Hi all experts, need some advise on focusing please. I am using Nikkor 35mm f1.8 DX. When I tried to take pics with f1.8 using A setting, sometimes I am not able to get a sharp focus on the face for portrait shots after focus and then frame. The sharpness could be on the shoulder or chest area instead. I set my camera to AF-S and I press half shutter and hold and do framing. Could it be that the shutter speed is too slow? Please help. Thank you.