How to make zoom +/- in ffmpeg?

Question

fmpeg

I'm trying to achieve the effect of "breathing" (first smooth zoom + to 1.2 then smooth zoom- to 1) of the background throughout the video. I am using zoompan. It turns out either zoom + or zoom- on a small segment of the video.
I use this filter:

zoompan=z='min(zoom+0.0005,1.2)':x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)'

Answer 1

And there to add sin (in_time) not?