FFMPEG converting to slow motion issues

[Sneil]

I've recently cancelled my adobe subscription for editing and made the switch to Davinci Resolve. Best thing I've ever done. However, I'm not quite ready to pay for the full version/Studio as of yet but as a result I can't use my 60fps or 120 fps 4k footage and slow it down when needed.

So I'm trying to find free ways of doing this and I came across FFMPEG. I installed it correctly, and ran the following command
C:\Users\Neil\Desktop>ffmpeg -i C7172.MP4 -vf "setpts=5*PTS" C7172_SLOW.mp4

[Attachment 72781 - Click to enlarge]

The idea is to slow the clip 5 times down from 120fps down to 24fps. I'm not even sure if this is the proper way to do this, but my 544MB file goes to 100MB, the footage is incredibly glitchy, from what I can see with all the glitchy and jumping is the footage has not been slowed and right clicking into the file properties it still showes it at 120fps.

What am I doing wrong or is this just not possible with FFMPEG? Is there another free method for doing this or am I stuck not using 4k slow motion in my videos until I can pony up the $295USD for the full version of Davinvi Reslove?

((I've attached the resultant glitchy video I got using the above command. This was trying to slow it only by half instead of 5x, hence the 55mb file size))

[davexnet]

Have you tried something like this

ffmpeg -itsscale 5.00 -i in.mp4 -vcodec copy -an  out5.mp4