I'm really struggling with the sharpness of my pictures. I use a Nikon D3100 with prime lens 35mm f/1.8. Every time I take a picture, doesn't matter outside or inside, they all come out not sharp enough. What am I doing wrong? Also, if I'm trying to photograph kids that move all the time, I need the camera to focus really fast, but seems like with these lenses you always have to focus yourself by rotating until it's sharp enough and sometimes it doesn't even come out sharp enough. How can auto focus quickly so the pictures come out really sharp?