once you have installed it, you can use the crop filter to crop your video.
the crop is a filter that accepts the parameters such as the following
By default, ffmpeg will crop towards the center if the positions of the output egde are not given.
Crop the central input area with size 100×100:
ffmpeg -i "input.mp4" -vf crop=100:100 "output.mp4"
ffmpeg -i "input.mp4" -vf crop=w=100:h=100 "output.mp4"
Crop area with size 100×100 at position (12,34). 3)
ffmpeg -i "input.mp4" -vf crop=100:100:12:34 "output.mp4"
ffmpeg -i "input.mp4" -vf crop=w=100:h=100:x=12:y=34 "output.mp4"
The standard procedure to eliminate these black areas is to run first the cropdetect filter 4) and then run the crop filter.
Example: 5) Find video area surrounded by black borders
ffmpeg -i file.mp4 -vf cropdetect,metadata=mode=print -f null -
A human eye give a better result quicker to detect the border.
This wiki page gives another alternative