How can you scale a background?

I’ve just learned about the Canvas Scaler for UI, and it’s very useful. I’d like to do the same thing with my scene’s background image so that when the aspect ratio changes, you still see the the same content of the background, only scaled appropriately.

I tried to put the background in a UI image on a World Space canvas, but that changes the scaler to a different type that doesn’t seem to scale. If I use any other type of canvas, the image goes in front of the game content, but it’s a background so I need it to go behind.

How do I make a background scale with the aspect ratio?

I figured it out: Screen Space - Camera has a “Order in Layer” field.