you should not always try to put your focus in the middle of your picture. For example, in this pic, if you can move girls to right a little bit, it should be better.
this is according to my understanding. I am not a fan or an expert. I took pictures for my friends in university. After we develop the pics, we could not find my friends head.
我也贴几张照片凑凑热闹，请大家指点指点，还有 Mr. elington