Precisely what's happening here is that the camera is moving forwards while it zooms out, so the background appears to widen as the field of view is getting larger. One can also achieve the opposite effect, a shrinking of the background, if the opposite is done, or if the camera is moved further away while it is being zoomed out.
Precisely what's happening here is that the camera is moving forwards while it zooms out, so the background appears to widen as the field of view is getting larger. One can also achieve the opposite effect, a shrinking of the background, if the opposite is done, or if the camera is moved further away while it is being zoomed out.