ffmpeg smooth video zoom-in to center over duration of video I'm trying to do a smooth zoom in to the center of a 4k video over the duration of that video, so basically zooming in 2.13x so that the zoom in stops when the zoom is equivalent to an HD file (4096 / 1920 = 2.13). 

I'm having trouble finding out how I can control the length of the zoom in effect dynamically, for different video lengths. 

Here's the command I have so far: 

ffmpeg -i "{filepath}" -vf "scale=w=(4096*4):h=(2160*4), zoompan=z='min(pzoom+0.00213,2.13)':d=1:x='iw/2-(iw/zoom/2)':y='ih/2-(ih/zoom/2)':s=1920x1080" "{output1}"

I'm guessing I need to do a calculation on the # of input frames and adjust the pzoom ratio accordingly, but I'm unsure how the pzoom ratio actually works.
pzoom is the calculated zoom value for the previous frame. It is initialized to 1 at the start of filtering. So, if total no. of frames is X, your zoom expr should be

z='min(pzoom+(2.13-1)/X,2.13)'