The photos taken show that people are transparent and can see objects behind their bodies. why

There is no result of long exposure with tripod. The image of this time will be recorded, while people are moving, and the movement track will be recorded by the camera, so there will be a transparent effect.

If you take a portrait at night, use the slow synchronization of the flash or the Hou Lian synchronization. You can ensure that the characters will not appear what you call "translucent effect". In addition, unless you are a machine with a high sense of NB. D700, D3, etc., always bring a tripod.