Page 127 - Photoshop for Lightroom Users – Scott Kelby 2nd Edition

Step Eight:
Open the background image you want her to appear on. This is a city street shot taken in Seattle. I focused on a friend standing in the foreground and held
my shutter button halfway down to lock the focus. Then, I recomposed so my friend was out of the frame, but my background was still out of focus and
took the shot, knowing I might want to use this as a background for portrait compositing in the future.