For some reason I'm having occasional focus difficulties with my recent purchase of a D5500. For example last night I attended a school open house and took many pictures of kids moving around a classroom. When I got home and transferred the images to my computer, I was surprised how many were not in focus for the subjects I was aiming at. Actually background subjects were in focus instead. What methods do you people use to make sure the focus is correct? The lens I was using was 35mm.