Maintaining aspect ratio with FFmpeg

Question

I need to convert a bunch of video files using FFmpeg. I run a Bash file that converts all the files nicely, however there is a problem if a file converted is not in 16:9 format

Answer 1

If '-aspect x:y' is present and output file format is ISO Media File Format (mp4) then ffmpeg adds pasp-atom (PixelAspectRatioBox) into stsd-box in the video track to indicate to players the expected aspect ratio. Players should scale video frames respectively. Not needed to scale video before encoding or transcoding to fit it to the aspect ratio, it should be performed by a player.