I have been happy with the lens at 1.8 but your focus has to be dead on, especially if close. At 4 feet your DOF is approx 1/3 inch in front of and behind focus plane. A running subject is not a fair test for such a shallow DOF. The 2/3 of an inch moving plane of sharpness is demanding on your focus system of both the camera and the lens. Bumping the f stop to 2.2 gives you about a 2 inch plane of sharp focus. This is still demanding but it worked for most shots.
"Cameras and lenses are simply tools to place our unique vision on film. Concentrate on equipment and you'll take technically good photographs. Concentrate on seeing the light's magic colors and your images will stir the soul." Jack Dykinga