Can you explain to me why when i record using 4k 60fps, 4k 30fps, 2.7k 60fps and 1080p 60fps the file size is almost the same. Whereas when i record using 1080p 30fps the file size drop significantly?

It is all about the bitrate, which actually is the only factor that determines the file size.

I know 4K 30fps and 4K 30fps are both recorded at 100Mb/s, which means they record the same amount of data every second (ergo every 60fps video frame is twice as compressed as a 30fps one).

Probably the other two modes mentioned have similar bitrates if the file sizes are similar (I never use them, so I don't know). Given their smaller resolution, they will be less compressed to fill up the bitrate allowance, and this is a good thing since the compression used is lossy.

100Mb/s would be an overkill for 1080p 30fps footage; so that uses a much lower bitrate (around 30Mb/s if I am not mistaken), resulting in noticeably smaller file size.

Hope this helps,

So basically you are saying that normally lower down the frame rate will reduce the file size significantly. But this case will not happen with 4k because shooting at 60fps and 30fps both will give result of100mb/sec.

Ya Aku Posted at 7-26 22:13
So basically you are saying that normally lower down the frame rate will reduce the file size significantly. But this case will not happen with 4k because shooting at 60fps and 30fps both will give result of100mb/sec.

Exactly! A lower frame rate (or resolution for that matter) video would generally utilize a lower bitrate, and that is why it would have a smaller file size.

What DJI chose to do here is a blessing if you ask me. Higher the bitrate, better the quality

Would you expect the image to be better on 24/30 FPS if it’s the same bit rate as 60fps ?

I must admit I don’t see one but then maybe it depends on the type of scene and movement etc .... ?

MKosmo Posted at 7-27 09:42
Would you expect the image to be better on 24/30 FPS if it’s the same bit rate as 60fps ?

I must admit I don’t see one but then maybe it depends on the type of scene and movement etc .... ?

"Would you expect the image to be better on 24/30 FPS if it’s the same bit rate as 60fps ?"

I would expect it to be better in that the exposure time for each frame at 30fps would be greater than @ 60fps, which I believe would give better colour. And then of course using the same max' bitrate would allow better compression (i.e. less) and better image detail perhaps.

Edit:- but there again fixed aperture...

MKosmo Posted at 7-27 09:42
Would you expect the image to be better on 24/30 FPS if it’s the same bit rate as 60fps ?

I must admit I don’t see one but then maybe it depends on the type of scene and movement etc .... ?

Well, yes and no Bitrate vs quality works on one principle: "More" does not make any noticeable difference as long as you have "enough". It is the times that you don't have enough, every single extra bit counts. So yes, having a much higher storage allowance per frame (by having less frames to store in the same amount of data) makes a difference, but no, not always...

Modern codecs perform both a spatial and a temporal compression. Which means they compress every frame within itself like a jpeg picture and compress multiple consequent frames together (oversimplified, but that is the gist of it). So when it comes to bitrate needs, every detail in the composition, and every big movement between the frames is our enemy.

This need increases considerably when you have a narrow aperture, wide angle lens camera with a fixed focus at infinity (a lot of things in composition, and in focus = a lot of detail) which is used for "Action" ( = a lot of movement)

This is why I think it is good to have high bitrates for OA, it is a blessing for it's intended use. And of course it would not be possible to see an apparent difference in every scenario, even with OA.

Sorry for the long explanation & cheers
  

CemAygun Posted at 7-27 17:44
Well, yes and no  Bitrate vs quality works on one principle: "More" does not make any noticeable difference as long as you have "enough". It is the times that you don't have enough, every single extra bit counts. So yes, having a much higher storage allowance per frame (by having less frames to store in the same amount of data) makes a difference, but no, not always...

Modern codecs perform both a spatial and a temporal compression. Which means they compress every frame within itself like a jpeg picture and compress multiple consequent frames together (oversimplified, but that is the gist of it). So when it comes to bitrate needs, every detail in the composition, and every big movement between the frames is our enemy.

Long explanation - not a problem !  All good info.

So in fact because the action camera has infinity focus - does this mean it actually has a harder job with its compression - compared to say the OP which has some amount of shallow DOF ?

I'm guessing the toughest scenes are pans and fast moving detailed scenes such as MTB.  I have noticed the OA was awesome filming in water rides but not quite as good with high detail such as grass and road tarmac detail.

MKosmo Posted at 7-28 03:35
Long explanation - not a problem !  All good info.

So in fact because the action camera has infinity focus - does this mean it actually has a harder job with its compression - compared to say the OP which has some amount of shallow DOF ?

I assume Osmo Action in a typical scenario would have to deal with more detail to compress compared to Pocket, but there are many factors involved here. Having quite a wide depth of field all the time is one of them, sure; but Action is also one stop slower/darker (out of necessity). Which means unless there is plenty of light, it has to use a higher ISO setting compared to pocket (at the same shutter speed), resulting in more noise.

In the end we are talking about "probability" here; it does not mean this will be the case every single time, or one device is better than the other.

Chaotic detail is the kryptonite of the codecs we use. Things like noise, rain, confetti, grass etc. all bite a huge chunk off the bitrate, sometimes to the point of rendering it insufficient. But most of the time, it is incredible to see the quality of video we can get from such small devices ...

Cheers,