Bear in mind also at f/1.8 the focusing must be spot on, as the DOF is so shallow, one tiny error in your focus can ruin the shot. For people/animals, focus on the closest eye, for other objects, pick the area of most interest. A flower, I'd pick the stamen for example.
Play with the different appertures, but remember the higher your f number, the longer the shutter speed will be, so you may want to pu the camera on a tripod.