Background fit controls how a background image or video behaves inside the available frame.
What background fit changes
Whether the full asset stays visible
Whether the asset fills the frame completely
Whether parts of the asset are cropped to avoid empty space
Background fit options
Cover makes the asset cover the full frame, which can crop some edges.
Contain keeps the full asset visible inside the frame, which can leave empty space if the aspect ratio does not match.
Contain and blur keeps the full asset visible while filling extra space with a blurred version of the same background.
Stretch makes the asset match the frame size exactly, which can distort the image or video.
How to choose the right option
Use Contain when keeping the full image visible matters most.
Use Cover when you want a full-bleed background with no empty borders.
Use Contain and blur when you want the whole asset visible without leaving hard empty bars.
Use Stretch only when distortion is acceptable or visually minor.
Tips
If a background subject is getting cut off, switch from Cover to Contain or crop the asset differently first.
If you see empty bars around the background, Cover or Contain and blur may be better options.
Background fit works best when the asset aspect ratio is already close to your video aspect ratio.