DoF is affected by aperture (wider = shallower), focal length (longer = shallower), and subject distance (shorter = shallower). So...50mm f/2.8 at 8' may be fine for two people, but a shot framing just their heads at 85mm f/2.8 at 6' may be too shallow. There are online DoF calculators (google DoFmaster), iPhone apps, etc., to play around with. After a while, selecting the aperture that's just narrow enough for your subject(s) becomes intuitive.