that, I finally understand! And I was using H264, yes. – meed96 Jul 26 '15 at 13:13
  • Yeah it took me some time to figure that out as well ... a while ago, I actually asked a quite similar question regarding the relation between bitrate and framerate. In case you're interested, the answer of AJ Henderson is a good read as well: video.stackexchange.com/questions/12459/… – MoritzLost Jul 26 '15 at 13:41
  • So Bitrate is attached to the framerate/per second? So a 1fps video that has a Bitrate of 5mbps would mean each frame is 5MB? 2fps would be 2.5MB/frame, and ect? – meed96 Jul 26 '15 at 18:16
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    Well, basically yes. You can consider the bitrate as the average amount of information available per second. A higher framerate means there is more information per second (more frames → more pixel to describe → more information in bits), so the level of compression gets higher. If you're using a constant bitrate (CPR), there is the same amount of bits available for each and every second of your video. To maximize efficiency, there is the variable bitrate (VBR), which will distribute the overall file size unevenly among the individual seconds. to be continued ... – MoritzLost Jul 26 '15 at 18:34
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    For example, if there are five seconds in your video where you only see a black screen, the bitrate can be significantly lowered for these five seconds, as the black screen contains very little information. That way, more bits can be used in scenes with much motion (→ much information). So using a variable bitrate, you achieve a better quality compared to a video of the same size that uses a constant bitrate. – MoritzLost Jul 26 '15 at 18:37

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