Change aspect ratio with ffmpeg without specifying width or height?

Question

I have a video that's 1920 x 1080 (16:9) and it looks stretched horizontally, because it should be 4:3.

That could be solved by converting it to a file that's 1440 x 1080, but in this case, the exact width and height needs specifying and this is what I am trying to avoid - what if a file in future is (for example) 1280 x 720 (16:9) and it's again stretched horizontally and should be 4:3, that means it should be 960 x 720 and would look normal with that width and height - but is there a way to tell ffmpeg to simply output a 4:3 file, regardless of what height and width the input file is?

I tried running this command with ffmpeg but nothing changes, even though this is the right setting as described in the documentation for ffmpeg:

ffmpeg -i "input.mp4" -aspect 4:3 "output.mp4"

In the past, this next command did change the aspect ratio on a video I was trying to (only slightly) correct from 16:10 to 16:9 or something like that, but on my current 1920 x 1080 file this also does nothing, it just comes out looking exactly the same:

ffmpeg -i "input.mp4" -aspect 1.8 "output.mp4"

I even converted 30 versions of it from "-aspect 0.1" all the way to "-aspect 3.0" and all 30 files came out looking the same, were the same size in KB, but had different checksums (proving ffmpeg is doing something different each time, although nothing looks any different in reality).

I am totally stuck, since the above "-aspect 1.4" did work a while back on some AVI files I was trying to change from 16:10 to 16:9 (or vice versa, can't remember).

The same command now isn't doing anything and "-aspect 4:3" also does nothing.

Thanks in advance.

Answer 1

Tom Yan thanks for trying to help but I figured it out.

Convert 16:9 to 4:3

ffmpeg -i "input.mp4" -vf scale=iw/1.3333:ih "output.mp4"

To convert 4:3 to 16:9 you would just reverse it:

ffmpeg -i "input.mp4" -vf scale=iw*1.3333:ih "output.mp4"