24 fps is just fine for your BluRay output. It's sort of become the new standard frame rate for movie-like discs. The program will interpolate the frames so it should look great.
There is no reason to use Fit to Disc if you're saving a 20 minute video to a BluRay disc. BluRays can hold nearly 2 hours of video, so you've got plenty of space. In any event, the difference in file size and change in bit rate you're seeing between Fit to Disc and don't Fit to Disc isn't significant. You won't see a difference -- but you may want to opt for the higher bit rate anyway.
And, yes, you can opt to create an AVCHD disc rather than a BluRay, since your movie is only 20 minutes long. An AVCHD disc is essentially BluRay disc files burned to a DVD disc. Just make sure the people receiving the disc know that the disc can only be played in a BluRay disc player (even if it is on a DVD player) and that some players may not play this type of disc at all.
That said, since you're producing several copies of this disc, I'd recommend you burn your BluRay to an ISO file on your hard drive rather than directly to a disc. You can then use a program like Img Burn or Nero to burn that ISO file as BluRay discs and you'll be able to spit out copies fairly quickly.
It was my intention to create ISO files & burn via ImgBurn (which I have done before).
I have already created a Blu-Ray ISO file with Frame Rate=25 (Recommended Settings) but not yet burnt.
What puzzel me (if AVCHD is essentially Bul-Ray burnt to a DVD disc) is that the predicted size of the ISO
for AVCHD is 2.76GB whilst for real Blu-Ray its predicted to be 3.62GB.
Anyhow, I ran into an unexpected problem with my method (1) above - the MP4 file.
Whilst the MP4 file plays back OK on PCs & after upload to YouTube when I copied that
MP$ file ontro a USB stick & inserted directly into our flatscreen TV the filename was shown but not recognised.
A file created by PE14.1 from a non-HD video source (.mpg) was recognised from the USB stick
as MPEG & played back. Same think happened when the USB stick was inserted into the Blu-Ray player.
My son says this is because some/many TV sets cannot process MP4 & I should try MPEG.
So back to PE 14.1 I select Devices => Computer => HD 1080 & in the Format pull-down it offers:
- M2T - H.264
- MOV - H.264
- MP4 - H.264
- MPEG - TS
I selected the latter & was suprised to see it created a file with ending .m2t (I really had selected MPEG)
and the WIN7 Explorer showed it to be an AVCHD video 4.47GB in size!
I have not yet tried this on the USB stick / TV.
So how should I create an MPEG?
I think your son is right.
In fact, Premiere Elements 14 and 15 have a specific output for creating video for a thumb drive to play on your TV (Export & Share/Devices/TV) and, yes, it creates an MPEG file.
MP4s are really more of a computer and portable device format.
I confirm that I was able to generate a MPEG file as you described above.
I copied this onto a USB stick & inserted that into out large flat-screen TV.
No luck BUT when inserted into the BluRay player attached to the TV the MPEG WAS recognised
and it was able to play back.
Since this flat-screen TV (Sony) is from 2011 it may just be too old. We also have a Panasonic TV
from 2013 & that recognised both the MPEG & MP4 files!
So one (hopefully) last question: any thoughts why the predicted AVCHD ISO file would be significantly
less in size than the Blu-Ray ISO if the former is supposed to be the same - just on a DVD rohling?
The BluRay disc files that Premiere Elements creates are MPEGs, I believe, while the AVCHD discs are AVCHD files, which are much more efficiently compressed.