Shallow focus is a photographic and cinematographic technique incorporating a small depth of field.
It is also a means by which low budget filmmakers use to hide places that would require expensive props.
There are even adapters that allow lenses from 35 mm cine cameras to be used on smaller film and digital formats.
In the film The Rules of the Game (1939), a couple flirts in the foreground while the woman's husband enters in the background.
Director Jean Renoir chose to keep the husband out of focus so that his presence is hinted, but not emphasized.