My solution for changing aspect ratio

[Hudson555x]

The easier solution is to use a 16:9 Video Mode, then Size, Position & Rotate filter if you don’t mind stretched video.
Set Size Mode to Distort.

This post may help you further:

Crop advice please

Nearly all SD video 4:3 or 16:9 has non-rectangular pixels. Shotcut has the video mode “DVD Widescreen PAL” that matches his timeline properties. It looks like there is something going on to cause an aspect ratio issue when 32 pixels are remove from the top. It is not as simple as 16 are taken from the top and 16 from the bottom even though it may look that way. Because it appears that black columns are added to the left and right. This is probably a defect in the crop filter and how its outpu…

And further down in the same post:

Crop advice please

Thank you so much for providing a sample file. We never would have solved this without it. TL;DR: Shotcut is working as expected. The problem is that these MXF files are not the direct output of a broadcast program. Somebody has first transcoded them with Vizrt and used some unconventional settings. (Vizrt is in the metadata.) Here are the problems: The video has PAL dimensions, but is using BT.709 colorspace which didn’t come into practice until HD resolutions. This is the first glari…