user20232: So I have some videos that are Full HD, h264 but they have over 30M bitrate at 30 FPS, they are SDR and their quality is not very impressive so it seems excessive and unnatural for such unimpressive quality - I have seen much better looking h.264 content with 60 fps with even half of this bitrate. So what I did try to do is use -crf 17 (yes, without encoding), to see how much this will reduce them and it usually does do a good job for example it made the file to about 12M, I also tried -fs with even a higher number of size than the original, and it brought the file down to 5M bitrate, I don't know what actually happens behind the scenes with this, but I think the video looks a bit worse, not much though compared to the 30M that it was.
So how, if even possible, do I correctly determine the bitrate of a video if it seems excessive?

user24473: Determining The best bit rate is more of an art form then a science. As soon as you go from a loss less to a lossy format, you’re accepting compromise. If the scene has a lot of natural blur, you may be able to get away with a surprisingly low bit rate before the effect is noticeable. Consumer playback systems may not be able to reproduce all the content contained in your  high bit rate stream. It’s a balance of audience, playback expectations and how much you can get away with.
With video, you’ll need a higher bit rate whenever there’s rapid change that you want to preserve or intricate detail that doesn’t compress well. With audio, there’s a point where you can hear the artifacts, but different people pick up the playback errors at different loss rates for different types of music. Classical guitar may not tolerate much compression because it’s high attack plucking. Rock guitar may compress well because the accuracy of the notes isn’t something you can pick out easily and, as Billy Joel would say...it’s still rock and roll to me. Animation with sharp lines may require a higher bit rate than expected because pixelation is immediately obvious.
Anyhow...my solution is to start with the cleanest, highest resolution uncompressed source, then drop bit rate until I can tell there are defects, then try to be a touch more generous.
If you’re doing video, you may be able to do a comparison mask that makes the discrepancies more obvious. With audio, if I can visually compare waveforms, I’ll reduce bitrate in the less important passages, but boost it if there’s a silent passage or one with lots of complexity. Almost any compression on a silky voice like Nat King Cole’s is quickly audible. 
I guess I’m saying that you need to do manual comparisons until you’re sick of watching/listening to the material.
And be careful with VBR. it’s not a cure-all, unless you have control over where bits are spent.

user3592: 
So how, if even possible, do I correctly determine the bitrate of a
  video if it seems excessive?

There's not a "correct" way.  That's why every encoder you come across presents bitrate as an option to the user.  The visual complexity of every video is different, and visual tolerance levels of artifacting is subjective in every case.  File size vs image quality will always be a tradeoff, and it takes an informed human being to decide where that tradeoff is most acceptable.
But to focus more on the "how" one best makes this decision, there's not much to say other than: trail and error.  
You can bifurcate.  That is, pick a high number, try it, pick a low number, try it, then pick a number between whichever direction was more suitable, rinse and repeat.  It's still a subjective process.  If you search the internet, it's not hard to find ballpark ranges to get you started, given the specifics of your output requirements (resolution, framerate, etc).  When you do this, you should use a representative subset of your total output to test, so that you can iterate more quickly.
Also, be sure to consider your destination platform.  You don't mention it in the question, but it will affect your decision.  If you're delivering to bluray, for example, you have a hard limit on total file size, and you'll probably want to optimize quality for that specific target.  On the other hand, if you're delivering to an online streaming service, you might want to go for maximum quality because YouTube, Vimeo and the like will automatically generate lower-quality versions to serve to people with slower connections.  Conversely, you might value high-speed upload for quick turnaround times.  Only you can decide.

20232: <p>So I have some videos that are Full HD, h264 but they have over 30M bitrate at 30 FPS, they are SDR and their quality is not very impressive so it seems excessive and unnatural for such unimpressive quality - I have seen much better looking h.264 content with 60 fps with even half of this bitrate. So what I did try to do is use -crf 17 (yes, without encoding), to see how much this will reduce them and it usually does do a good job for example it made the file to about 12M, I also tried -fs with even a higher number of size than the original, and it brought the file down to 5M bitrate, I don't know what actually happens behind the scenes with this, but I think the video looks a bit worse, not much though compared to the 30M that it was.</p>

<p>So how, if even possible, do I correctly determine the bitrate of a video if it seems excessive?</p>
 24473: <p>Determining The best bit rate is more of an art form then a science. As soon as you go from a loss less to a lossy format, you’re accepting compromise. If the scene has a lot of natural blur, you may be able to get away with a surprisingly low bit rate before the effect is noticeable. Consumer playback systems may not be able to reproduce all the content contained in your  high bit rate stream. It’s a balance of audience, playback expectations and how much you can get away with.</p>

<p>With video, you’ll need a higher bit rate whenever there’s rapid change that you want to preserve or intricate detail that doesn’t compress well. With audio, there’s a point where you can hear the artifacts, but different people pick up the playback errors at different loss rates for different types of music. Classical guitar may not tolerate much compression because it’s high attack plucking. Rock guitar may compress well because the accuracy of the notes isn’t something you can pick out easily and, as Billy Joel would say...it’s still rock and roll to me. Animation with sharp lines may require a higher bit rate than expected because pixelation is immediately obvious.</p>

<p>Anyhow...my solution is to start with the cleanest, highest resolution uncompressed source, then drop bit rate until I can tell there are defects, then try to be a touch more generous.</p>

<p>If you’re doing video, you may be able to do a comparison mask that makes the discrepancies more obvious. With audio, if I can visually compare waveforms, I’ll reduce bitrate in the less important passages, but boost it if there’s a silent passage or one with lots of complexity. Almost any compression on a silky voice like Nat King Cole’s is quickly audible. </p>

<p>I guess I’m saying that you need to do manual comparisons until you’re sick of watching/listening to the material.</p>

<p>And be careful with VBR. it’s not a cure-all, unless you have control over where bits are spent.</p>
 3592: <blockquote>
  <p>So how, if even possible, do I correctly determine the bitrate of a
  video if it seems excessive?</p>
</blockquote>

<p>There's not a "correct" way.  That's why every encoder you come across presents bitrate as an option to the user.  The visual complexity of every video is different, and visual tolerance levels of artifacting is subjective in every case.  File size vs image quality will always be a tradeoff, and it takes an informed human being to decide where that tradeoff is most acceptable.</p>

<p>But to focus more on the "how" one best makes this decision, there's not much to say other than: trail and error.  </p>

<p>You can bifurcate.  That is, pick a high number, try it, pick a low number, try it, then pick a number between whichever direction was more suitable, rinse and repeat.  It's still a subjective process.  If you search the internet, it's not hard to find ballpark ranges to get you started, given the specifics of your output requirements (resolution, framerate, etc).  When you do this, you should use a representative subset of your total output to test, so that you can iterate more quickly.</p>

<p>Also, be sure to consider your destination platform.  You don't mention it in the question, but it will affect your decision.  If you're delivering to bluray, for example, you have a hard limit on total file size, and you'll probably want to optimize quality for that specific target.  On the other hand, if you're delivering to an online streaming service, you might want to go for maximum quality because YouTube, Vimeo and the like will automatically generate lower-quality versions to serve to people with slower connections.  Conversely, you might value high-speed upload for quick turnaround times.  Only you can decide.</p>
