Using ffmpeg to change framerate

With re-encoding:

ffmpeg -y -i seeing_noaudio.mp4 -vf "setpts=1.25*PTS" -r 24 seeing.mp4

Without re-encoding:

First step - extract video to raw bitstream

ffmpeg -y -i seeing_noaudio.mp4 -c copy -f h264 seeing_noaudio.h264

Remux with new framerate

ffmpeg -y -r 24 -i seeing_noaudio.h264 -c copy seeing.mp4

You may consider using fps filter. It won't change the video playback speed:

ffmpeg -i <input> -filter:v fps=fps=30 <output>

Worked nice for reducing fps from 59.6 to 30.

Simply specify the desired framerate in "-r " option before the input file:

ffmpeg -y -r 24 -i seeing_noaudio.mp4 seeing.mp4

Options affect the next file AFTER them. "-r" before an input file forces to reinterpret its header as if the video was encoded at the given framerate. No recompression is necessary. There was a small utility avifrate.exe to patch avi file headers directly to change the framerate. ffmpeg command above essentially does the same, but has to copy the entire file.

_{In general, to set a video's FPS to 24, almost always you can do:}

With Audio and without re-encoding:

# Extract video stream
ffmpeg -y -i input_video.mp4 -c copy -f h264 output_raw_bitstream.h264
# Extract audio stream
ffmpeg -y -i input_video.mp4 -vn -acodec copy output_audio.aac
# Remux with new FPS 
ffmpeg -y -r 24 -i output_raw_bitstream.h264 -i output_audio.aac -c copy output.mp4

If you want to find the video format (H264 in this case), you can use FFprobe, like this

ffprobe -loglevel error -select_streams v -show_entries stream=codec_name -of default=nw=1:nk=1 input_video.mp4

which will output:

h264

Read more in How can I analyze file and detect if the file is in H.264 video format?

With re-encoding:

ffmpeg -y -i input_video.mp4 -vf -r 24 output.mp4