Bit rate reduction when converting 60fps footage into 30fps?

Question

I'm shooting at 60fps, the result is video playing back normal speed but displaying 60fps.

I then have stretched the clip so that i get 30fps playback speed with a slow motion clip twice as long as the original.

The camera is recording the original video at 50mbps bit rate, my question is this.

If both clips have the same number of total frames, yet one is serving half as many of them for every second, will the bitrate have reduced roughly by half as well?

I ask this because i have rendered both version of the clip. one technically twice as long but with the same number of frames, i rendered them both with the same bit rate as the original source, one of course turned out twice the size of the other.

I would like the slow motion version to be roughly the same file-size as the original, i figure with the same number of frames it should be more or less the same size.

But because i encoded them both at the source files bit rate one is twice as large.

Can i reduce the bit rate for the slow motion one without losing quality?

Many thanks for your time and expertise should you answer!

P.S i'm new (as a user) to stack exchange i first posted this in the superuser section, since exploring i found this dedicated area so thought it best to put it here!

Regards, Cal

Answer 1

In theory, if the number of keyframes and their placement were about the same, as decided by the encoder, then yes, you can reduce the bitrate to half and retain the same quality. In practice, the encoder may be configured to place a keyframe every N seconds, in which case, you should use a bitrate a bit higher than half the original bitrate, say, around 55%.